hypothesis for natural selection

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AP®︎/College Biology

Course: ap®︎/college biology   >   unit 7.

• Introduction to evolution and natural selection
• Natural selection and the owl butterfly
• Biodiversity and natural selection
• Variation in a species

Darwin, evolution, & natural selection

Natural selection.

Key points:

• Charles Darwin was a British naturalist who proposed the theory of biological evolution by natural selection.
• Darwin defined evolution as "descent with modification," the idea that species change over time, give rise to new species, and share a common ancestor.
• The mechanism that Darwin proposed for evolution is natural selection . Because resources are limited in nature, organisms with heritable traits that favor survival and reproduction will tend to leave more offspring than their peers, causing the traits to increase in frequency over generations.
• Natural selection causes populations to become adapted , or increasingly well-suited, to their environments over time. Natural selection depends on the environment and requires existing heritable variation in a group.

What is evolution?

Early ideas about evolution, influences on darwin, darwin and the voyage of the beagle.

• Traits are often heritable. In living organisms, many characteristics are inherited, or passed from parent to offspring. (Darwin knew this was the case, even though he did not know that traits were inherited via genes.) A diagram with text reading parents pass on heritable traits to their offspring. On the left a dark blue and a light blue butterfly are crossed to produce offspring with wings of varying shades of blue. On the right a dark red and a light red butterfly are crossed to produce offspring with wings of varying shades of red.
• More offspring are produced than can survive. Organisms are capable of producing more offspring than their environments can support. Thus, there is competition for limited resources in each generation. A diagram with a box reading limited resources. Arrows point away from the box to bubbles reading lack of food, lack of habitat, and lack of mates. Text below reads …not all individuals will survive and reproduce. A group of 16 butterflies with wings of varying shades of blue and red is shown. A text bubble reading gleep! comes from 4 of the butterflies.
• Offspring vary in their heritable traits. The offspring in any generation will be slightly different from one another in their traits (color, size, shape, etc.), and many of these features will be heritable. A group of 16 butterflies with wings of varying shades of blue and red is shown. A text bubble reading Hey, are you red? That's pretty sweet! comes from one of the blue butterflies. A text bubble reading Whoa! Love that blue wing color comes from one of the red butterflies. Text at the bottom reads Butterflies do not actually talk! Cartoon for cute illustration purposes only. A smiley face is shown next to the text.
• In a population, some individuals will have inherited traits that help them survive and reproduce (given the conditions of the environment, such as the predators and food sources present). The individuals with the helpful traits will leave more offspring in the next generation than their peers, since the traits make them more effective at surviving and reproducing.
• Because the helpful traits are heritable, and because organisms with these traits leave more offspring, the traits will tend to become more common (present in a larger fraction of the population) in the next generation.
• Over generations, the population will become adapted to its environment (as individuals with traits helpful in that environment have consistently greater reproductive success than their peers).

Example: How natural selection can work

Key points about natural selection, natural selection depends on the environment, natural selection acts on existing heritable variation, heritable variation comes from random mutations, natural selection and the evolution of species, attribution:, works cited:.

• Wilkin, D. and Akre, B. (2016, March 23). Influences on Darwin - Advanced. In CK-12 biology advanced concepts . Retrieved from http://www.ck12.org/book/CK-12-Biology-Advanced-Concepts/section/10.18/ .
• Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., and Jackson, R. B. (2011). The voyage of the Beagle . In Campbell Biology (10th ed., p. 466). San Francisco, CA: Pearson.
• Darwin's finches. (2016, April 25). Retrieved March 16, 2016 from Wikipedia: https://en.wikipedia.org/wiki/Darwin%27s_finches .
• Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., and Jackson, R. B. (2011). Figure 1.18. Natural selection. In Campbell biology (10th ed., p. 14). San Francisco, CA: Pearson.

Additional references:

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Natural Selection

Natural selection is one of the basic mechanisms of evolution, along with mutation, migration, and genetic drift.

Darwin’s grand idea of evolution by natural selection is relatively simple but often misunderstood. To see how it works, imagine a population of beetles:

If you have variation, differential reproduction, and heredity, you will have evolution by natural selection as an outcome. It is as simple as that.

• More Details
• Evo Examples
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See how the simple mechanisms of natural selection can  produce complex structures , learn about  misconceptions regarding natural selection , or review the history of the idea of natural selection .

Learn more about natural selection in context:

• Angling for evolutionary answers: The work of David O. Conover , a research profile.
• Battling bacterial evolution: The work of Carl Bergstrom , a research profile.
• Natural slection from the gene up: The work of Elizabeth Dahlhoff and Nathan Rank , a research profile.

Teach your students about natural selection:

• Clipbirds , a classroom activity for grades 6-12.
• Breeding bunnies , a classroom activity for grades 9-12.

Find  additional lessons, activities, videos, and articles  that focus on natural selection.

Reviewed and updated June, 2020.

Genetic drift

Natural selection at work

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Charles Darwin: Theory of Natural Selection

• Living reference work entry
• First Online: 23 October 2019
• Cite this living reference work entry

• David Stack 3  

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Survival of the fittest

Natural selection was the term Darwin used to describe the evolutionary process by which favorable or advantageous traits and characteristics are preserved and unfavorable or disadvantageous ones discarded.

Introduction

Natural selection was the term Charles Darwin (1809–1882) used for the main mechanism by which he understood evolution to work. Natural selection was first announced publicly in a joint reading of his and Alfred Russel Wallace’s papers at the Linnean Society in July 1858 (Darwin and Wallace 1858 ) and first developed in published form in Darwin’s most important book, On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life ( 1859 ). As the full title of the Origin indicates, natural selection was key to Darwin’s evolutionary argument, and the fourth chapter of the Origin was devoted to an exposition of its operation. Darwin’s definition of natural selection was...

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Al-Zahrani, A. (2007). Darwin’s metaphors revisited: Conceptual metaphors, conceptual blends, and idealized cognitive models in the theory of evolution. Metaphor and Symbol, 23 , 50–82.

Article   Google Scholar  

Barlow, N. (Ed.). (1958). The autobiography of Charles Darwin 1809–1882. With the original omissions restored. Edited and with appendix and notes by his grand-daughter Nora Barlow . London: Collins.

Google Scholar  

Browne, J. (2002). The power of place. Volume II of a biography . Princeton: Princeton University Press.

Darwin, C. (1859). On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life . London: John Murray.

Darwin, C. (1861). On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life (3rd ed.). London: John Murray.

Darwin, C. (1868). The variation of animals and plants under domestication . London: John Murray.

Darwin, C. (1869). On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life (5th ed.). London: John Murray.

Darwin, C. (1871). The descent of man, and selection in relation to sex . London: John Murray.

Book   Google Scholar  

Darwin, C. (1872). The origin of species by means of natural selection, or the preservation of favoured races in the struggle for life (6th ed.). London: John Murray.

Darwin, C. (1874). The descent of man, and selection in relation to sex (2nd ed.). London: John Murray.

Darwin, F. (Ed.). (1909). The foundations of the origin of species. Two essays written in 1842 and 1844 . Cambridge: Cambridge University Press.

Darwin, C. R., & Wallace, A. R. (1858). On the tendency of species to form varieties; and on the perpetuation of varieties and species by natural means of selection. Journal of the Proceedings of the Linnean Society of London, Zoology, 3 , 45–62.

Dobzhansky, T. (1937). Genetics and the origin of species . Columbia: Columbia University Press.

Fisher, R. (1930). The genetical theory of natural selection . Oxford: Clarendon Press.

Galton, F. (1869). Hereditary genius. An inquiry into its laws and consequences . London: Macmillan.

Goodman, L. E. (2019). Darwin’s heresy. Philosophy: The Journal of the Royal Institute of Philosophy, 94 , 43–86.

Gruber, H. E. (1974). Darwin on man. A psychological study of scientific creativity, together with Darwin’s early and unpublished notebooks, transcribed and annotated by Paul H. Barrett . London: Wildwood House.

Jenkin, F. (1867). The origin of species. The North British Review, 46 , 277–318.

Pullen, J. M. (1987). Malthus, Jesus, and Darwin. Religious Studies, 23 , 233–246.

Romanes, G. J. (1895). Darwin and after Darwin (Vol. II). London: Longmans, Green.

Spencer, H. (1864). The principles of biology (Vol. 1). London: William and Norgate.

Stack, D. (2012). Charles Darwin’s liberalism in ‘Natural selection as affecting civilised nations’. History of Political Thought, 33 , 525–554.

Stauffer, R. C. (Ed.). (1975). Charles Darwin’s natural selection; being the second part of his big species book written from 1856 to 1858 . Cambridge: Cambridge University Press.

Stott, R. (2012). Darwin’s Ghosts: In search of the first evolutionists. London: Bloomsbury.

van Wyhe, J. (2007). Mind the gap: Did Darwin avoid publishing his theory for many years? Notes and Records of the Royal Society, 61 , 177–205.

Wallace, A. R. (1864). The origin of human races and the antiquity of man deduced from the theory of “Natural Selection”. Journal of the Anthropological Society of London, 2 , clviii–clxxxvii.

Wallace, A. R. (1889). Darwinism: An exposition of the theory of natural selection, with some of its applications . London: Macmillan.

Young, R. M. (1971). Darwin’s metaphor: Does nature select? The Monist, 55 , 442–503.

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Stack, D. (2019). Charles Darwin: Theory of Natural Selection. In: Shackelford, T., Weekes-Shackelford, V. (eds) Encyclopedia of Evolutionary Psychological Science. Springer, Cham. https://doi.org/10.1007/978-3-319-16999-6_1382-1

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Natural Selection

Charles Darwin and Alfred Wallace are the two co-discoverers of natural selection (Darwin & Wallace 1858), though, between the two, Darwin is the principal theorist of the notion whose most famous work on the topic is On the Origin of Species (Darwin 1859). For Darwin, natural selection is a drawn-out, complex process involving multiple interconnected causes. Natural selection requires variation in a population of organisms. For the process to work, at least some of that variation must be heritable and passed on to organisms’ descendants in some way. That variation is acted upon by the struggle for existence, a process that in effect “selects” variations conducive to the survival and reproduction of their bearers. Much like breeders choose which of their animals will reproduce and thereby create the various breeds of domesticated dogs, pigeons, and cattle, nature effectively “selects” which animals will breed and creates evolutionary change just as breeders do. Such “selection” by nature, natural selection, occurs as a result of the struggle for existence and, in the case of sexual populations, the struggle for mating opportunities. That struggle is itself the result of checks on the geometric population increase that would occur in the absence of the checks. All populations, even slow-breeding ones such as those of elephants, will increase in size in the absence of limitations on growth that are imposed by nature. These checks take different forms in different populations. Such limitations may take the form of limited food supply, limited nesting sites, predation, disease, harsh climactic conditions, and much else besides. One way or another, only some of the candidate reproducers in natural populations actually do reproduce, often because others simply die before maturity. Owing to the variations among the candidate reproducers, some have better chances of making it into the sample of actual reproducers than do others. If such variations are heritable, the offspring of those with the “beneficial” traits will be likely to produce especially many further descendants themselves. To use one of Darwin’s own examples, wolves with especially long legs that allow them to run more quickly will be more likely to catch prey and thereby avoid starvation and so produce offspring that have especially long legs that allow them, in turn, to breed and produce still more long-legged descendants, and so on. By means of this iterative process, a trait conducive to reproduction that is initially found in one or a few population members will spread through the population.

Multiple bouts of Darwin’s process involving different traits, acting sequentially or in concert, may then explain both how speciation and the evolution of complex adaptations occur through the gradual evolution (change over time) of natural populations. Darwin aimed to convince his audience that even such structures as the vertebrate eye, which at first seem explicable only as the product of design, could instead be explained by incremental, directional evolution, a complex but still naturalistic process (Darwin 1859: ch. 6). What is initially a light sensitive patch may be transformed into an eye by means of a great many bouts of selection that gradually improve and enhance its sensitivity.

Showing that something is explicable is importantly different from explaining it (Lennox 1991); still, a theory must be an explanatory sort of theory for it to accomplish either task. After Darwin, the appearance of novel species in the geological record and the existence of designed-appearing adaptations cannot be used as grounds for invoking supernatural causes as a matter of last explanatory resort.

1. Two Conceptions of Natural Selection

2.1 replicator selectionism, 2.2 is evolution necessary for natural selection, 3. natural selection as the interpretation of a component of a formalism, 4.1 explanatory scope, 4.2 challenges to explanatoriness, 4.3 natural selection as a mechanism, 5. causation, 6. conclusion, other internet resources, related entries.

Natural selection is chiefly discussed in two different ways among contemporary philosophers and biologists. One usage, the “focused” one, aims to capture only a single element of one iteration of Darwin’s process under the rubric “natural selection”, while the other, the “capacious” usage, aims to capture a full cycle under the same rubric. These are clearly alternative, non-competing uses of the term, and distinct philosophical controversies surround each one. This section distinguishes these two uses and the following two sections are dedicated to the debates that surround each one. Sections 4 and 5 consider how natural selection connects to explanation and causation.

In Darwin’s wake, theorists have developed formal, quantitative approaches to modeling Darwin’s process. The “focused” usage of natural selection finds its home as an interpretation of a single term in some of these formalisms (and only some of them). Below, we will consider two formal approaches, type recursions and the Price Equation, elements of which have been interpreted as quantifying selection. In the Price Equation, the covariance of offspring number and phenotype is interpreted as quantifying selection; in type recursions, fitness variables (or, equivalently, selection coefficients) are interpreted as quantifying selection. What makes these interpretations focused is that they quantify only a single element of Darwin’s process using the notion natural selection; other facets of Darwin’s process are handled in other ways. So, in type recursions for instance, type frequency variables quantify how the population varies, and “spontaneous” variations are quantified by mutation parameters. Similarly, in the Price Equation, inheritance is captured by a different term than the term that is interpreted as quantifying natural selection. The point is that each formal apparatus as a whole is understood to capture Darwin’s process, while only a single element of that apparatus is said to refer to natural selection.

Some philosophers’ definitions of natural selection are clearly intended to capture this focused usage of the term. Millstein, for instance, characterizes selection as a discriminate sampling process (Millstein 2002: 35). Otsuka identifies natural selection with the causal influence of traits on offspring number in causal-graphical models (Otsuka 2016: 265). Okasha interprets the covariance of offspring number and offspring phenotype as quantifying the causal influence of selection (Okasha 2006: 28) Clearly, these uses of “natural selection” are meant to capture only an element of Darwin’s process; they make no mention of inheritance or replication. As discussed further below, controversies over the focused notion of selection have to do with whether the focused notion of selection can be distinguished from that of drift ( section 3 ), and whether selection, in the focused sense, should be counted as a cause ( section 5 ).

The alternative, “capacious”, usage of the notion of natural selection is to capture Darwin’s process in its entirety, rather than a single contributor to it. Because Darwin’s process is cyclical, specifying what is sufficient for a single cycle of it, a single instance of, say, replication of genes for long legs caused by long-legged wolves making narrow captures, is sufficient to specify a process that may explain adaptation and speciation. This is true, anyway, when it is added that the process gets repeated. The capacious notion, capturing a cycle of Darwin’s process, is used by Lewontin and later authors working in the same vein, who put forward conditions for evolution by natural selection: these include variation, inheritance, and reproduction. While falling within the scope of “natural selection” in the capacious sense used by Lewontin, these elements of Darwin’s process are treated as distinct from natural selection when that notion is used in its focused sense.

2. Evolution and the Conditions for Natural Selection

Philosophers and biologists have been concerned to state the conditions for evolution by natural selection, many because they take it that there is a single theory, evolutionary theory, that governs Darwin’s process. In many ways, the attempt to state the conditions for natural selection is a typical philosophical undertaking. We know, for instance, that confirmatory evidence may be used to raise our confidence in what it confirms and this recognition spawns a debate over what, exactly, should count as confirmation (see entry on confirmation ). Similarly, Darwin’s theory shows how some natural phenomena may be explained (including at least adaptations and speciation), and thus it is a similar philosophical concern to state exactly when the deployment of the theory is licensed. Such a statement would then issue in a verdict on what, beyond the phenomena targeted by Darwin, is equally explicable using his theory. Such a verdict could be used to arbitrate whether the spread of cultural variations, “memes”, is genuinely explicable using the theory, as Dawkins (1982) suggested. The mammalian immune system may equally involve dynamics that are explicable as selection processes (see section 4.1 of the entry on replication and reproduction ). Zurek (2009) has even defended using the theory to explain phenomena in quantum mechanics. Charlat et al. (2021) offer a critical appraisal of the operation of natural selection outside the context of living organisms.

Here are the three principles that form the “logical skeleton” of “Darwin’s argument”, according to Lewontin (1970: 1):

• Different individuals in a population have different morphologies, physiologies, and behaviors (phenotypic variation).
• Different phenotypes have different rates of survival and reproduction in different environments (differential fitness).
• There is a correlation between parents and offspring in the contribution of each to future generations (fitness is heritable)

Lewontin’s principles invoke, implicitly or explicitly, a number of causal processes, including development, reproduction, survival, and inheritance. Though it is controversial whether Lewontin succeeds, clearly his three principles aim to capture at least what is sufficient for a cycle of evolutionary change by natural selection, something which, if repeated, could be used to explain adaptation and speciation.

Lewontin’s 1970 statement of the requirements for selection should be contrasted with a similar statement in a later work, in which that author once again states that “the theory of evolution by natural selection” rests on three principles:

Different individuals within a species differ from one another in physiology, morphology, and behavior (the principle of variation); the variation is in some way heritable, so that on the average offspring resemble their parents more than they resemble other individuals (the principle of heredity); different variants leave different numbers of offspring either immediately or in remote generations (the principle of natural selection). (Lewontin 1978: 220)

This later statement is in many ways similar to the earlier one, but there are some crucial differences in the formulations. According to the later article, the different individuals must be within the same species, and it is phenotypic variations, rather than fitness, that must be inherited. Many authors have echoed Lewontin’s influential principles (see Godfrey-Smith (2007) for a thorough review); for markedly different approach to formalizing the theory, see Tennant (2014)).

Lewontin’s conditions have been criticized from many directions. For one thing, they make no mention of the populations in which selection occurs, and though the second set of conditions refers to species, all the members of a single species may not form a single population for the purposes of applying selection theory. Populations must be appropriately circumscribed for some of the key vocabulary of evolutionary theory (focused selection, drift) to be deployed in a non-arbitrary fashion (Millstein 2009). Lewontin’s formulations also make no mention of the struggle for existence, a seminal notion for Darwin (see discussion in Lennox & Wilson 1994). The principles have also been subject to counterexamples.

On the one hand, authors put forward cases of evolving populations that fail to meet Lewontin’s conditions. Earnshaw-White (2012) puts forward a population in which offspring do not inherit their parents’ traits, but whose members exhibit differential survival such that the population will evolve to a stable equilibrium. Equally, differential heritability may lead to evolutionary change without differential fitness (Earnshaw-White 2012; but see Bourrat 2015). Despite failing to meet Lewontin’s conditions, populations will evolve that feature variants that differ not in average offspring number, but only in generation time, such that one variant reproduces twice as fast as the other (Godfrey-Smith 2007: 495). On the other hand, authors put forward cases of systems that fail to evolve despite meeting Lewontin’s conditions. A system exhibiting stabilizing selection, such that it tends to evolve toward an equilibrium state where multiple variants remain within the population, may meet either set of Lewontin’s conditions despite failing to evolve (Godfrey-Smith 2007: 504). Counterexamples of these sorts depend upon the assumption that we can tell whether a system is undergoing selection without applying any set of criteria for doing so (see Jantzen 2019 for a discussion of how a Lewontin-style approach to whether a system undergoes selection is question-begging).

Okasha makes an important point about the intransitivity of covariance that is relevant to Lewontin’s principles. Okasha writes that Lewontin’s last two conditions essentially require covariance between offspring number and parental character (Principle 2), along with covariance between parental character and offspring character (Principle 3). (Here, Okasha is interpreting “fitness” in Lewontin’s conditions as offspring number rather than as a variable in a formalism quantifying selection in its focused sense.) Both those last two covariances might be positive without the system exhibiting an evolutionary response (as would occur if some of the parents with a given character have especially many offspring that do not especially resemble them, while other parents with the same character have offspring that especially resemble them but do not have especially many of them). For a system to exhibit an evolutionary response, Okasha requires that the covariance between parental offspring number and average offspring character be positive (2006: 37). Papale (2021) offers a reformulation of Leontin’s principles rejecting reproduction and relying on a pair of principles, fitness-level variation and population-level inheritance, to formulate conditions for evolution by natural selection.

Okasha’s alternative condition is intended to replace a pair of Lewontin’s principles. Interestingly, Okasha further differs from Lewontin in allowing that systems that do not evolve may meet his requirements. A system in which selection is exactly offset by transmission bias will not evolve but will undergo natural selection, according to Okasha (2006: 39).

An alternative approach to stating conditions for natural selection involves attention to replicators, of which genes are the paradigm instance. This approach was motivated by the discovery of genetic variations that spread despite not being conducive to the reproduction of the organisms that bear them, for instance, genes that exhibit meiotic drive. Replicator selectionism, which makes genes/replicators the units of selection, was developed in contrast with the Lewontin-style approaches, which make larger entities, organisms and even species, units of selection (see entry on units and levels of selection ).

Dawkins defines replicators as anything in the universe of which copies are made (1982: 82). Hull has a similar definition: a replicator is an entity that passes on its structure largely intact in successive replications (Hull 1988: 408). Germ-line replicators have the potential to have indefinitely many descendants; they contrast with somatic replicators, the genes found in body cells, which produce copies only as part of mitosis and whose lineages of descendants end when the body dies. Active replicators have some causal influence on their probability of being copied in contrast to inactive, “neutral” ones, which do not have any effects on development. Natural selection will occur wherever we find active germ-line replicators (Dawkins 1982: 83). Dawkins distinguished replicators from vehicles, his notion meant to replace and generalize that of organism. Hull proposed the notion of interactor as a similar complement to the notion of replicator. Neither notion, however, is meant to further delineate the circumstances in which selection occurs, or to narrow the scope of application of evolutionary theory (for further discussion of these notions, see entry on units and levels of selection ). This is evident, for Hull at least, insofar as genes may be both replicators and interactors (1988: 409).

The view that evolutionary theory is a theory that applies to active germ-line replicators has come under fire from a multitude of directions. Genes need not be germ-line to undergo selection, as it is at least arguable that the immune system exhibits selection processes (Okasha 2006: 11). Copying is beside the point, since only similarity across generations, rather than identity, is necessary for evolutionary change (Godfrey-Smith 2000, 2007 and entries on units and levels of selection and replication and reproduction ). For his part, Hull seems to agree with this last point, as he allows that organisms might well count as replicators, at least in cases in which they reproduce asexually (Hull 2001: 28–29).

Despite the bevy of attacks on replicator selectionism, replicator selectionists have not, to my knowledge at least, been criticized for being too permissive and allowing that systems that do not evolve count as undergoing selection. But germ-line replicators may exert a causal influence on their probability of being copied without spreading in a natural population as a result, as in some cases of frequency-dependent selection of systems already at equilibrium. In cases of frequency-dependent selection, variant genes cause their own reproduction, but the extent of influence on reproduction is a function of their frequency. Suppose each type spreads when it is rarer. At 50% frequency, the influences of each gene on reproduction cancel each other out, and a stable intermediate frequency is the result (for a case of a natural system that behaves this way, see Hori 1993). Because causing replication may not lead to differential replication in these and other cases, replicator selectionists do not effectively take evolution to be necessary for selection while Lewontin and those who follow his basic approach typically do do so.

One natural way to arbitrate the issue of whether systems that undergo selection must evolve is to attend to the point of statements of principles of natural selection, or statements of the requirements for selection. Many theorists take it that the point of these principles is to set out the scope of a theory in the special sciences that deals with selection and evolution, evolutionary theory. Lewontin claims that the theory of evolution by natural selection rests on his three principles (1978: 220). Equally, Godfrey-Smith claims that statements of conditions for evolution by natural selection exhibit the coherence of evolutionary theory and capture some of its core principles (2007: 489). Finally, Maynard-Smith (1991) offered a statement of conditions for selection that include evolution as a necessary component, calling the theory so delineated, “Darwin’s theory”. For these writers, the (or at least a ) point of the principles seems to be to capture the domain of application of the theory we have inherited from Darwin.

Darwin would have been surprised to hear that his theory of natural selection was circumscribed so as to apply only to evolving populations. He himself constructed an explanation of a persistent polymorphism, heterostyly, using his own theory. Plants exhibiting heterostyly develop two, or sometimes three, different forms of flower whose reproductive organisms vary in a number of ways, principally length. Some plants exhibit different forms of flower on the same plant, while some are dimorphic and trimorphic, with only one sort of flower per plant. Darwin interpreted the flower variations as conducive to intercrossing, which he thought was beneficial, at least for many organisms. Populations should not evolve directionally such that a single form of flower spreads throughout the population; instead, multiple variants should be retained, a polymorphism. Darwin thinks it clear that heterostyly is an adaptation:

The benefit which heterostyled dimorphic plants derive from the existence of the two forms is sufficiently obvious [….] Nothing can be better adapted for this end than the relative positions of the anthers and stigmas in the two forms. (Darwin 1877: 30; thanks to Jim Lennox for this reference)

Even though the population is not evolving, but instead remaining the same over time, it exhibits an adaptation that consists in this persistent lack of change, an adaptation that Darwin thought explicable using his theory. For a more recent and especially compelling case of a selectionist explanation of a polymorphism, see Bayliss, Field, and Moxon’s selectionist explanation of a fimbriae polymorphism produced by contingency genes (Bayliss, Field, & Moxon 2001).

Darwin thought his theory could explain a lack of evolution, and Darwinists in Darwin’s wake have explained not only stable polymorphisms, but unstable ones, cyclical behaviors, protected polymorphisms, and a variety of other behaviors that differ from simple directional evolution. These sorts of behaviors result from specific assignments of values for theoretical parameters in many of the very same models that are used to explain simple directional selection (where a single variant spreads throughout a population, as in the wolf case discussed in the introduction). The point is that systems seemingly governed by evolutionary theory exhibit a variety of different sorts of dynamics, and this variety includes both different sorts of evolution, including at least cyclical and directional, as well as a lack of evolution at all, as in cases of stabilizing selection.

Consider in particular how the difference between stabilizing and directional selection in the simplest deterministic models of diploid evolution lies in the value of a single parameter in the genotypic selection model, heterozygote fitness:

• \(p'\) is the frequency of one allele (the \(A\) allele) in the next generation
• \(q'\) is the frequency of the alternative allele (the \(a\) allele) in the next generation
• \(p\) is the frequency of \(A\) alleles in this generation
• \(q\) is the frequency of \(a\) alleles in this generation
• \(w_{D}\) is the fitness of the organisms bearing 2 \(A\) alleles
• \(w_{H}\) is the fitness of the organisms bearing 1 \(A\) allele and 1 \(a\) allele
• \(w_{R}\) is the fitness of the organisms bearing 2 \(a\) alleles.

In the above simple, “deterministic” models, if we set the fitness parameters such that \(w_{D} > w_{H} > w_{R},\) the \(A\) allele spreads throughout the population, together with whatever traits it causes. Suppose we include within the purview of Darwin’s theory models of this sort, together with the systems they (approximately) govern. If we change the value of the fitness coefficients such that \(w_{D} < w_{H} > w_{R},\) the system will exhibit a different sort of dynamics. It will evolve to an equilibrium point, where there exist some \(A\) alleles as well as some \(a\) alleles in the population, and remain there, resulting in a stable polymorphism. If we hold evolution as a condition for selection, we will issue the curious ruling that a system governed by the first sort of model falls within the scope of evolutionary theory while a system governed by the second sort of model only does so up until it reaches a stable intermediate state but then no longer. Moreover, populations exhibiting stable polymorphisms resulting from heterozygote superiority, or overdominance, are just one case among many different sorts of systems that equally exhibit stable polymorphisms.

Consider further that it is more realistic for systems’ dynamics to be a function of effective population size in the binomial sampling equation, as well as fitness. The above models are deterministic, while the dynamics of natural systems are to some extent random. A more realistic systems of equations, one capturing the randomness involved in evolutionary change, would feed the \(p'\) value issued by the above equations into the binomial sampling equation by making \(p' = j\):

where \(x_{ij}\) is the probability of \(i\) \(A\) alleles in the next generation given \(j\) \(A\) alleles in this generation, and \(N\) is effective population size. A system governed by both the deterministic equations and the binomial sampling equation is said to undergo drift; all natural systems do so. (For more on drift, effective population size, and randomness in evolutionary theory, see entry on genetic drift ). A system exhibiting heterozygote superiority whose dynamics are a function of the binomial sampling equation will not simply rest at its stable intermediate frequency but will hover around it, in some generations evolving toward it, more rarely evolving away, and in some generations exhibiting no evolution at all. Which of these cases are cases in which the system undergoes natural selection in the capacious sense? That is, which cases are cases in which the system falls within the purview of evolutionary theory? A natural answer is all of them. To answer in this way, however, we must not make evolution necessary for natural selection. If we do make evolution necessary, we issue the verdict that systems governed by the above systems of equations sometimes fall within this purview of evolutionary theory and sometimes fall outside it, on a generation to generation basis, despite being governed the whole time by a single same system of equations, ones undeniably developed as an outgrowth of Darwin’s initial theorizing.

This last pattern of argument can be extended. Indeed, given that every natural system undergoing selection also undergoes drift, evolutionary theory is arguably applicable also to systems that undergo drift even in the absence of selection (in the focused sense). Gradually reduce the importance of focused selection in the above system of equations, that is, gradually reduce the differences between \(w_{D}\), \(w_{H}\), and \(w_{R}\). Is the point at which the values equalize so momentous that it marks the point at which systems governed by the equations cease to fall within the purview of one theory and instead fall within the purview of another? Does Kimura’s neutral theory of genetic polymorphism belong in a different theory than does Ohta’s (1973) competing nearly neutral theory? Brandon (2006) argues that the principle of drift is biology’s first law, writing that neutral evolution is to evolutionary theory what inertia is to Newtonian mechanics: both being the natural or default states of the systems to which they apply. If Brandon is right, then conditions for the application of evolutionary theory must not even include conditions for selection in the focused sense, much less conditions for evolutionary change.

The point of stating conditions for evolution by natural selection need not be to state the conditions of deployment of a particular theory in the special sciences. Godfrey-Smith mentions that the principles may be important to discussions of extensions of evolutionary principles to new domains. Statements of the conditions for evolution by natural selection might have value for other reasons. But evolutionary theory is, despite the name, at least arguably a theory that is applicable to more systems than just those that evolve, as the replicator selectionists would have it.

One of the two chief uses of the notion of natural selection is as an interpretation of one or another quantity in formal models of evolutionary processes; this is the focused sense distinguished above. Two different quantities are called selection in different formal models widely discussed by philosophers. On the one hand, fitness coefficients are said to quantify selection in type recursion models of selection; the \(w\)’s in the above genotypic selection model are fitness coefficients. This is standard textbook usage (Rice 2004; Hedrick 2011). The recursive structure of these models is important. They can be used to infer how a system will behave into the future (though of course only if causes of the variables in the system do not change their values in dynamically-relevant ways that are not explicitly modeled in the recursive equations). At their simplest, type recursions make system dynamics a function of fitness coefficients weighting type frequencies/numbers together with effective population size (quantifying drift), as in the genotypic selection model discussed above. More complex variants of type recursions include frequency-dependent selection models, for cases in which population members’ fitness is a function of the type frequency variables; density-dependent selection models, for cases in which fitness is a function of population size; spatially and temporally variable selection models, for cases in which fitness varies as a function of a varying environmental variable that interacts with type differences, and many more. Writers working with type recursion models have developed explicit interpretations of their theoretical terms, including the fitness variables quantifying selection. So, for instance, Beatty and Millstein defend the view that the fitness coefficients representing selection in type recursions should be understood as modeling a discriminate sampling process, while drift, controlled by effective population size, should be understood as indiscriminate sampling (Beatty 1992; Millstein 2002).

Philosophers have also contended that particular terms in models of systems featuring the formation of groups (or collectives) should be understood as quantifying the influence of selection at different levels. So, some type recursions of systems involving groups feature “group fitness” or “collective fitness” parameters, ones analogous to individual or particle fitness parameters. Kerr and Godfrey-Smith (2002) discuss one such system of recursions; Jantzen (2019) defends an alternative parameterization of group selection as part of different system of equations. (See also Krupp 2016 for causal-graphical conceptualization of the notion of group selection.) For much more on multi-level selection, see entry on units and levels of selection .

The other formal model of particular interest to philosophers is the Price Equation. The Price Equation represents the extent of evolution in a system with respect to a given trait across a single generation using statistical functions:

• \(z_i\) denotes the character value of the \(i\) th population member
• \(\Delta Z\) is the change in average character value in the population
• \(w_i\) denotes the number of offspring produced by (the fitness of) the \(i\) th population member
• \(W\) is the average number of offspring produced by population members
• E is expectation
• Cov is covariance

In the Price Equation, selection is associated with the first right-hand side quantity, while the second represents transmission bias.

Identities among algebraic functions of statistical functions make possible the mathematical manipulation of the Price Equation such that it may feature a variety of different quantities. As with type recursions, quantities in various transformations of the Price Equation are equated with selection at different levels for different systems; Okasha, following Price, treats the covariance of the fitness of collectives with the phenotype of collectives as collective-level selection, while the average of the within-collective covariances between particle character and particular fitness is identified with particle-level selection. The persistence of altruistic variants in natural populations has been explained as the result of the stabilizing conflict between selection at these different levels (Sober & Wilson 1998; and again see entry on units and levels of selection . The Price Equation can equally be manipulated to yield distinct notions of inheritance; Bourrat distinguishes temporal, persistence, and generational heritabilities and argues for the temporal notion as appropriate for the purposes of stating conditions for evolution by natural selection (Bourrat 2015).

The distinction between type recursions and the Price Equation is important, because selection is interpreted differently in each. The two formalisms will issue in different verdicts about whether, and the extent to which, focused selection operates within a single system. If we fix upon a single natural system, and ask how selection operates within it, we will get different answers if we associate selection with fitness variables in type recursions rather than \(\textrm{Cov}(w_{i},z_{i})\) in the Price Equation. To see this, consider how type recursions are structured such that inferences about dynamics over multiple generations may be made by means of them. If fitness coefficients in these models quantify selection, and these take fixed values (as they do in the genotypic selection model considered above and a great many others), then the extent of selection will remain the same over the time period governed by the model: the fitness variables remain at fixed values so selection remains an unchanging influence. But, in just the same cases, the value of \(\textrm{Cov}(w_{i},z_{i})\) in the Price Equation will change across generations as the system evolves: the covariance function in the Price Equation will be highest at intermediate frequencies, when evolution proceeds quickly, and lowest at liminal ones, when evolution goes more slowly. Consider, for instance, the extent to which the population evolves, according to the genotypic selection model above, when the following values are plugged into the model:

In that case, the frequency of the \(A\) alleles moves from 0.9 to 0.92 across a single generation; there is some covariance between \(A\) types and reproduction. Inputting \(p = 0.5,\) and \(q = 0.5,\) the frequency change is greater, \(A\) types go from being half the population to frequency 0.56, and the covariance between offspring production and being an \(A\) type is correspondingly greater as well.

To see even more starkly how what is called selection in the Price Equation differs from what is called selection in type recursions, consider a system exhibiting heterozygote superiority like the one from earlier, where \(w_{D} = w_{R} < w_{H}.\) Recall that a system of this sort will evolve to a stable equilibrium (provided drift is idealized away). Using the recursive equations above, the reader can input values that meet the constraint that heterozygotes have the highest fitness, set \(p' = p,\) and compute the stable equilibrium value for \(p\), the frequency of the \(A\) alleles. If we understand selection as quantified by the fitness coefficients in this sort of set-up, then the whole time, selection operates in a constant fashion, since the fitness coefficients remain fixed. In particular, the operation of selection is the same when the system is evolving toward its stable equilibrium as when it remains at that stable equilibrium. By contrast, the covariance term in Price Equation model of the system will diminish in value until it reaches zero as the system evolves to its equilibrium state. When selection is identified with the covariance between type and reproduction, the frequency of the different types matters to the extent of selection. When selection is identified with fitness variables in type recursions, the frequency of different types has no influence on the extent of selection in the system. Thus, the different interpretations of selection that correspond to different quantities in different formal models are actually incompatible. We should expect, then, at least one of these interpretations of selection to fail, since focused selection cannot be two different things at once, at least if what counts as natural selection is non-arbitrary.

One way to reconcile these competing interpretations of selection is to make first right-hand side term in the Price Equation quantify the extent of the influence of selection in a system. If we assume that focused selection accounts for whatever covariance exists between parental offspring number and phenotype, then we may treat the first right-hand side term of the Price Equation as a measure of the extent of the influence of focused selection, at least at a given type frequency (see Okasha 2006: 26). This approach puts the logical house in order, allowing for a univocal concept of selection, but it does so at the expense of other commitments. To note just one, the Price Equation will no longer be causally interpretable, since its quantities may no longer be said to represent causes (but instead measure the extents of their influences given further limiting assumptions). There exists a sizable literature on which of multiple alternative manipulations of the Price Equation represents the actual causal structure of different sorts of system (see Okasha 2016 and section 5 below for more on this issue).

It was noted earlier that if what counts as selection is non-arbitrary, then it cannot be the case that what is quantified by the \(w\)’s in type recursions and the covariance of offspring number and phenotype both count as selection. A substantial debate has arisen over the question of whether what counts as selection is indeed non-arbitrary. In particular, some philosophers dubbing themselves “statisticalists” have challenged the non-arbitrary character of the distinction, claiming that it is model-relative (Walsh, Lewens, & Ariew 2002; Walsh 2004, 2007; Walsh, Ariew, & Matthen 2017). (For more on the arbitrariness of the selection/drift distinction, see entry on genetic drift ). A related issue, discussed in the subsequent section, concerns the causal interpretability of the theory: Advocates of the non-arbitrary character of selection also typically treat selection and drift not only as non-arbitrary quantities, but also as causes, while those who allege that the distinction is arbitrary typically equally challenge the treatment of selection and drift as causes.

There exists very little discussion of formal models in statisticalist writings; various toy scenarios are put forward instead, and these are not analyzed using the techniques of formal population genetics (Matthen & Ariew 2002; Walsh, Lewens, & Ariew 2002; Walsh 2007; Walsh, Ariew, & Matthen 2017). When biologically realistic scenarios are discussed, systems of equations for inferring how such systems behave are not made part of the discussion (for more on population genetics, see entry on population genetics ). We consider next a case they discuss because it provides a way of contrasting how the contrast between selection and drift is made in type recursions and how it is made in the Price Equation. There is a sort of arbitrariness here, but it emerges only from analysis of a hypothetical system using population genetics modeling techniques.

In a recent paper, Walsh, Ariew, and Matthen put forward a case of temporally variable selection and claim that it could be treated as a case either of selection or of drift (2017). The case is of a discrete-generation system with yearly reproduction in which each of two types of organisms produce different numbers of offspring depending on whether the year is warm or cold, with each type of year being equally probable: the H types produce 6 offspring in warm years while the T types produce 4, and the reverse holds for cold years. The authors take the position that the system “looks like” both a case of selection and a case of drift, but without consideration of any formal model of it. Instead, we are invited to think it is a case of selection because the fitnesses of the different types vary year-to-year; we are invited to think it is a case of drift because, allegedly, over long stretches of time the two types have equal fitnesses and we should, allegedly, predict no net increase in frequency for either type (Walsh, Ariew, & Matthen 2017: 12–13).

The scenario is illuminating because it involves randomness that cannot be quantified by effective population size in a type recursion but can be quantified as such by the drift parameter in Price Equation. When deploying type recursions, we must treat cases of temporally variable selection as cases of selection, but we are under no similar constraint when it comes to the Price Equation.

The dynamics of the warm/cold system cannot be inferred from a type recursion in which fitness coefficients are set equal and next-generation frequencies are determined entirely by the “drift” term, effective population size in the binomial sampling equation. When fitnesses are equal, the frequency of each type is determined solely by the binomial sampling equation above (since post-selection frequency, the input to the sampling equation, is just pre-selection frequency). Such a determination makes next-generation frequency a normal, bell-shaped distribution whose mean is the initial frequency of the types in the system. The distribution of next-generation frequencies in the warm/cold system does not look like this. The type recursions for the warm/cold system must be calculated using a temporally variable selection model:

• \(p\) is the frequency of warm types
• \(q\) is the frequency of cold types
• \(w^{t}_{w}\) is the fitness of warm types in year \(t\)
• \(w^{t}_{c}\) is the fitness of cold types in year \(t\)

What is inputted into the binomial sampling equation, \(p'\), when the dynamics of the system are calculated using the temporally variable selection model is never the same as what is inputted into the binomial sampling equation when the dynamics of the system are treated as solely a function of drift. In the latter case, the input into the equation is just \(p\). But since the temperature always favors one or another type, post-selection frequency, \(p'\) is never equal to pre-selection frequency, \(p\). Treating a system of warm/cold type that undergoes temporally variable selection as though it undergoes drift would lead to making false inferences about its dynamics. Cases of temporally variable selection of the warm/cold sort simply must be modeled using type recursions in which fitness parameters are a function of time; the pioneering analysis of such systems was done by Dempster (1955). The above point generalizes: what is quantifiable as selection and drift in type recursions is made definite by how fitness variables and effective population size function in those models.

The story is different, however, with the Price Equation, owing to how randomness is handled in that formalism. A version of the Price Equation in which both selection and drift are represented is this (Okasha 2006: 32):

• \(w_{i}^{*}\) is the expected fitness of the \(i\) th population member
• \(z_{i}'\) is the average character value of the \(i\) th population member’s offspring
• \(\mu_{i}\) is deviation from expectation of the \(i\) th population member’s offspring production

Here, the second term quantifies change due to drift (Okasha 2006: 33). Note how the \(\mu_{i}\) parameter quantifies deviation from expectation, and hence drift is a function of how the expectation is determined. Nothing about the Price Equation formalism constrains such determinations. In the warm/cold population, whether a given year’s evolution is quantified by the first right-hand side term, and hence constitutes change due to selection, or the second one, and hence constitutes change due to drift, will be determined by whether the theorist treats the weather as contributing to expected fitness. If, for instance, she is ignorant of how the weather changes year to year, she may treat the weather as drift; if she can predict it, she may not do so. Deployment of the Price Equation is compatible with both treating the weather as contributing to expected fitness and treating it as causing deviation from expectation.

The result is that a theorist deploying the Price Equation may treat as drift (that is, quantify as deviation from expectation) what a theorist deploying type recursions must treat as selection (quantify by fitness variables). It is possible to make assumptions using the Price Equation such that the drift term quantifies what is quantified by the drift term in type recursions, but nothing about the Price Equation proper forces one to do this, and indeed proponents of the Price Equation, such as Grafen (2000), tout how the drift term in the Price Equation may quantify all sorts of randomness, explicitly including randomness that is not quantifiable as drift in type recursions.

As noted earlier, selection and drift are construed in logically distinct fashions in type recursions and the Price Equation. What is called is selection in type recursions is one sort of definite thing: it’s whatever it is that must be quantified by fitness variables. What is called selection in the Price Equation is another thing, and this will be determined by researchers’ understanding of the system, in particular how they generate expectations about offspring production.

Ultimately, the conflict between the two modeling approaches with respect to what counts as selection may be resolvable in at least a couple of ways. Perhaps one modeling approach is simply wrong about what selection is. Alternatively, something having to do with selection is arbitrary here. In particular, whether some or another form of randomness should be treated as selection or drift is a function of one’s choice of which modeling approach to use, where distinct approaches provide logically distinct understandings of “selection” and “drift”. Once one chooses to model the system using type recursions, one’s hand is forced: the temperature must be modeled as selection. Once one chooses to model the system using the Price Equation, one’s hand remains free: one may treat the changing temperature as relevant to expected fitness, and hence treat it as selection, or instead treat it as producing deviation from expectation, and thereby treat it as drift. According to this second way of resolving the conflict, the choice between the two modeling approaches is not dictated by nature, and is thus at least metaphysically (if not pragmatically) arbitrary.

4. Natural Selection and Explanation

As noted above, evolutionary theory cannot do its job unless it has an explanatory structure. Philosophers have contended selection explains a variety of things in a variety of ways. Philosophers have detected many types of explanation used in the study of evolution, including selection explanations (McLoone, 2013), phylogenetic inertia explanations (Griffiths, 1996; Orzack and Sober, 2001), lineage explanations (Brown 2014; Calcott, 2009), homology explanations (Ereshefsky, 2012), evolvability-based explanations (Brown, 2014), part-whole explanations (Winther, 2011), topological explanations (Huneman, 2010; 2018). Huneman (2018) argues for pluralism with respect to explanation in evolutionary biology. Reydon (2023) argues that evolutionary explanations consist of a pair of explanations, each of which involve natural selection. Below, I consider some controversies concerning whether and how selection explains.

There exists a longstanding debate among both scientists and philosophers over exactly what natural selection can explain, one begun by Sober and Neander who were concerned with what natural selection can explain (Sober 1984, 1995; Neander 1988, 1995). In an expansive treatment, Razeto-Barry & Frick (2011) distinguish between the creative and non-creative views of natural selection; Beatty (2016; 2019) provides a historical perspective on the issue. On the non-creative view, natural selection merely eliminates traits while doing nothing to create new ones; the latter phenomenon is the result of mutation. Proponents of the creative view see natural selection as a creative force that makes probable combinations of mutations that are necessary for the development of at least some traits. While Razeto-Barry and Frick grant that natural selection cannot explain the origin of traits that arise by a single mutation, they argue that it can explain the occurrence of sequences of phenotypic changes that would otherwise be wildly unlikely to occur without selection operating to cause the spread of the changes prior to the final one in the sequence. Additionally, it is disputed whether natural selection can explain why an arbitrary individual has the traits it has (Walsh 1998; Pust 2004; Birch 2012). On the positive view, selection affects the identity of individual organisms, and it is part of the identity of an individual to have been produced by the parents that produced it, so natural selection explains why individuals have the traits they do. On the negative view, the explanatory scope of natural selection is limited to population level properties. Razeto-Barry and Frick further consider the question of whether natural selection can explain the existence of individuals, ultimately arguing against it. Reydon (2011) critiques the metaphor that natural selection functions as a sieve by which it eliminates variations.

The capacity for natural selection to explain has come under fire from several directions. An important challenge to the explanatory power of selection was made by Gould and Lewontin who impugn the veracity of selectionist accounts of the origin of traits whose correctness is pinned on nothing more than their plausibility and capacity to explain (Gould & Lewontin 1979: 588). Gould and Lewontin offer several examples of traits for which such “just so” stories are easy to construct but nonetheless taken to be true on the basis of little or no evidence beyond the plausibility of the accounts and their consistency with the theory. Lennox argues that, when not taken as even purportedly true, “just so stories” can play a genuine, if modest, role in the evaluation of scientific theories as thought experiments used to assess the explanatory potential of the theory (see also Mayr 1983; Lennox 1991; entry on adaptationism ). In the debate over Darwin’s theory, thought experiments of this sort were deployed by both Darwin and his critics in ways both legitimate and rhetorically effective (Lennox 1991).

Another attack, or set of attacks, on the ability of natural selection to explain have to do with the threat that selectionist explanations are circular. This sort of problem with a would-be explanation arises when one must already know what one means to explain in order to construct one’s (thereby failed) explanation of it. This problem surfaces, in different ways, in the contexts of Lewontin’s requirements, type recursions, and the Price Equation.

Lewontin’s requirements for evolution by natural selection contain an ambiguity. “Fitness” could refer either to offspring number or to fitness variables of the sort found in the genotypic selection equation above (the \(w\)’s). For instance, Godfrey-Smith, in noting that some formalisms do not feature fitness variables at all, is using the term in the latter sense (2007: 496), while Okasha’s treatment of Lewontin’s requirements uses fitness in the former sense. Suppose fitness means offspring number and suppose further that the requirements play the role of determining under what circumstances evolutionary theory may be deployed. If these things hold, then the circularity problem arises: we must know that the variants in the system have different numbers of offspring in order to apply Lewontin’s requirements to deploy the theory that would explain the very same offspring production despite such production serving to license the theory’s deployment in the first place.

Consider type recursions next. Suppose we must know actual reproduction rates to assign relative fitnesses values in type recursions. Were this so, an alleged explanation of the extent of evolutionary change in the system that makes crucial use of type recursions would be circular. The circularity problem does not come up for practicing biologists deploying type recursions, as those workers rely upon fitness estimates that are inferred from statistical facts about a target system during the estimation phase in order to assign values to variables in type recursions that are then deployed over the system during the projection phase. What such biologists mean to explain, the behavior during the projection phase, is different from what they use to construct the explanation, the behavior during the estimation phase (Mills & Beatty 1979; Bouchard & Rosenberg 2004; the estimation/projection phase contrast is due to Glymour 2006).

That the scientists largely agree about the practice of statistical estimation shows that they largely share some tacit concepts of selection and fitness, ones it would be an advance for philosophers to define. Philosophers have developed definitions of fitness. Brandon was the first to defend the propensity interpretation of fitness and it has earned much discussion since (Brandon 1978; Mills & Beatty 1979; Rosenberg 1982; Brandon & Beatty 1984; Brandon 1990; Sober 2000). Rosenberg and Bouchard have developed a recent definition of fitness based on the relationships between an individual and its environment that contribute to the individual’s success (Bouchard & Rosenberg 2004; for more on “fitness” see entry on fitness ).

Turning finally to how the tautology problem surfaces in the context of the Price Equation, consider how that equation formally represents the extent of evolution across some time period in which reproduction occurs. Assigning values to its statistical functions requires information that spans that time period, such as information about the offspring number of the parents and the phenotypes of the offspring. The deployment of the Price Equation does not involve any estimation/projection period contrast. It cannot be used a source of new information about some time period that remains otherwise unexamined. The Price Equation could not, for instance, be used to make a prediction about the dynamics of some system into the future in the same way that type recursions can do. This is because “an application of the Price equation for the purpose of prediction would presuppose the very information you want to predict with it” (Otsuka 2016: 466). For this reason, Otsuka claims that the equation is not explanatory (2016: 466).

Another difficulty faced by selectionist explanations has to do with their reliability. Glymour has argued against the reliability of explanations constructed using type recursions: “population genetics models evolving populations with the wrong variables related by the wrong equations employing the wrong kinds of parameters” (Glymour 2006: 371). Glymour demonstrates that exogenous fitness variables cannot be used to quantify environmental causes of reproduction that change in their influence during the projection period. Fitness cannot be a kind of summary variable that abstracts away from the causal details of the natural population while also functioning to reliably predict and explain the dynamics of the systems to which type recursions apply. Gildenhuys (Gildenhuys 2011) provides a response to Glymour, acceding that his arguments are valid but contending that there are a variety of ways to complexify type recursions to formally handle the influence of environmental causes; Glymour (2013) provides a reply.

Philosophers interested in explanation have recently turned their attention to mechanisms: “mechanisms are sought to explain how a phenomenon comes about or how some significant process works” (Machamer, Darden, & Craver 2000: 2). One way to secure explanatory status for selection would be to show that it functions as a mechanism. Whether natural selection qualifies as a mechanism is controversial. Skipper and Millstein gainsay the position: natural selection not organized in the right sort of way to count as a mechanism, exhibits a lack of productive continuity between stages, and further lacks the requisite sort regularity of operation (Skipper & Millstein 2005: 335). Barros (2008) has argued that natural selection may be characterized as a two-level mechanism, with a population-level mechanism and an individual-level mechanism working together. Havstad (2011) responds that the account Barros offers is too general and so includes any selective process, not just natural selection. Moreover, the parts/entities and activities/interactions that supposedly make-up the mechanism of selection can only be specified by the roles in the selective process, whereas mechanistic explanation works by associating these roles with specific phenomena (Havstad 2011: 522–523). Matthews (2016) offers a case study of the debate over selection as a mechanism, while DesAutels (2016) provides a defense of a mechanistic view. Villegas (2024) integrates the mechanistic view of evolutionary developmental biology with the population-level approach of evolutionary theory.

Godfrey-Smith makes the point that some character could fit Lewontin’s conditions for evolution by natural selection by mere chance. Parents with a particular trait could have more offspring than parents without it, even though the trait does not causally influence offspring number (Godfrey-Smith 2007: 511). In such cases, Lewontin would rule there is evolution by natural selection, provided that the trait is heritable, since individuals with it have a de facto higher rate of survival and reproduction, if only by chance. Equally, a trait could spread by hitchhiking and meet Lewontin’s requirements for selection (or Okasha’s for that matter). A hitchhiking trait is correlated with one that causes reproduction, and as a result, individuals with the trait exhibit increased reproduction, despite the trait not causing reproduction at all. But, for Godfrey-Smith, there is no natural selection, at least in the focused sense, on the trait in these sorts of cases because differences in the character had no causal role in producing variation in offspring number (Godfrey-Smith 2007: 513). Thus, for Godfrey-Smith, anyway, natural selection in the focused sense is definitely a causal notion.

As noted earlier, theorists working on type recursions who defend the view that selection is a distinct quantity from drift typically defend the view that it is also a distinct cause. Forber and Reisman explicitly invoke the interventionist account of causation and argue that natural selection (and drift) qualify as causes (Reisman & Forber 2005). They do so on the basis of an examination of experimental work in which selection (and drift) are manipulated to produce changes in population-level behavior. “Causalists” working with type recursions must confront the natural philosophical challenge of stating, precisely, what cause or causes count as selection, that is, what exactly fitness coefficients quantify. The Beatty/Millstein interpretation of selection as discriminate sampling is explicitly causal. Equally, Otsuka’s position that fitness coefficients should be interpreted as linear path coefficients from phenotype to offspring number in causal graphs is clearly causal as well.

The Price Equation may be deployed for some system without regard to what causal relationships hold within it. Okasha argues that variants of the equation may constitute causally adequate representations under special circumstances. It is important to distinguish this contention from the contention that the Price Equation is causally interpretable. The right-hand side terms of the Price Equation are (algebraic functions of) statistical functions, while, in causally interpretable equations, causes are variables (Woodward 2003: 93). A causally interpretable system of equations is one that one features causal variables on its right-hand side and effects on the left: \(a = b\) is causally interpretable if \(b\) causes \(a\) but not if the reverse. Luque and Baravalle (2021) see the Price Equation in analogy to classical mechanics, with the Price Equation quantifying trait diffusion as the effect of forces, where distinct specifications of the Price Equation are identified with distinct types of selection, such as directional or stabilizing selection.

Okasha’s basic approach is to assess whether some formulation of the Price Equation as a causally adequate representation in terms of whether the statistical functions in the equation take positive values while their arguments are causally related (2016: 447). In a multi-level, causally adequate version of the Price Equation,

• \(W\) is average offspring number
• \(\Delta P\) is change in average character value
• \(W_{k}\) is the average fitness of the individuals in the \(k\) th group
• \(P_{k}\) is the average character value of the individuals in the \(k\) th group
• \(\mathrm{E}_{k}\) is the average across groups
• \(w_{jk}\) is the offspring number of the \(j\) th individual in the \(k\) th group
• \(p_{jk}\) is the character value of the \(j\) th individual in the \(k\) th group

Okasha proposes a test for causal adequacy that works this way: if the first term takes a positive value, then \(W_{k}\) and \(P_{k}\) must be causally related, or else the equation is not causally adequate. Equally, a version of the Price Equation is not causally adequate if its statistical functions take a positive value despite their components failing to be causally related, or if its statistical functions take value zero when their components are causally related. Whether a version of the Price Equation is causally adequate is thus a function of the causal relationships among the variables in the system. On this approach, the concept of causal adequacy means something different than causal interpretability does for equations that exhibit dependencies between variables that represent causes.

This entry initially distinguished two usages of “natural selection”, the focused and the capacious one, the focused one picking out a single formal element of the process captured by the capacious sense. There are some intermediate cases. Brandon offers this definition of selection: selection is differential reproduction that is due to differential adaptedness to a common selective environment (Brandon 2005: 160). This definition does not attempt to capture the entirety of Darwin’s process: while it identifies natural selection with a type of reproduction, there is no mention of inheritance or replication. Repeat a process specified to fit Brandon’s definition as often as you like and we still might not have something that makes adaptation or speciation explicable. But neither is Brandon’s definition of selection appropriate as an interpretation of a theoretical term in any formalism. Instead, that definition connects with an effort to develop a Principle of Natural Selection which would serve a seminal role in a qualitative version of Darwin’s theory. The Principle is formulated using probability talk along with the notion of fitness. The historical development of the Principle of Natural Selection, beginning with Brandon together with Beatty and Mills (Brandon 1978; Mills & Beatty 1979), has been guided by debates over the interpretation of probability and fitness addressed elsewhere (Brandon 1990: ch. 1; Sober 2000; Campbell & Robert 2005; entry on fitness ).

“Natural selection” has been used to pick out one, or multiple, or all the elements of a single cycle of the recursive process that we learned from Darwin. Seemingly, there is arbitrariness in how one decides to deploy the term natural selection, such that any part of Darwin’s recursive process could be treated as the natural selection part . Cycles of Darwin’s process are important theoretical elements; variables in systems of equations that specify elements of Darwin’s process are equally important theoretical elements. It is difficult to say that either Brandon, or Okasha, or Otsuka, or Millstein is wrong in their characterizations of selection, even though the characterizations are superficially logically incompatible. The several definitions pick out genuine elements of genuine processes, each with their own significant theoretical importance.

• Barros, D. Benjamin, 2008, “Natural Selection as a Mechanism”, Philosophy of Science , 75(3): 306–322. doi:10.1086/593075
• Bayliss, Christopher D., Dawn Field, and Richard Moxon, “The Simple Sequence Contingency Loci of Haemophilus Influenzae and Neisseria Meningitidis ”, Journal of Clinical Investigation , 107(6): 657–662. doi:10.1172/JCI12557
• Beatty, John, 1992, “Random Drift”, in Evelyn Fox Keller and Elisabeth A. Lloyd (ed.), Keywords in Evolutionary Biology , Cambridge, MA: Harvard University Press, pp. 273–281.
• –––, 2016, “The Creativity of Natural Selection? Part 1: Darwin, Darwinism, and the mutationists”, Journal of the history of Biology , 49: 659–684. doi:10.1007/s10739-016-9456-5
• –––, 2019, “The Creativity of Natural Selection? Part 2: the synthesis and since”, Journal of the history of Biology , 52(4): 705–731. doi:10.1007/s10739-019-09583-4
• Birch, Jonathan, 2012, “The Negative View of Natural Selection”, Studies in History and Philosophy of Science Part C: Studies in History and Philosophy of Biological and Biomedical Sciences , 43(2): 569–573. doi:10.1016/j.shpsc.2012.02.002
• Bouchard, Frédéric and Alex Rosenberg, 2004, “Fitness, Probability and the Principles of Natural Selection”, The British Journal for the Philosophy of Science , 55(4): 693–712. doi:10.1093/bjps/55.4.693
• Bourrat, Pierrick, 2015, “How to Read ‘Heritability’ in the Recipe Approach to Natural Selection”, The British Journal for the Philosophy of Science , 66(4): 883–903. doi:10.1093/bjps/axu015
• Brandon, Robert N., 1978, “Adaptation and Evolutionary Theory”, Studies in History and Philosophy of Science Part A , 9(3): 181–206. doi:10.1016/0039-3681(78)90005-5
• –––, 1990, Adaptation and Environment , Princeton, NJ: Princeton University Press.
• –––, 2005, “The Difference Between Selection and Drift: A Reply to Millstein”, Biology & Philosophy , 20(1): 153–170. doi:10.1007/s10539-004-1070-9
• –––, 2006, “The Principle of Drift: Biology’s First Law”, Journal of Philosophy , 103(7): 319–335. doi:10.5840/jphil2006103723
• Brandon, Robert N. and John Beatty, 1984, “The Propensity Interpretation of ‘Fitness’ – No Interpretation is No Substitute”, Philosophy of Science , 51(2): 342–347.
• Campbell, Richmond and Jason Scott Robert, 2005, “The Structure of Evolution by Natural Selection”, Biology & Philosophy , 20(4): 673–696. doi:10.1007/s10539-004-2439-5
• Charlat, Sylvain, Andre Ariew, Pierrick Bourrat, Maria Ferreira Ruiz, Thomas Heams, Phillipe Heneman, Sandeep Krishna, Michael Lachmann, Nicholas Lartillot, Louis Le Sergeant d’Hendecourt, Christophe Malterre, Phillipe Nghe, Etienne Rajon, Olivier rivoire, Matteo Smerlak, Zorana Zeravcic, 2021, “Natural Selection Beyond Life? A Workshop Report”, Life , 11(10). doi:10.3390/life11101051
• Darwin, Charles, 1859, On the Origin of Species , London: John Murray.
• –––, 1877, The Different Forms of Flowers on Plants of the Same Species , London: John Murray. New York: New York University Press. [ Darwin 1877 available online ]
• Darwin, Charles and Alfred Wallace, 1858, “On the Tendency of Species to Form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection”, Journal of the Proceedings of the Linnean Society of London. Zoology , 3(9): 45–62. doi:10.1111/j.1096-3642.1858.tb02500.x
• Dawkins, Richard, 1982, The Extended Phenotype: the Gene as the Unit of Selection , New York: Oxford University Press.
• Dempster, Everett R., 1955, “Maintenance of Genetic Heterogeneity”, Cold Spring Harbor Symposia on Quantitative Biology , 20: 25–32. doi:10.1101/SQB.1955.020.01.005
• DesAutels, Lane, 2016, “Natural Selection and Mechanistic Regularity”, Studies in History and Philosophy of Science Part C: Studies in History and Philosophy of Biological and Biomedical Sciences , 57: 13–23. doi:10.1016/j.shpsc.2016.01.004
• Earnshaw-Whyte, Eugene, 2012, “Increasingly Radical Claims about Heredity and Fitness”, Philosophy of Science , 79(3): 396–412. doi:10.1086/666060
• Gildenhuys, Peter, 2011, “Righteous Modeling: The Competence of Classical Population Genetics”, Biology & Philosophy , 26(6): 813–835. doi:10.1007/s10539-011-9268-0
• Glymour, Bruce, 2006, “Wayward Modeling: Population Genetics and Natural Selection”, Philosophy of Science , 73(4): 369–389. doi:10.1086/516805
• –––, 2013, “The Wrong Equations: A Reply to Gildenhuys”, Biology & Philosophy , 28(4): 675–681. doi:10.1007/s10539-013-9362-6
• Godfrey-Smith, Peter, 2000, “The Replicator in Retrospect”, Biology & Philosophy , 15(3): 403–423. doi:10.1023/A:1006704301415
• –––, 2007, “Conditions for Evolution by Natural Selection”:, Journal of Philosophy , 104(10): 489–516. doi:10.5840/jphil2007104103
• Gould, Stephen J. and Richard C. Lewontin, 1979, “The Spandrels of San Marco and the Panglossian Paradigm”, Proceedings of the Royal Society of London , 205(1161): 581–598. doi:10.1098/rspb.1979.0086
• Grafen, Alan, 2000, “Developments of the Price Equation and Natural Selection under Uncertainty”, Proceedings of the Royal Society of London. Series B: Biological Sciences , 267(1449): 1223–1227. doi:10.1098/rspb.2000.1131
• Havstad, Joyce C., 2011, “Problems for Natural Selection as a Mechanism”, Philosophy of Science , 78(3): 512–523. doi:10.1086/660734
• Hedrick, Philip W., 2011, Genetics of Populations , fourth edition, Sudbury, MA: Jones and Bartlett Publishers.
• Hori, Michio, 1993, “Frequency-Dependent Natural Selection in the Handedness of Scale-Eating Cichlid Fish”, Science , 260(5105): 216–219. doi:10.1126/science.260.5105.216
• Hull, David L., 1988, Science as a Process: An Evolutionary Account of the Social and Conceptual Development of Science , Chicago: University of Chicago Press.
• –––, 2001, Science and Selection: Essays on Biological Evolution and the Philosophy of Science , Cambridge: Cambridge University Press.
• Jantzen, Benjamin C., 2019, “Kinds of Process and the Levels of Selection”, Synthese , 196(6): 2407–2433. doi:10.1007/s11229-017-1546-1
• Kerr, Benjamin and Peter Godfrey-Smith, 2002, “Individualist and Multi-Level Perspectives on Selection in Structured Populations”, Biology & Philosophy , 17(4): 477–517. doi:10.1023/A:1020504900646
• Krupp, D.B., 2016, “Causality and the Levels of Selection”, Trends in Ecology & Evolution , 31(4): 255–257. doi:10.1016/j.tree.2016.01.008
• Lennox, James G., 1991, “Darwinian Thought Experiments: A Function For Just So Stories”, in Tamara Horowitz and Gerald J. Massey (eds.), Thought Experiments in Science and Philosophy , Savage, MD: Rowman and Littlefield, pp. 173–195.
• Lennox, James G. and Bradley E. Wilson, 1994, “Natural Selection and the Struggle for Existence”, Studies in History and Philosophy of Science Part A , 25(1): 65–80. doi:10.1016/0039-3681(94)90020-5
• Lewontin, Richard C, 1970, “The Units of Selection”, Annual Review of Ecology and Systematics , 1(1): 1–18. doi:10.1146/annurev.es.01.110170.000245
• –––, 1978, “Adaptation”, Scientific American , 239(3): 212–230. doi:10.1038/scientificamerican0978-212
• Machamer, Peter, Lindley Darden, and Carl F. Craver, 2000, “Thinking about Mechanisms”, Philosophy of Science , 67(1): 1–25. doi:10.1086/392759
• Matthen, Mohan, Andre Ariew, 2002, “Two Ways of Thinking about Fitness and Natural Selection”, The Journal of Philosophy , XCIX(2): 55–83. doi:10.2307/3655552
• Matthews, Lucas J., 2016, “On Closing the Gap between Philosophical Concepts and Their Usage in Scientific Practice: A Lesson from the Debate about Natural Selection as Mechanism”, Studies in History and Philosophy of Science Part C: Studies in History and Philosophy of Biological and Biomedical Sciences , 55: 21–28. doi:10.1016/j.shpsc.2015.11.012
• Maynard Smith, John, 1991, “A Darwinian View of Symbiosis”, in Lynn Margulis and René Fester (eds), Symbiosis as a Source of Evolutionary Innovation , Cambridge, MA: MIT Press, pp. 26–39.
• Mayr, Ernst, 1983, “How to Carry Out the Adaptationist Program?”, The American Naturalist , 121(3): 324–334. doi:10.1086/284064
• Mills, Susan K. and John H. Beatty, 1979, “The Propensity Interpretation of Fitness”, Philosophy of Science , 46(2): 263–286. doi:10.1086/288865
• Millstein, Roberta L., 2002, “Are Random Drift and Natural Selection Conceptually Distinct?”, Biology & Philosophy , 17(1): 33–53. doi:10.1023/A:1012990800358
• –––, 2009, “Populations as Individuals”, Biological Theory , 4(3): 267–273. doi:10.1162/biot.2009.4.3.267
• Neander, Karen, 1988, “What Does Natural Selection Explain? Correction to Sober”, Philosophy of Science , 55(3): 422–426. doi:10.1086/289446
• –––, 1995, “Explaining Complex Adaptations: A Reply to Sober’s ‘Reply to Neander’”, The British Journal for the Philosophy of Science , 46(4): 583–587. doi:10.1093/bjps/46.4.583
• Ohta, T., 1973, “Slightly Deleterious Mutant Substitutions in Evolution”, Nature , 246(5428): 96–8. doi:10.1038/246096a0
• Okasha, Samir, 2006, Evolution and the Levels of Selection , New York: Oxford University Press. doi:10.1093/acprof:oso/9780199267972.001.0001
• –––, 2016, “The Relation between Kin and Multilevel Selection: An Approach Using Causal Graphs”, The British Journal for the Philosophy of Science , 67(2): 435–470. doi:10.1093/bjps/axu047
• Otsuka, Jun, 2016, “Causal Foundations of Evolutionary Genetics”, The British Journal for the Philosophy of Science , 67(1): 247–269. doi:10.1093/bjps/axu039
• Papale, Francois, 2021, “Evolution by Means of Natural Selection without Reproduction: revamping Lewontin’s account”, Synthese , 198(11): 10429–10455. doi:10.1007/s11229-020-02729-6
• Pust, Joel, 2004, “Natural Selection and the Traits of Individual Organisms”, Biology & Philosophy , 19(5): 765–779. doi:10.1007/s10539-005-0888-0
• Razeto-Barry, P. and R. Frick, “Probabilistic causation and the explanatory role of natural selection”, Studies in History and Philosophy of Science Part C: Studies in History and Philosophy of Biological and Biomedical Sciences , 42(3): 344–355. doi:10.1016/j.shpsc.2011.03.001
• Reisman, Kenneth and Patrick Forber, 2005, “Manipulation and the Causes of Evolution”, Philosophy of Science , 72(5): 1113–1123. doi:10.1086/508120
• Reydon, Thomas A. C., 2011, “The Arrival of the Fittest What?”, in D. Diecks, W.J. Gonzales, S. Hartmann, T. Uebel & M. Weber (eds.), Explanation, Prediction, and Confirmation , Dordrecht: Springer, pp. 162–187.
• –––, 2023, “The Proper Role of History in Evolutionary Explanations”, Nous , 57(1): 162–187. doi: 10.1111/nous.12402.
• Rice, Sean H., 2004, Evolutionary Theory: Mathematical and Conceptual Foundations , Sunderland, MA: Sinauer and Associates.
• Rosenberg, Alexander, 1982, “On the Propensity Definition of Fitness”, Philosophy of Science , 49(2): 268–273.
• Skipper, Robert A. and Roberta L. Millstein, 2005, “Thinking about Evolutionary Mechanisms: Natural Selection”, Studies in History and Philosophy of Science Part C: Studies in History and Philosophy of Biological and Biomedical Sciences , 36(2): 327–347. doi:10.1016/j.shpsc.2005.03.006
• Sober, Elliott, 1984, The Nature Of Selection: Evolutionary Theory In Philosophical Focus , Cambridge, MA: MIT Press.
• –––, 1995, “Natural Selection and Distributive Explanation: A Reply to Neander”, The British Journal for the Philosophy of Science , 46(3): 384–397. doi:10.1093/bjps/46.3.384
• –––, 2000, Philosophy of Biology , Boulder, CO: Westview Press.
• Sober, Elliott and David Sloan Wilson, 1998, Unto Others: The Evolution and Psychology of Unselfish Behavior , Cambridge, MA: Harvard University Press.
• Tennant, Neil, 2014, “The Logical Structure of Evolutionary Explanation and Prediction: Darwinism’s Fundamental Schema”, Biology & Philosophy , 29(5): 611–655. doi:10.1007/s10539-014-9444-0
• Villegas, Cristina, 2024, “Causing and Composing Evolution: lessons from evo-devo mechanisms”, in J.L. Cordovil, G. Santos, D. Vecchi (eds.), New Mechanism. History, Philosophy and Theory of the Life Sciences (Volume 35), Cham: Springer. doi:10.1007/978-3-031-46917-6_4.
• Walsh, D.M., 1998, “The Scope of Selection: Sober and Neander on What Natural Selection Explains”, Australasian Journal of Philosophy , 76(2): 250–264. doi:10.1080/00048409812348391
• –––, 2004, “Bookkeeping or Metaphysics? The Units of Selection Debate”, Synthese , 138(3): 337–361. doi:10.1023/B:SYNT.0000016426.73707.92
• –––, 2007, “The Pomp of Superfluous Causes: The Interpretation of Evolutionary Theory*”, Philosophy of Science , 74(3): 281–303. doi:10.1086/520777
• Walsh, Denis M., André Ariew, and Mohan Matthen, 2017, “Four Pillars of Statisticalism”, Philosophy, Theory, and Practice in Biology , 9(1). doi:10.3998/ptb.6959004.0009.001 [ Walsh, Ariew, & Matthen 2017 available online ]
• Walsh, Denis, Tim Lewens, and André Ariew, 2002, “The Trials of Life: Natural Selection and Random Drift*”, Philosophy of Science , 69(3): 429–446. doi:10.1086/342454
• Woodward, James, 2003, Making Things Happen: A Theory of Causal Explanation , New York: Oxford University Press. doi:10.1093/0195155270.001.0001
• Zurek, Wojciech Hubert, 2009, “Quantum Darwinism”, Nature Physics , 5(3): 181–188. doi:10.1038/nphys1202

How to cite this entry . Preview the PDF version of this entry at the Friends of the SEP Society . Look up topics and thinkers related to this entry at the Internet Philosophy Ontology Project (InPhO). Enhanced bibliography for this entry at PhilPapers , with links to its database.

• Brandon, Robert, “Natural Selection,” Stanford Encyclopedia of Philosophy (Fall 2019 Edition), Edward N. Zalta (ed.), URL = < https://plato.stanford.edu/archives/fall2019/entries/natural-selection/ >. [This was the previous entry on natural selection in the Stanford Encyclopedia of Philosophy — see the version history .]

adaptationism | confirmation | fitness | genetic drift | genetics: population | natural selection: units and levels of | replication and reproduction

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Natural selection is one of the ways to account for the millions of species on Earth. For example, the beetle family Curculionidae (snout beetles) is extremely diverse, comprising an estimated 83,000 species.

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What is natural selection?

Natural selection is a mechanism of evolution. Organisms that are more adapted to their environment are more likely to survive and pass on the genes that aided their success. This process causes species to change and diverge over time.

Natural selection is one of the ways to account for the millions of species that have lived on Earth.

Charles Darwin (1809-1882) and Alfred Russel Wallace (1823-1913) are jointly credited with coming up with the theory of evolution by natural selection, having co-published on it in 1858. Darwin has generally overshadowed Wallace since the publication of On the Origin of Species in 1859, however.

The Museum's Library holds the world's largest concentration of Darwin works, with 478 editions of On the Origin of Species in 38 languages. 

In Darwin and Wallace's time, most believed that organisms were too complex to have natural origins and must have been designed by a transcendent God. Natural selection, however, states that even the most complex organisms occur by totally natural processes.

Prof Adrian Lister , a researcher at the Museum says, 'It's not that biologists don't understand that organisms are complex and functional, and it does seem almost miraculous that they exist. We realise that, but we think we've found another way of explaining it.'

Wallace (L) and Darwin (R) came up with very similar theories on evolution. Darwin has generally overshadowed Wallace's contributions, however.

How does natural selection work?

In natural selection, genetic mutations that are beneficial to an individual's survival are passed on through reproduction. This results in a new generation of organisms that are more likely to survive to reproduce.

For example, evolving long necks has enabled giraffes to feed on leaves that others can't reach, giving them a competitive advantage. Thanks to a better food source, those with longer necks were able to survive to reproduce and so pass on the characteristic to the succeeding generation. Those with shorter necks and access to less food would be less likely to survive to pass on their genes.

The evolution of a long neck is an adaptation that helps giraffes survive in their environment © FluffyCreature via Flickr ( CC BY-NC 2.0 )

Adrian explains, 'If you took 1,000 giraffes and measured their necks, they're all going to be slightly different from one another. Those differences are at least in part determined by their genes.

'The ones with longer necks may leave proportionally more offspring, because they have fed better and have maybe been better in competing for mates because they are stronger. Then, if you were to measure the necks of the next generation, they're also going to vary, but the average will have shifted slightly towards the longer ones. The process carries on generation after generation.'

What is an adaptation?

An adaptation is a physical or behavioural characteristic that helps an organism to survive in its environment. 

But not all characteristics of an animal are adaptations.

Adaptations for one purpose can be co-opted for another. For instance, feathers were an adaptation for thermoregulation - their use for flight only came later. This means that feathers are an exaptation for flight, rather than an adaptation.

Adaptations can also become outdated, such as the tough exterior of the calabash fruit ( Crescentia cujete ). This gourd is generally thought to have evolved to avoid being eaten by Gomphotheres, a family of elephant-like animals. But these animals went extinct around 10,000 years ago, so the fruit's adaptation no longer has a survival benefit. 

The large, spherical calabash fruit has an extremely tough exterior. But this adaptation is now outdated  © Wendy Cutler via Flickr ( CC BY-NC 2.0 )

Selection for adaptation is not the only cause of evolution. Species change can also be caused by neutral mutations that have no detriment or benefit to an individual, genetic drift or gene flow.

What does 'survival of the fittest' mean?

In terms of evolution, an animal that is 'fit' is one that is adapted to its environment. This concept is at the core of natural selection, although the term 'survival of the fittest' has often been misunderstood and may be best avoided.

There is also a degree of randomness to evolution, so the best-adapted animal won't always be the one to survive.

Adrian explains, 'If you're going to get hit by a rock or something, it's just bad luck. But on average and over time, the ones that survive are the ones that are fittest - the ones that have the best adaptations.'

Peppered moths ( Biston betularia ) are difficult to see when they perch on tree bark. Those that blend in best are less likely to be preyed on, so have advantage for survival.

What are Darwin's finches?

Darwin collected many animal specimens during the voyage of HMS Beagle (1831-1836). Among his best-known are the finches, of which he collected around 14 species from the Galápagos Islands. The birds sit within the same taxonomic family and have a diverse array of beak sizes and shapes. These correspond to both their differing primary food sources and divergence due to isolation on different islands.

The green warbler-finch ( Certhidea olivacea ), for example, has a sharp, slender beak which is perfect for feeding on small insects. In comparison, the large ground finch ( Geospiza magnirostris ) has a short, stocky beak to crack seeds and nuts. 

The Galápagos finches have distinctly different beak shapes and sizes, as can be seen here from specimens of a green warbler-finch  (L) and a large ground finch (R)

Darwin's finches are often thought of as inspiring a 'eureka moment', but it was actually mockingbirds that impacted Darwin's thoughts on evolution.

Darwin had collected mockingbirds in South America before travelling to the Galápagos. On the first island, San Cristóbal (then known as Chatham Island), he saw a bird he recognised as a mockingbird. But on nearby Floreana Island he saw that the mockingbirds were considerably different.

Darwin realised that differences between species of mockingbird on the islands were greater than between those he'd seen across the continent. He began contemplating while aboard HMS Beagle, but it took several years before he came up with his theory of evolution by natural selection.

The finches - once they had been identified as different species by the British ornithologist John Gould - became one useful example among the many other animals he saw.

Charles Darwin collected these three mockingbird specimens during his time on the Galápagos Islands in 1835, during the voyage of HMS Beagle

The finches are of scientific interest today. The study of Daphne Major , a volcanic island in the Galápagos archipelago, began in 1972 and found that natural selection has resulted in changes in the beak shape and size of two species of finch: the medium ground finch ( Geospiza fortis ) and common cactus finch ( Geospiza scandens ). Both species' beaks have been seen to shrink over time, but followed different patterns.

Darwin thought that natural selection progressed slowly and only occurred over a long period of time. This may often be true, but it has been shown that in some cases a new species can evolve within a lifetime .

For 31 years, scientists studied the survival of a male finch that emigrated from Santa Cruz Island as well as six generations of its descendants on Daphne Major. From the second generation onwards, the birds behaved as a separate species to the others on the island. 

The Daphne Major cactus finches have been studied for over 30 years. In that time the size of their beaks has fluctuated, eventually decreasing in size over a period of 15 years.

What is Lamarckism?

Lamarckism is a theory named after French naturalist Jean-Baptiste Lamarck (1744-1829). It proposes that animals acquire characteristics based on use or disuse during their lives, rather than through hard-coded genetic changes.

In Lamarckian theory, giraffes stretch their necks to make them longer. These animal's offspring would inherit longer necks as a result of their parents' efforts.

Adrian says, 'If you tried to stretch your neck for 10 minutes each morning, then you would probably end up with your neck being a few millimetres longer for a few years. But your children would not inherit it. That's where this theory fails.'

Are we still evolving?

For millennia, the world was viewed as static. The ideas that mountains could rise, and climate and organisms could change didn't exist. Earth was thought to exist in an optimal form.

But natural selection relies on the fact that the world is constantly changing. Evolution occurs automatically for survival and for millions of years it has been playing catch-up with our dynamic world. 

Poaching and habitat loss have had huge impacts on the now critically endangered saiga antelope ( Saiga tatarica ). Natural selection stands little chance in cases like this. © Andrey Giljov via Wikimedia Commons ( CC BY-SA 4.0 )

'Organisms are either adapted enough to survive and reproduce, or they are sub-optimal and the population shrinks. It may even shrink to zero, and that means extinction,' states Adrian.

Scientists have been able to predict natural selection over short terms. But it is almost impossible to accurately determine its effects in the future due to unpredictable fluctuations of the environment.

Natural selection implies that if organisms are surviving, they are adapted. But as the environment changes, we may find that what was once an adaptation may no longer be useful.

Although it is possible for evolution to occur quickly, the more rapidly the planet changes, the harder it is for evolution to keep pace and the more serious the risk of a massive rise in extinctions becomes.

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Volume 2 Supplement 2

Special Issue: Transitional Fossils

• Evolutionary Concepts
• Open access
• Published: 09 April 2009

Understanding Natural Selection: Essential Concepts and Common Misconceptions

• T. Ryan Gregory 1  

Evolution: Education and Outreach volume  2 ,  pages 156–175 ( 2009 ) Cite this article

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Natural selection is one of the central mechanisms of evolutionary change and is the process responsible for the evolution of adaptive features. Without a working knowledge of natural selection, it is impossible to understand how or why living things have come to exhibit their diversity and complexity. An understanding of natural selection also is becoming increasingly relevant in practical contexts, including medicine, agriculture, and resource management. Unfortunately, studies indicate that natural selection is generally very poorly understood, even among many individuals with postsecondary biological education. This paper provides an overview of the basic process of natural selection, discusses the extent and possible causes of misunderstandings of the process, and presents a review of the most common misconceptions that must be corrected before a functional understanding of natural selection and adaptive evolution can be achieved.

“There is probably no more original, more complex, and bolder concept in the history of ideas than Darwin's mechanistic explanation of adaptation.” Ernst Mayr ( 1982 , p.481)

Introduction

Natural selection is a non-random difference in reproductive output among replicating entities, often due indirectly to differences in survival in a particular environment, leading to an increase in the proportion of beneficial, heritable characteristics within a population from one generation to the next. That this process can be encapsulated within a single (admittedly lengthy) sentence should not diminish the appreciation of its profundity and power. It is one of the core mechanisms of evolutionary change and is the main process responsible for the complexity and adaptive intricacy of the living world. According to philosopher Daniel Dennett ( 1995 ), this qualifies evolution by natural selection as “the single best idea anyone has ever had.”

Natural selection results from the confluence of a small number of basic conditions of ecology and heredity. Often, the circumstances in which those conditions apply are of direct significance to human health and well-being, as in the evolution of antibiotic and pesticide resistance or in the impacts of intense predation by humans (e.g., Palumbi 2001 ; Jørgensen et al. 2007 ; Darimont et al. 2009 ). Understanding this process is therefore of considerable importance in both academic and pragmatic terms. Unfortunately, a growing list of studies indicates that natural selection is, in general, very poorly understood—not only by young students and members of the public but even among those who have had postsecondary instruction in biology.

As is true with many other issues, a lack of understanding of natural selection does not necessarily correlate with a lack of confidence about one's level of comprehension. This could be due in part to the perception, unfortunately reinforced by many biologists, that natural selection is so logically compelling that its implications become self-evident once the basic principles have been conveyed. Thus, many professional biologists may agree that “[evolution] shows how everything from frogs to fleas got here via a few easily grasped biological processes ” (Coyne 2006 ; emphasis added). The unfortunate reality, as noted nearly 20 years ago by Bishop and Anderson ( 1990 ), is that “the concepts of evolution by natural selection are far more difficult for students to grasp than most biologists imagine.” Despite common assumptions to the contrary by both students and instructors, it is evident that misconceptions about natural selection are the rule, whereas a working understanding is the rare exception.

The goal of this paper is to enhance (or, as the case may be, confirm) readers' basic understanding of natural selection. This first involves providing an overview of the basis and (one of the) general outcomes of natural selection as they are understood by evolutionary biologists Footnote 1 . This is followed by a brief discussion of the extent and possible causes of difficulties in fully grasping the concept and consequences of natural selection. Finally, a review of the most widespread misconceptions about natural selection is provided. It must be noted that specific instructional tools capable of creating deeper understanding among students generally have remained elusive, and no new suggestions along these lines are presented here. Rather, this article is aimed at readers who wish to confront and correct any misconceptions that they may harbor and/or to better recognize those held by most students and other non-specialists.

The Basis and Basics of Natural Selection

Though rudimentary forms of the idea had been presented earlier (e.g., Darwin and Wallace 1858 and several others before them), it was in On the Origin of Species by Means of Natural Selection that Darwin ( 1859 ) provided the first detailed exposition of the process and implications of natural selection Footnote 2 . According to Mayr ( 1982 , 2001 ), Darwin's extensive discussion of natural selection can be distilled to five “facts” (i.e., direct observations) and three associated inferences. These are depicted in Fig.  1 .

The basis of natural selection as presented by Darwin ( 1859 ), based on the summary by Mayr ( 1982 )

Some components of the process, most notably the sources of variation and the mechanisms of inheritance, were, due to the limited available information in Darwin's time, either vague or incorrect in his original formulation. Since then, each of the core aspects of the mechanism has been elucidated and well documented, making the modern theory Footnote 3 of natural selection far more detailed and vigorously supported than when first proposed 150 years ago. This updated understanding of natural selection consists of the elements outlined in the following sections.

Overproduction, Limited Population Growth, and the “Struggle for Existence”

A key observation underlying natural selection is that, in principle, populations have the capacity to increase in numbers exponentially (or “geometrically”). This is a simple function of mathematics: If one organism produces two offspring, and each of them produces two offspring, and so on, then the total number grows at an increasingly rapid rate (1 → 2 → 4 → 8 → 16 → 32 → 64... to 2 n after n rounds of reproduction).

The enormity of this potential for exponential growth is difficult to fathom. For example, consider that beginning with a single Escherichia coli bacterium, and assuming that cell division occurs every 30 minutes, it would take less than a week for the descendants of this one cell to exceed the mass of the Earth. Of course, exponential population expansion is not limited to bacteria. As Nobel laureate Jacques Monod once quipped, “What is true for E. coli is also true for the elephant,” and indeed, Darwin ( 1859 ) himself used elephants as an illustration of the principle of rapid population growth, calculating that the number of descendants of a single pair would swell to more than 19,000,000 in only 750 years Footnote 4 . Keown ( 1988 ) cites the example of oysters, which may produce as many as 114,000,000 eggs in a single spawn. If all these eggs grew into oysters and produced this many eggs of their own that, in turn, survived to reproduce, then within five generations there would be more oysters than the number of electrons in the known universe.

Clearly, the world is not overrun with bacteria, elephants, or oysters. Though these and all other species engage in massive overproduction (or “superfecundity”) and therefore could in principle expand exponentially, in practice they do not Footnote 5 . The reason is simple: Most offspring that are produced do not survive to produce offspring of their own. In fact, most population sizes tend to remain relatively stable over the long term. This necessarily means that, on average, each pair of oysters produces only two offspring that go on to reproduce successfully—and that 113,999,998 eggs per female per spawn do not survive (see also Ridley 2004 ). Many young oysters will be eaten by predators, others will starve, and still others will succumb to infection. As Darwin ( 1859 ) realized, this massive discrepancy between the number of offspring produced and the number that can be sustained by available resources creates a “struggle for existence” in which often only a tiny fraction of individuals will succeed. As he noted, this can be conceived as a struggle not only against other organisms (especially members of the same species, whose ecological requirements are very similar) but also in a more abstract sense between organisms and their physical environments.

Variation and Inheritance

Variation among individuals is a fundamental requirement for evolutionary change. Given that it was both critical to his theory of natural selection and directly counter to much contemporary thinking, it should not be surprising that Darwin ( 1859 ) expended considerable effort in attempting to establish that variation is, in fact, ubiquitous. He also emphasized the fact that some organisms—namely relatives, especially parents and their offspring—are more similar to each other than to unrelated members of the population. This, too, he realized is critical for natural selection to operate. As Darwin ( 1859 ) put it, “Any variation which is not inherited is unimportant for us.” However, he could not explain either why variation existed or how specific characteristics were passed from parent to offspring, and therefore was forced to treat both the source of variation and the mechanism of inheritance as a “black box.”

The workings of genetics are no longer opaque. Today, it is well understood that inheritance operates through the replication of DNA sequences and that errors in this process (mutations) and the reshuffling of existing variants (recombination) represent the sources of new variation. In particular, mutations are known to be random (or less confusingly, “undirected”) with respect to any effects that they may have. Any given mutation is merely a chance error in the genetic system, and as such, its likelihood of occurrence is not influenced by whether it will turn out to be detrimental, beneficial, or (most commonly) neutral.

As Darwin anticipated, extensive variation among individuals has now been well established to exist at the physical, physiological, and behavioral levels. Thanks to the rise of molecular biology and, more recently, of genomics, it also has been possible to document variation at the level of proteins, genes, and even individual DNA nucleotides in humans and many other species.

Non-random Differences in Survival and Reproduction

Darwin saw that overproduction and limited resources create a struggle for existence in which some organisms will succeed and most will not. He also recognized that organisms in populations differ from one another in terms of many traits that tend to be passed on from parent to offspring. Darwin's brilliant insight was to combine these two factors and to realize that success in the struggle for existence would not be determined by chance, but instead would be biased by some of the heritable differences that exist among organisms. Specifically, he noted that some individuals happen to possess traits that make them slightly better suited to a particular environment, meaning that they are more likely to survive than individuals with less well suited traits. As a result, organisms with these traits will, on average, leave more offspring than their competitors.

Whereas the origin of a new genetic variant occurs at random in terms of its effects on the organism, the probability of it being passed on to the next generation is absolutely non-random if it impacts the survival and reproductive capabilities of that organism. The important point is that this is a two-step process: first, the origin of variation by random mutation, and second, the non-random sorting of variation due to its effects on survival and reproduction (Mayr 2001 ). Though definitions of natural selection have been phrased in many ways (Table  1 ), it is this non-random difference in survival and reproduction that forms the basis of the process.

Darwinian Fitness

The meaning of fitness in evolutionary biology.

In order to study the operation and effects of natural selection, it is important to have a means of describing and quantifying the relationships between genotype (gene complement), phenotype (physical and behavioral features), survival, and reproduction in particular environments. The concept used by evolutionary biologists in this regard is known as “Darwinian fitness,” which is defined most simply as a measure of the total (or relative) reproductive output of an organism with a particular genotype (Table  1 ). In the most basic terms, one can state that the more offspring an individual produces, the higher is its fitness. It must be emphasized that the term “fitness,” as used in evolutionary biology, does not refer to physical condition, strength, or stamina and therefore differs markedly from its usage in common language.

“Survival of the Fittest” is Misleading

In the fifth edition of the Origin (published in 1869), Darwin began using the phrase “survival of the fittest”, which had been coined a few years earlier by British economist Herbert Spencer, as shorthand for natural selection. This was an unfortunate decision as there are several reasons why “survival of the fittest” is a poor descriptor of natural selection. First, in Darwin's context, “fittest” implied “best suited to a particular environment” rather than “most physically fit,” but this crucial distinction is often overlooked in non-technical usage (especially when further distorted to “only the strong survive”). Second, it places undue emphasis on survival: While it is true that dead organisms do not reproduce, survival is only important evolutionarily insofar as it affects the number of offspring produced. Traits that make life longer or less difficult are evolutionarily irrelevant unless they also influence reproductive output. Indeed, traits that enhance net reproduction may increase in frequency over many generations even if they compromise individual longevity. Conversely, differences in fecundity alone can create differences in fitness, even if survival rates are identical among individuals. Third, this implies an excessive focus on organisms, when in fact traits or their underlying genes equally can be identified as more or less fit than alternatives. Lastly, this phrase is often misconstrued as being circular or tautological (Who survives? The fittest. Who are the fittest? Those who survive). However, again, this misinterprets the modern meaning of fitness, which can be both predicted in terms of which traits are expected to be successful in a specific environment and measured in terms of actual reproductive success in that environment.

Which Traits Are the Most Fit?

Directional natural selection can be understood as a process by which fitter traits (or genes) increase in proportion within populations over the course of many generations. It must be understood that the relative fitness of different traits depends on the current environment. Thus, traits that are fit now may become unfit later if the environment changes. Conversely, traits that have now become fit may have been present long before the current environment arose, without having conferred any advantage under previous conditions. Finally, it must be noted that fitness refers to reproductive success relative to alternatives here and now —natural selection cannot increase the proportion of traits solely because they may someday become advantageous. Careful reflection on how natural selection actually works should make it clear why this is so.

Natural Selection and Adaptive Evolution

Natural selection and the evolution of populations.

Though each has been tested and shown to be accurate, none of the observations and inferences that underlies natural selection is sufficient individually to provide a mechanism for evolutionary change Footnote 6 . Overproduction alone will have no evolutionary consequences if all individuals are identical. Differences among organisms are not relevant unless they can be inherited. Genetic variation by itself will not result in natural selection unless it exerts some impact on organism survival and reproduction. However, any time all of Darwin's postulates hold simultaneously—as they do in most populations—natural selection will occur. The net result in this case is that certain traits (or, more precisely, genetic variants that specify those traits) will, on average , be passed on from one generation to the next at a higher rate than existing alternatives in the population. Put another way, when one considers who the parents of the current generation were, it will be seen that a disproportionate number of them possessed traits beneficial for survival and reproduction in the particular environment in which they lived.

The important points are that this uneven reproductive success among individuals represents a process that occurs in each generation and that its effects are cumulative over the span of many generations. Over time, beneficial traits will become increasingly prevalent in descendant populations by virtue of the fact that parents with those traits consistently leave more offspring than individuals lacking those traits. If this process happens to occur in a consistent direction—say, the largest individuals in each generation tend to leave more offspring than smaller individuals—then there can be a gradual, generation-by-generation change in the proportion of traits in the population. This change in proportion and not the modification of organisms themselves is what leads to changes in the average value of a particular trait in the population. Organisms do not evolve; populations evolve.

The term “adaptation” derives from ad + aptus , literally meaning “toward + fit”. As the name implies, this is the process by which populations of organisms evolve in such a way as to become better suited to their environments as advantageous traits become predominant. On a broader scale, it is also how physical, physiological, and behavioral features that contribute to survival and reproduction (“adaptations”) arise over evolutionary time. This latter topic is particularly difficult for many to grasp, though of course a crucial first step is to understand the operation of natural selection on smaller scales of time and consequence. (For a detailed discussion of the evolution of complex organs such as eyes, see Gregory 2008b .)

On first pass, it may be difficult to see how natural selection can ever lead to the evolution of new characteristics if its primary effect is merely to eliminate unfit traits. Indeed, natural selection by itself is incapable of producing new traits, and in fact (as many readers will have surmised), most forms of natural selection deplete genetic variation within populations. How, then, can an eliminative process like natural selection ever lead to creative outcomes?

To answer this question, one must recall that evolution by natural selection is a two-step process. The first step involves the generation of new variation by mutation and recombination, whereas the second step determines which randomly generated variants will persist into the next generation. Most new mutations are neutral with respect to survival and reproduction and therefore are irrelevant in terms of natural selection (but not, it must be pointed out, to evolution more broadly). The majority of mutations that have an impact on survival and reproductive output will do so negatively and, as such, will be less likely than existing alternatives to be passed on to subsequent generations. However, a small percentage of new mutations will turn out to have beneficial effects in a particular environment and will contribute to an elevated rate of reproduction by organisms possessing them. Even a very slight advantage is sufficient to cause new beneficial mutations to increase in proportion over the span of many generations.

Biologists sometimes describe beneficial mutations as “spreading” or “sweeping” through a population, but this shorthand is misleading. Rather, beneficial mutations simply increase in proportion from one generation to the next because, by definition, they happen to contribute to the survival and reproductive success of the organisms carrying them. Eventually, a beneficial mutation may be the only alternative left as all others have ultimately failed to be passed on. At this point, that beneficial genetic variant is said to have become “fixed” in the population.

Again, mutation does not occur in order to improve fitness—it merely represents errors in genetic replication. This means that most mutations do not improve fitness: There are many more ways of making things worse than of making them better. It also means that mutations will continue to occur even after previous beneficial mutations have become fixed. As such, there can be something of a ratcheting effect in which beneficial mutations arise and become fixed by selection, only to be supplemented later by more beneficial mutations which, in turn, become fixed. All the while, neutral and deleterious mutations also occur in the population, the latter being passed on at a lower rate than alternatives and often being lost before reaching any appreciable frequency.

Of course, this is an oversimplification—in species with sexual reproduction, multiple beneficial mutations may be brought together by recombination such that the fixation of beneficial genes need not occur sequentially. Likewise, recombination can juxtapose deleterious mutations, thereby hastening their loss from the population. Nonetheless, it is useful to imagine the process of adaptation as one in which beneficial mutations arise continually (though perhaps very infrequently and with only minor positive impacts) and then accumulate in the population over many generations.

The process of adaptation in a population is depicted in very basic form in Fig.  2 . Several important points can be drawn from even such an oversimplified rendition:

Mutations are the source of new variation. Natural selection itself does not create new traits; it only changes the proportion of variation that is already present in the population. The repeated two-step interaction of these processes is what leads to the evolution of novel adaptive features.

Mutation is random with respect to fitness. Natural selection is, by definition, non-random with respect to fitness. This means that, overall, it is a serious misconception to consider adaptation as happening “by chance”.

Mutations occur with all three possible outcomes: neutral, deleterious, and beneficial. Beneficial mutations may be rare and deliver only a minor advantage, but these can nonetheless increase in proportion in the population over many generations by natural selection. The occurrence of any particular beneficial mutation may be very improbable, but natural selection is very effective at causing these individually unlikely improvements to accumulate. Natural selection is an improbability concentrator.

No organisms change as the population adapts. Rather, this involves changes in the proportion of beneficial traits across multiple generations.

The direction in which adaptive change occurs is dependent on the environment. A change in environment can make previously beneficial traits neutral or detrimental and vice versa.

Adaptation does not result in optimal characteristics. It is constrained by historical, genetic, and developmental limitations and by trade-offs among features (see Gregory 2008b ).

It does not matter what an “ideal” adaptive feature might be—the only relevant factor is that variants that happen to result in greater survival and reproduction relative to alternative variants are passed on more frequently. As Darwin wrote in a letter to Joseph Hooker (11 Sept. 1857), “I have just been writing an audacious little discussion, to show that organic beings are not perfect, only perfect enough to struggle with their competitors.”

The process of adaptation by natural selection is not forward-looking, and it cannot produce features on the grounds that they might become beneficial sometime in the future. In fact, adaptations are always to the conditions experienced by generations in the past.

A highly simplified depiction of natural selection ( Correct ) and a generalized illustration of various common misconceptions about the mechanism ( Incorrect ). Properly understood, natural selection occurs as follows: ( A ) A population of organisms exhibits variation in a particular trait that is relevant to survival in a given environment. In this diagram, darker coloration happens to be beneficial, but in another environment, the opposite could be true. As a result of their traits, not all individuals in Generation 1 survive equally well, meaning that only a non-random subsample ultimately will succeed in reproducing and passing on their traits ( B ). Note that no individual organisms in Generation 1 change, rather the proportion of individuals with different traits changes in the population. The individuals who survive from Generation 1 reproduce to produce Generation 2. ( C ) Because the trait in question is heritable, this second generation will (mostly) resemble the parent generation. However, mutations have also occurred, which are undirected (i.e., they occur at random in terms of the consequences of changing traits), leading to both lighter and darker offspring in Generation 2 as compared to their parents in Generation 1. In this environment, lighter mutants are less successful and darker mutants are more successful than the parental average. Once again, there is non-random survival among individuals in the population, with darker traits becoming disproportionately common due to the death of lighter individuals ( D ). This subset of Generation 2 proceeds to reproduce. Again, the traits of the survivors are passed on, but there is also undirected mutation leading to both deleterious and beneficial differences among the offspring ( E ). ( F ) This process of undirected mutation and natural selection (non-random differences in survival and reproductive success) occurs over many generations, each time leading to a concentration of the most beneficial traits in the next generation. By Generation N , the population is composed almost entirely of very dark individuals. The population can now be said to have become adapted to the environment in which darker traits are the most successful. This contrasts with the intuitive notion of adaptation held by most students and non-biologists. In the most common version, populations are seen as uniform, with variation being at most an anomalous deviation from the norm ( X ). It is assumed that all members within a single generation change in response to pressures imposed by the environment ( Y ). When these individuals reproduce, they are thought to pass on their acquired traits. Moreover, any changes that do occur due to mutation are imagined to be exclusively in the direction of improvement ( Z ). Studies have revealed that it can be very difficult for non-experts to abandon this intuitive interpretation in favor of a scientifically valid understanding of the mechanism. Diagrams based in part on Bishop and Anderson ( 1990 )

Natural Selection Is Elegant, Logical, and Notoriously Difficult to Grasp

The extent of the problem.

In its most basic form, natural selection is an elegant theory that effectively explains the obviously good fit of living things to their environments. As a mechanism, it is remarkably simple in principle yet incredibly powerful in application. However, the fact that it eluded description until 150 years ago suggests that grasping its workings and implications is far more challenging than is usually assumed.

Three decades of research have produced unambiguous data revealing a strikingly high prevalence of misconceptions about natural selection among members of the public and in students at all levels, from elementary school pupils to university science majors (Alters 2005 ; Bardapurkar 2008 ; Table  2 ) Footnote 7 . A finding that less than 10% of those surveyed possess a functional understanding of natural selection is not atypical. It is particularly disconcerting and undoubtedly exacerbating that confusions about natural selection are common even among those responsible for teaching it Footnote 8 . As Nehm and Schonfeld ( 2007 ) recently concluded, “one cannot assume that biology teachers with extensive backgrounds in biology have an accurate working knowledge of evolution, natural selection, or the nature of science.”

Why is Natural Selection so Difficult to Understand?

Two obvious hypotheses present themselves for why misunderstandings of natural selection are so widespread. The first is that understanding the mechanism of natural selection requires an acceptance of the historical fact of evolution, the latter being rejected by a large fraction of the population. While an improved understanding of the process probably would help to increase overall acceptance of evolution, surveys indicate that rates of acceptance already are much higher than levels of understanding. And, whereas levels of understanding and acceptance may be positively correlated among teachers (Vlaardingerbroek and Roederer 1997 ; Rutledge and Mitchell 2002 ; Deniz et al. 2008 ), the two parameters seem to be at most only very weakly related in students Footnote 9 (Bishop and Anderson 1990 ; Demastes et al. 1995 ; Brem et al. 2003 ; Sinatra et al. 2003 ; Ingram and Nelson 2006 ; Shtulman 2006 ). Teachers notwithstanding, “it appears that a majority on both sides of the evolution-creation debate do not understand the process of natural selection or its role in evolution” (Bishop and Anderson 1990 ).

The second intuitive hypothesis is that most people simply lack formal education in biology and have learned incorrect versions of evolutionary mechanisms from non-authoritative sources (e.g., television, movies, parents). Inaccurate portrayals of evolutionary processes in the media, by teachers, and by scientists themselves surely exacerbate the situation (e.g., Jungwirth 1975a , b , 1977 ; Moore et al. 2002 ). However, this alone cannot provide a full explanation, because even direct instruction on natural selection tends to produce only modest improvements in students' understanding (e.g., Jensen and Finley 1995 ; Ferrari and Chi 1998 ; Nehm and Reilly 2007 ; Spindler and Doherty 2009 ). There also is evidence that levels of understanding do not differ greatly between science majors and non-science majors (Sundberg and Dini 1993 ). In the disquieting words of Ferrari and Chi ( 1998 ), “misconceptions about even the basic principles of Darwin's theory of evolution are extremely robust, even after years of education in biology.”

Misconceptions are well known to be common with many (perhaps most) aspects of science, including much simpler and more commonly encountered phenomena such as the physics of motion (e.g., McCloskey et al. 1980 ; Halloun and Hestenes 1985 ; Bloom and Weisberg 2007 ). The source of this larger problem seems to be a significant disconnect between the nature of the world as reflected in everyday experience and the one revealed by systematic scientific investigation (e.g., Shtulman 2006 ; Sinatra et al. 2008 ). Intuitive interpretations of the world, though sufficient for navigating daily life, are usually fundamentally at odds with scientific principles. If common sense were more than superficially accurate, scientific explanations would be less counterintuitive, but they also would be largely unnecessary.

Conceptual Frameworks Versus Spontaneous Constructions

It has been suggested by some authors that young students simply are incapable of understanding natural selection because they have not yet developed the formal reasoning abilities necessary to grasp it (Lawson and Thompson 1988 ). This could be taken to imply that natural selection should not be taught until later grades; however, those who have studied student understanding directly tend to disagree with any such suggestion (e.g., Clough and Wood-Robinson 1985 ; Settlage 1994 ). Overall, the issue does not seem to be a lack of logic (Greene 1990 ; Settlage 1994 ), but a combination of incorrect underlying premises about mechanisms and deep-seated cognitive biases that influence interpretations.

Many of the misconceptions that block an understanding of natural selection develop early in childhood as part of “naïve” but practical understandings of how the world is structured. These tend to persist unless replaced with more accurate and equally functional information. In this regard, some experts have argued that the goal of education should be to supplant existing conceptual frameworks with more accurate ones (see Sinatra et al. 2008 ). Under this view, “Helping people to understand evolution...is not a matter of adding on to their existing knowledge, but helping them to revise their previous models of the world to create an entirely new way of seeing” (Sinatra et al. 2008 ). Other authors suggest that students do not actually maintain coherent conceptual frameworks relating to complex phenomena, but instead construct explanations spontaneously using intuitions derived from everyday experience (see Southerland et al. 2001 ). Though less widely accepted, this latter view gains support from the observation that naïve evolutionary explanations given by non-experts may be tentative and inconsistent (Southerland et al. 2001 ) and may differ depending on the type of organisms being considered (Spiegel et al. 2006 ). In some cases, students may attempt a more complex explanation but resort to intuitive ideas when they encounter difficulty (Deadman and Kelly 1978 ). In either case, it is abundantly clear that simply describing the process of natural selection to students is ineffective and that it is imperative that misconceptions be confronted if they are to be corrected (e.g., Greene 1990 ; Scharmann 1990 ; Settlage 1994 ; Ferrari and Chi 1998 ; Alters and Nelson 2002 ; Passmore and Stewart 2002 ; Alters 2005 ; Nelson 2007 ).

A Catalog of Common Misconceptions

Whereas the causes of cognitive barriers to understanding remain to be determined, their consequences are well documented. It is clear from many studies that complex but accurate explanations of biological adaptation typically yield to naïve intuitions based on common experience (Fig.  2 ; Tables  2 and 3 ). As a result, each of the fundamental components of natural selection may be overlooked or misunderstood when it comes time to consider them in combination, even if individually they appear relatively straightforward. The following sections provide an overview of the various, non-mutually exclusive, and often correlated misconceptions that have been found to be most common. All readers are encouraged to consider these conceptual pitfalls carefully in order that they may be avoided. Teachers, in particular, are urged to familiarize themselves with these errors so that they may identify and address them among their students.

Teleology and the “Function Compunction”

Much of the human experience involves overcoming obstacles, achieving goals, and fulfilling needs. Not surprisingly, human psychology includes a powerful bias toward thoughts about the “purpose” or “function” of objects and behaviors—what Kelemen and Rosset ( 2009 ) dub the “human function compunction.” This bias is particularly strong in children, who are apt to see most of the world in terms of purpose; for example, even suggesting that “rocks are pointy to keep animals from sitting on them” (Kelemen 1999a , b ; Kelemen and Rosset 2009 ). This tendency toward explanations based on purpose (“teleology”) runs very deep and persists throughout high school (Southerland et al. 2001 ) and even into postsecondary education (Kelemen and Rosset 2009 ). In fact, it has been argued that the default mode of teleological thinking is, at best, suppressed rather than supplanted by introductory scientific education. It therefore reappears easily even in those with some basic scientific training; for example, in descriptions of ecological balance (“fungi grow in forests to help decomposition”) or species survival (“finches diversified in order to survive”; Kelemen and Rosset 2009 ).

Teleological explanations for biological features date back to Aristotle and remain very common in naïve interpretations of adaptation (e.g., Tamir and Zohar 1991 ; Pedersen and Halldén 1992 ; Southerland et al. 2001 ; Sinatra et al. 2008 ; Table  2 ). On the one hand, teleological reasoning may preclude any consideration of mechanisms altogether if simply identifying a current function for an organ or behavior is taken as sufficient to explain its existence (e.g., Bishop and Anderson 1990 ). On the other hand, when mechanisms are considered by teleologically oriented thinkers, they are often framed in terms of change occurring in response to a particular need (Table  2 ). Obviously, this contrasts starkly with a two-step process involving undirected mutations followed by natural selection (see Fig.  2 and Table  3 ).

Anthropomorphism and Intentionality

A related conceptual bias to teleology is anthropomorphism, in which human-like conscious intent is ascribed either to the objects of natural selection or to the process itself (see below). In this sense, anthropomorphic misconceptions can be characterized as either internal (attributing adaptive change to the intentional actions of organisms) or external (conceiving of natural selection or “Nature” as a conscious agent; e.g., Kampourakis and Zogza 2008 ; Sinatra et al. 2008 ).

Internal anthropomorphism or “intentionality” is intimately tied to the misconception that individual organisms evolve in response to challenges imposed by the environment (rather than recognizing evolution as a population-level process). Gould ( 1980 ) described the obvious appeal of such intuitive notions as follows:

Since the living world is a product of evolution, why not suppose that it arose in the simplest and most direct way? Why not argue that organisms improve themselves by their own efforts and pass these advantages to their offspring in the form of altered genes—a process that has long been called, in technical parlance, the “inheritance of acquired characters.” This idea appeals to common sense not only for its simplicity but perhaps even more for its happy implication that evolution travels an inherently progressive path, propelled by the hard work of organisms themselves.

The penchant for seeing conscious intent is often sufficiently strong that it is applied not only to non-human vertebrates (in which consciousness, though certainly not knowledge of genetics and Darwinian fitness, may actually occur), but also to plants and even to single-celled organisms. Thus, adaptations in any taxon may be described as “innovations,” “inventions,” or “solutions” (sometimes “ingenious” ones, no less). Even the evolution of antibiotic resistance is characterized as a process whereby bacteria “learn” to “outsmart” antibiotics with frustrating regularity. Anthropomorphism with an emphasis on forethought is also behind the common misconception that organisms behave as they do in order to enhance the long-term well-being of their species. Once again, a consideration of the actual mechanics of natural selection should reveal why this is fallacious.

All too often, an anthropomorphic view of evolution is reinforced with sloppy descriptions by trusted authorities (Jungwirth 1975a , b , 1977 ; Moore et al. 2002 ). Consider this particularly egregious example from a website maintained by the National Institutes of Health Footnote 10 :

As microbes evolve, they adapt to their environment. If something stops them from growing and spreading—such as an antimicrobial—they evolve new mechanisms to resist the antimicrobials by changing their genetic structure. Changing the genetic structure ensures that the offspring of the resistant microbes are also resistant.

Fundamentally inaccurate descriptions such as this are alarmingly common. As a corrective, it is a useful exercise to translate such faulty characterizations into accurate language Footnote 11 . For example, this could read:

Bacteria that cause disease exist in large populations, and not all individuals are alike. If some individuals happen to possess genetic features that make them resistant to antibiotics, these individuals will survive the treatment while the rest gradually are killed off. As a result of their greater survival, the resistant individuals will leave more offspring than susceptible individuals, such that the proportion of resistant individuals will increase each time a new generation is produced. When only the descendants of the resistant individuals are left, the population of bacteria can be said to have evolved resistance to the antibiotics.

Use and Disuse

Many students who manage to avoid teleological and anthropomorphic pitfalls nonetheless conceive of evolution as involving change due to use or disuse of organs. This view, which was developed explicitly by Jean-Baptiste Lamarck but was also invoked to an extent by Darwin ( 1859 ), emphasizes changes to individual organisms that occur as they use particular features more or less. For example, Darwin ( 1859 ) invoked natural selection to explain the loss of sight in some subterranean rodents, but instead favored disuse alone as the explanation for loss of eyes in blind, cave-dwelling animals: “As it is difficult to imagine that eyes, though useless, could be in any way injurious to animals living in darkness, I attribute their loss wholly to disuse.” This sort of intuition remains common in naïve explanations for why unnecessary organs become vestigial or eventually disappear. Modern evolutionary theory recognizes several reasons that may account for the loss of complex features (e.g., Jeffery 2005 ; Espinasa and Espinasa 2008 ), some of which involve direct natural selection, but none of which is based simply on disuse.

Soft Inheritance

Evolution involving changes in individual organisms, whether based on conscious choice or use and disuse, would require that characteristics acquired during the lifetime of an individual be passed on to offspring Footnote 12 , a process often termed “soft inheritance.” The notion that acquired traits can be transmitted to offspring remained a common assumption among thinkers for more than 2,000 years, including into Darwin's time (Zirkle 1946 ). As is now understood, inheritance is actually “hard,” meaning that physical changes that occur during an organism's lifetime are not passed to offspring. This is because the cells that are involved in reproduction (the germline) are distinct from those that make up the rest of the body (the somatic line); only changes that affect the germline can be passed on. New genetic variants arise through mutation and recombination during replication and will often only exert their effects in offspring and not in the parents in whose reproductive cells they occur (though they could also arise very early in development and appear later in the adult offspring). Correct and incorrect interpretations of inheritance are contrasted in Fig.  3 .

A summary of correct ( left ) and incorrect ( right ) conceptions of heredity as it pertains to adaptive evolutionary change. The panels on the left display the operation of “hard inheritance”, whereas those on the right illustrate naïve mechanisms of “soft inheritance”. In all diagrams, a set of nine squares represents an individual multicellular organism and each square represents a type of cell of which the organisms are constructed. In the left panels, the organisms include two kinds of cells: those that produce gametes (the germline, black ) and those that make up the rest of the body (the somatic line, white ). In the top left panel , all cells in a parent organism initially contain a gene that specifies white coloration marked W ( A ). A random mutation occurs in the germline, changing the gene from one that specifies white to one that specifies gray marked G ( B ). This mutant gene is passed to the egg ( C ), which then develops into an offspring exhibiting gray coloration ( D ). The mutation in this case occurred in the parent (specifically, in the germline) but its effects did not become apparent until the next generation. In the bottom left panel , a parent once again begins with white coloration and the white gene in all of its cells ( H ). During its lifetime, the parent comes to acquire a gray coloration due to exposure to particular environmental conditions ( I ). However, because this does not involve any change to the genes in the germline, the original white gene is passed into the egg ( J ), and the offspring exhibits none of the gray coloration that was acquired by its parent ( K ). In the top right panel , the distinction between germline and somatic line is not understood. In this case, a parent that initially exhibits white coloration ( P ) changes during its lifetime to become gray ( Q ). Under incorrect views of soft inheritance, this altered coloration is passed on to the egg ( R ), and the offspring is born with the gray color acquired by its parent ( S ). In the bottom right panel , a more sophisticated but still incorrect view of inheritance is shown. Here, traits are understood to be specified by genes, but no distinction is recognized between the germline and somatic line. In this situation, a parent begins with white coloration and white-specifying genes in all its cells ( W ). A mutation occurs in one type of body cells to change those cells to gray ( X ). A mixture of white and gray genes is passed on to the egg ( Y ), and the offspring develops white coloration in most cells but gray coloration in the cells where gray-inducing mutations arose in the parent ( Z ). Intuitive ideas regarding soft inheritance underlie many misconceptions of how adaptive evolution takes place (see Fig.  2 )

Studies have indicated that belief in soft inheritance arises early in youth as part of a naïve model of heredity (e.g., Deadman and Kelly 1978 ; Kargbo et al. 1980 ; Lawson and Thompson 1988 ; Wood-Robinson 1994 ). That it seems intuitive probably explains why the idea of soft inheritance persisted so long among prominent thinkers and why it is so resistant to correction among modern students. Unfortunately, a failure to abandon this belief is fundamentally incompatible with an appreciation of evolution by natural selection as a two-step process in which the origin of new variation and its relevance to survival in a particular environment are independent considerations.

Nature as a Selecting Agent

Thirty years ago, widely respected broadcaster Sir David Attenborough ( 1979 ) aptly described the challenge of avoiding anthropomorphic shorthand in descriptions of adaptation:

Darwin demonstrated that the driving force of [adaptive] evolution comes from the accumulation, over countless generations, of chance genetical changes sifted by the rigors of natural selection. In describing the consequences of this process it is only too easy to use a form of words that suggests that the animals themselves were striving to bring about change in a purposeful way–that fish wanted to climb onto dry land, and to modify their fins into legs, that reptiles wished to fly, strove to change their scales into feathers and so ultimately became birds.

Unlike many authors, Attenborough ( 1979 ) admirably endeavored to not use such misleading terminology. However, this quote inadvertently highlights an additional challenge in describing natural selection without loaded language. In it, natural selection is described as a “driving force” that rigorously “sifts” genetic variation, which could be misunderstood to imply that it takes an active role in prompting evolutionary change. Much more seriously, one often encounters descriptions of natural selection as a processes that “chooses” among “preferred” variants or “experiments with” or “explores” different options. Some expressions, such as “favored” and “selected for” are used commonly as shorthand in evolutionary biology and are not meant to impart consciousness to natural selection; however, these too may be misinterpreted in the vernacular sense by non-experts and must be clarified.

Darwin ( 1859 ) himself could not resist slipping into the language of agency at times:

It may be said that natural selection is daily and hourly scrutinizing, throughout the world, every variation, even the slightest; rejecting that which is bad, preserving and adding up all that is good; silently and insensibly working, whenever and wherever opportunity offers, at the improvement of each organic being in relation to its organic and inorganic conditions of life. We see nothing of these slow changes in progress, until the hand of time has marked the long lapse of ages, and then so imperfect is our view into long past geological ages, that we only see that the forms of life are now different from what they formerly were.

Perhaps recognizing the ease with which such language can be misconstrued, Darwin ( 1868 ) later wrote that “The term ‘Natural Selection’ is in some respects a bad one, as it seems to imply conscious choice; but this will be disregarded after a little familiarity.” Unfortunately, more than “a little familiarity” seems necessary to abandon the notion of Nature as an active decision maker.

Being, as it is, the simple outcome of differences in reproductive success due to heritable traits, natural selection cannot have plans, goals, or intentions, nor can it cause changes in response to need. For this reason, Jungwirth ( 1975a , b , 1977 ) bemoaned the tendency for authors and instructors to invoke teleological and anthropomorphic descriptions of the process and argued that this served to reinforce misconceptions among students (see also Bishop and Anderson 1990 ; Alters and Nelson 2002 ; Moore et al. 2002 ; Sinatra et al. 2008 ). That said, a study of high school students by Tamir and Zohar ( 1991 ) suggested that older students can recognize the distinction between an anthropomorphic or teleological formulation (i.e., merely a convenient description) versus an anthropomorphic/teleological explanation (i.e., involving conscious intent or goal-oriented mechanisms as causal factors; see also Bartov 1978 , 1981 ). Moore et al. ( 2002 ), by contrast, concluded from their study of undergraduates that “students fail to distinguish between the relatively concrete register of genetics and the more figurative language of the specialist shorthand needed to condense the long view of evolutionary processes” (see also Jungwirth 1975a , 1977 ). Some authors have argued that teleological wording can have some value as shorthand for describing complex phenomena in a simple way precisely because it corresponds to normal thinking patterns, and that contrasting this explicitly with accurate language can be a useful exercise during instruction (Zohar and Ginossar 1998 ). In any case, biologists and instructors should be cognizant of the risk that linguistic shortcuts may send students off track.

Source Versus Sorting of Variation

Intuitive models of evolution based on soft inheritance are one-step models of adaptation: Traits are modified in one generation and appear in their altered form in the next. This is in conflict with the actual two-step process of adaptation involving the independent processes of mutation and natural selection. Unfortunately, many students who eschew soft inheritance nevertheless fail to distinguish natural selection from the origin of new variation (e.g., Greene 1990 ; Creedy 1993 ; Moore et al. 2002 ). Whereas an accurate understanding recognizes that most new mutations are neutral or harmful in a given environment, such naïve interpretations assume that mutations occur as a response to environmental challenges and therefore are always beneficial (Fig.  2 ). For example, many students may believe that exposure to antibiotics directly causes bacteria to become resistant, rather than simply changing the relative frequencies of resistant versus non-resistant individuals by killing off the latter Footnote 13 . Again, natural selection itself does not create new variation, it merely influences the proportion of existing variants. Most forms of selection reduce the amount of genetic variation within populations, which may be counteracted by the continual emergence of new variation via undirected mutation and recombination.

Typological, Essentialist, and Transformationist Thinking

Misunderstandings about how variation arises are problematic, but a common failure to recognize that it plays a role at all represents an even a deeper concern. Since Darwin ( 1859 ), evolutionary theory has been based strongly on “population” thinking that emphasizes differences among individuals. By contrast, many naïve interpretations of evolution remain rooted in the “typological” or “essentialist” thinking that has existed since the ancient Greeks (Mayr 1982 , 2001 ; Sinatra et al. 2008 ). In this case, species are conceived of as exhibiting a single “type” or a common “essence,” with variation among individuals representing anomalous and largely unimportant deviations from the type or essence. As Shtulman ( 2006 ) notes, “human beings tend to essentialize biological kinds and essentialism is incompatible with natural selection.” As with many other conceptual biases, the tendency to essentialize seems to arise early in childhood and remains the default for most individuals (Strevens 2000 ; Gelman 2004 ; Evans et al. 2005 ; Shtulman 2006 ).

The incorrect belief that species are uniform leads to “transformationist” views of adaptation in which an entire population transforms as a whole as it adapts (Alters 2005 ; Shtulman 2006 ; Bardapurkar 2008 ). This contrasts with the correct, “variational” understanding of natural selection in which it is the proportion of traits within populations that changes (Fig.  2 ). Not surprisingly, transformationist models of adaptation usually include a tacit assumption of soft inheritance and one-step change in response to challenges. Indeed, Shtulman ( 2006 ) found that transformationists appeal to “need” as a cause of evolutionary change three times more often than do variationists.

Events and Absolutes Versus Processes and Probabilities

A proper understanding of natural selection recognizes it as a process that occurs within populations over the course of many generations. It does so through cumulative, statistical effects on the proportion of traits differing in their consequences for reproductive success. This contrasts with two major errors that are commonly incorporated into naïve conceptions of the process:

Natural selection is mistakenly seen as an event rather than as a process (Ferrari and Chi 1998 ; Sinatra et al. 2008 ). Events generally have a beginning and end, occur in a specific sequential order, consist of distinct actions, and may be goal-oriented. By contrast, natural selection actually occurs continually and simultaneously within entire populations and is not goal-oriented (Ferrari and Chi 1998 ). Misconstruing selection as an event may contribute to transformationist thinking as adaptive changes are thought to occur in the entire population simultaneously. Viewing natural selection as a single event can also lead to incorrect “saltationist” assumptions in which complex adaptive features are imagined to appear suddenly in a single generation (see Gregory 2008b for an overview of the evolution of complex organs).

Natural selection is incorrectly conceived as being “all or nothing,” with all unfit individuals dying and all fit individuals surviving. In actuality, it is a probabilistic process in which some traits make it more likely—but do not guarantee—that organisms possessing them will successfully reproduce. Moreover, the statistical nature of the process is such that even a small difference in reproductive success (say, 1%) is enough to produce a gradual increase in the frequency of a trait over many generations.

Concluding Remarks

Surveys of students at all levels paint a bleak picture regarding the level of understanding of natural selection. Though it is based on well-established and individually straightforward components, a proper grasp of the mechanism and its implications remains very rare among non-specialists. The unavoidable conclusion is that the vast majority of individuals, including most with postsecondary education in science, lack a basic understanding of how adaptive evolution occurs.

While no concrete solutions to this problem have yet been found, it is evident that simply outlining the various components of natural selection rarely imparts an understanding of the process to students. Various alternative teaching strategies and activities have been suggested, and some do help to improve the level of understanding among students (e.g., Bishop and Anderson 1986 ; Jensen and Finley 1995 , 1996 ; Firenze 1997 ; Passmore and Stewart 2002 ; Sundberg 2003 ; Alters 2005 ; Scharmann 1990 ; Wilson 2005 ; Nelson 2007 , 2008 ; Pennock 2007 ; Kampourakis and Zogza 2008 ). Efforts to integrate evolution throughout biology curricula rather than segregating it into a single unit may also prove more effective (Nehm et al. 2009 ), as may steps taken to make evolution relevant to everyday concerns (e.g., Hillis 2007 ).

At the very least, it is abundantly clear that teaching and learning natural selection must include efforts to identify, confront, and supplant misconceptions. Most of these derive from deeply held conceptual biases that may have been present since childhood. Natural selection, like most complex scientific theories, runs counter to common experience and therefore competes—usually unsuccessfully—with intuitive ideas about inheritance, variation, function, intentionality, and probability. The tendency, both outside and within academic settings, to use inaccurate language to describe evolutionary phenomena probably serves to reinforce these problems.

Natural selection is a central component of modern evolutionary theory, which in turn is the unifying theme of all biology. Without a grasp of this process and its consequences, it is simply impossible to understand, even in basic terms, how and why life has become so marvelously diverse. The enormous challenge faced by biologists and educators in correcting the widespread misunderstanding of natural selection is matched only by the importance of the task.

For a more advanced treatment, see Bell ( 1997 , 2008 ) or consult any of the major undergraduate-level evolutionary biology or population genetics textbooks.

The Origin was, in Darwin's words, an “abstract” of a much larger work he had initially intended to write. Much of the additional material is available in Darwin ( 1868 ) and Stauffer ( 1975 ).

See Gregory ( 2008a ) for a discussion regarding the use of the term “theory” in science.

Ridley ( 2004 ) points out that Darwin's calculations require overlapping generations to reach this exact number, but the point remains that even in slow-reproducing species the rate of potential production is enormous relative to actual numbers of organisms.

Humans are currently undergoing a rapid population expansion, but this is the exception rather than the rule. As Darwin ( 1859 ) noted, “Although some species may now be increasing, more or less rapidly, in numbers, all cannot do so, for the world would not hold them.”

It cannot be overemphasized that “evolution” and “natural selection” are not interchangeable. This is because not all evolution occurs by natural selection and because not all outcomes of natural selection involve changes in the genetic makeup of populations. A detailed discussion of the different types of selection is beyond the scope of this article, but it can be pointed out that the effect of “stabilizing selection” is to prevent directional change in populations.

Instructors interested in assessing their own students' level of understanding may wish to consult tests developed by Bishop and Anderson ( 1986 ), Anderson et al. ( 2002 ), Beardsley ( 2004 ), Shtulman ( 2006 ), or Kampourakis and Zogza ( 2009 ).

Even more alarming is a recent indication that one in six teachers in the USA is a young Earth creationist, and that about one in eight teaches creationism as though it were a valid alternative to evolutionary science (Berkman et al. 2008 ).

Strictly speaking, it is not necessary to understand how evolution occurs to be convinced that it has occurred because the historical fact of evolution is supported by many convergent lines of evidence that are independent of discussions about particular mechanisms. Again, this represents the important distinction between evolution as fact and theory. See Gregory ( 2008a ).

http://www3.niaid.nih.gov/topics/antimicrobialResistance/Understanding/history.htm , accessed February 2009.

One should always be wary of the linguistic symptoms of anthropomorphic misconceptions, which usually include phrasing like “so that” (versus “because”) or “in order to” (versus “happened to”) when explaining adaptations (Kampourakis and Zogza 2009 ).

It must be noted that the persistent tendency to label the inheritance of acquired characteristics as “Lamarckian” is false: Soft inheritance was commonly accepted long before Lamarck's time (Zirkle 1946 ). Likewise, mechanisms involving organisms' conscious desires to change are often incorrectly attributed to Lamarck. For recent critiques of the tendency to describe various misconceptions as Lamarckian, see Geraedts and Boersma ( 2006 ) and Kampourakis and Zogza ( 2007 ). It is unfortunate that these mistakenly attributed concepts serve as the primary legacy of Lamarck, who in actuality made several important contributions to biology (a term first used by Lamarck), including greatly advancing the classification of invertebrates (another term he coined) and, of course, developing the first (albeit ultimately incorrect) mechanistic theory of evolution. For discussions of Lamarck's views and contributions to evolutionary biology, see Packard ( 1901 ), Burkhardt ( 1972 , 1995 ), Corsi ( 1988 ), Humphreys ( 1995 , 1996 ), and Kampourakis and Zogza ( 2007 ). Lamarck's works are available online at http://www.lamarck.cnrs.fr/index.php?lang=en .

One may wonder how this misconception is reconciled with the common admonition by medical doctors to complete each course of treatment with antibiotics even after symptoms disappear—would this not provide more opportunities for bacteria to “develop” resistance by prolonging exposure?

Alters B. Teaching biological evolution in higher education. Boston: Jones and Bartlett; 2005.

Google Scholar  

Alters BJ, Nelson CE. Teaching evolution in higher education. Evolution. 2002;56:1891–901.

Anderson DL, Fisher KM, Norman GJ. Development and evaluation of the conceptual inventory of natural selection. J Res Sci Teach. 2002;39:952–78. doi: 10.1002/tea.10053 .

Asghar A, Wiles JR, Alters B. Canadian pre-service elementary teachers' conceptions of biological evolution and evolution education. McGill J Educ. 2007;42:189–209.

Attenborough D. Life on earth. Boston: Little, Brown and Company; 1979.

Banet E, Ayuso GE. Teaching of biological inheritance and evolution of living beings in secondary school. Int J Sci Edu 2003;25:373–407.

Bardapurkar A. Do students see the “selection” in organic evolution? A critical review of the causal structure of student explanations. Evo Edu Outreach. 2008;1:299–305. doi: 10.1007/s12052-008-0048-5 .

Barton NH, Briggs DEG, Eisen JA, Goldstein DB, Patel NH. Evolution. Cold Spring Harbor: Cold Spring Harbor Laboratory Press; 2007.

Bartov H. Can students be taught to distinguish between teleological and causal explanations? J Res Sci Teach. 1978;15:567–72. doi: 10.1002/tea.3660150619 .

Bartov H. Teaching students to understand the advantages and disadvantages of teleological and anthropomorphic statements in biology. J Res Sci Teach. 1981;18:79–86. doi: 10.1002/tea.3660180113 .

Beardsley PM. Middle school student learning in evolution: are current standards achievable? Am Biol Teach. 2004;66:604–12. doi: 10.1662/0002-7685(2004)066[0604:MSSLIE]2.0.CO;2 .

Bell G. The basics of selection. New York: Chapman & Hall; 1997.

Bell G. Selection: the mechanism of evolution. 2nd ed. Oxford: Oxford University Press; 2008.

Berkman MB, Pacheco JS, Plutzer E. Evolution and creationism in America's classrooms: a national portrait. PLoS Biol. 2008;6:e124. doi: 10.1371/journal.pbio.0060124 .

Bishop BA, Anderson CW. Evolution by natural selection: a teaching module (Occasional Paper No. 91). East Lansing: Institute for Research on Teaching; 1986.

Bishop BA, Anderson CW. Student conceptions of natural selection and its role in evolution. J Res Sci Teach. 1990;27:415–27. doi: 10.1002/tea.3660270503 .

Bizzo NMV. From Down House landlord to Brazilian high school students: what has happened to evolutionary knowledge on the way? J Res Sci Teach. 1994;31:537–56.

Bloom P, Weisberg DS. Childhood origins of adult resistance to science. Science. 2007;316:996–7. doi: 10.1126/science.1133398 .

CAS   Google Scholar  

Brem SK, Ranney M, Schindel J. Perceived consequences of evolution: college students perceive negative personal and social impact in evolutionary theory. Sci Educ. 2003;87:181–206. doi: 10.1002/sce.10105 .

Brumby M. Problems in learning the concept of natural selection. J Biol Educ. 1979;13:119–22.

Brumby MN. Misconceptions about the concept of natural selection by medical biology students. Sci Educ. 1984;68:493–503. doi: 10.1002/sce.3730680412 .

Burkhardt RW. The inspiration of Lamarck's belief in evolution. J Hist Biol. 1972;5:413–38. doi: 10.1007/BF00346666 .

Burkhardt RW. The spirit of system. Cambridge: Harvard University Press; 1995.

Chinsamy A, Plaganyi E. Accepting evolution. Evolution. 2007;62:248–54.

Clough EE, Wood-Robinson C. How secondary students interpret instances of biological adaptation. J Biol Educ. 1985;19:125–30.

Corsi P. The age of Lamarck. Berkeley: University of California Press; 1988.

Coyne JA. Selling Darwin. Nature. 2006;442:983–4. doi: 10.1038/442983a .

Creedy LJ. Student understanding of natural selection. Res Sci Educ. 1993;23:34–41. doi: 10.1007/BF02357042 .

Curry A. Creationist beliefs persist in Europe. Science. 2009;323:1159. doi: 10.1126/science.323.5918.1159 .

Darimont CT, Carlson SM, Kinnison MT, Paquet PC, Reimchen TE, Wilmers CC. Human predators outpace other agents of trait change in the wild. Proc Natl Acad Sci U S A. 2009;106:952–4. doi: 10.1073/pnas.0809235106 .

Darwin C. On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life. London: John Murray; 1859.

Darwin, C. The variation of animals and plants under domestication. London: John Murray; 1868.

Darwin C, Wallace AR. On the tendency of species to form varieties; and on the perpetuation of varieties and species by natural means of selection. Proc Linn Soc. 1858;3:46–62.

Deadman JA, Kelly PJ. What do secondary school boys understand about evolution and heredity before they are taught the topic? J Biol Educ. 1978;12:7–15.

Demastes SS, Settlage J, Good R. Students' conceptions of natural selection and its role in evolution: cases of replication and comparison. J Res Sci Teach. 1995;32:535–50. doi: 10.1002/tea.3660320509 .

Deniz H, Donelly LA, Yilmaz I. Exploring the factors related to acceptance of evolutionary theory among Turkish preservice biology teachers: toward a more informative conceptual ecology for biological evolution. J Res Sci Teach. 2008;45:420–43. doi: 10.1002/tea.20223 .

Dennett DC. Darwin's dangerous idea. New York: Touchstone Books; 1995.

Espinasa M, Espinasa L. Losing sight of regressive evolution. Evo Edu Outreach. 2008;1:509–16. doi: 10.1007/s12052-008-0094-z .

Evans EM, Mull MS, Poling DA, Szymanowski K. Overcoming an essentialist bias: from metamorphosis to evolution. In Biennial meeting of the Society for Research in Child Development , Atlanta, GA; 2005.

Evans EM, Spiegel A, Gram W, Frazier BF, Thompson S, Tare M, Diamond J. A conceptual guide to museum visitors’ understanding of evolution. In Annual Meeting of the American Education Research Association , San Francisco; 2006.

Ferrari M, Chi MTH. The nature of naive explanations of natural selection. Int J Sci Educ. 1998;20:1231–56. doi: 10.1080/0950069980201005 .

Firenze R. Lamarck vs. Darwin: dueling theories. Rep Natl Cent Sci Educ. 1997;17:9–11.

Freeman S, Herron JC. Evolutionary analysis. 4th ed. Upper Saddle River: Prentice Hall; 2007.

Futuyma DJ. Evolution. Sunderland: Sinauer; 2005.

Gelman SA. Psychological essentialism in children. Trends Cogn Sci. 2004;8:404–9. doi: 10.1016/j.tics.2004.07.001 .

Geraedts CL, Boersma KT. Reinventing natural selection. Int J Sci Educ. 2006;28:843–70. doi: 10.1080/09500690500404722 .

Gould SJ. Shades of Lamarck. In: The Panda's Thumb. New York: Norton; 1980. p. 76–84.

Greene ED. The logic of university students' misunderstanding of natural selection. J Res Sci Teach. 1990;27:875–85. doi: 10.1002/tea.3660270907 .

Gregory TR. Evolution as fact, theory, and path. Evo Edu Outreach. 2008a;1:46–52. doi: 10.1007/s12052-007-0001-z .

Gregory TR. The evolution of complex organs. Evo Edu Outreach. 2008b;1:358–89. doi: 10.1007/s12052-008-0076-1 .

Gregory TR. Artificial selection and domestication: modern lessons from Darwin's enduring analogy. Evo Edu Outreach. 2009;2:5–27. doi: 10.1007/s12052-008-0114-z .

Hall BK, Hallgrimsson B. Strickberger's evolution. 4th ed. Sudbury: Jones and Bartlett; 2008.

Halldén O. The evolution of the species: pupil perspectives and school perspectives. Int J Sci Educ. 1988;10:541–52. doi: 10.1080/0950069880100507 .

Halloun IA, Hestenes D. The initial knowledge state of college physics students. Am J Phys. 1985;53:1043–55. doi: 10.1119/1.14030 .

Hillis DM. Making evolution relevant and exciting to biology students. Evolution. 2007;61:1261–4. doi: 10.1111/j.1558-5646.2007.00126.x .

Humphreys J. The laws of Lamarck. Biologist. 1995;42:121–5.

Humphreys J. Lamarck and the general theory of evolution. J Biol Educ. 1996;30:295–303.

Ingram EL, Nelson CE. Relationship between achievement and students' acceptance of evolution or creation in an upper-level evolution course. J Res Sci Teach. 2006;43:7–24. doi: 10.1002/tea.20093 .

Jeffery WR. Adaptive evolution of eye degeneration in the Mexican blind cavefish. J Heredity. 2005;96:185–96. doi: 10.1093/jhered/esi028 .

Jensen MS, Finley FN. Teaching evolution using historical arguments in a conceptual change strategy. Sci Educ. 1995;79:147–66. doi: 10.1002/sce.3730790203 .

Jensen MS, Finley FN. Changes in students' understanding of evolution resulting from different curricular and instructional strategies. J Res Sci Teach. 1996;33:879–900. doi: 10.1002/(SICI)1098-2736(199610)33:8<879::AID-TEA4>3.0.CO;2-T .

Jiménez-Aleixandre MP. Thinking about theories or thinking with theories?: a classroom study with natural selection. Int J Sci Educ. 1992;14:51–61. doi: 10.1080/0950069920140106 .

Jiménez-Aleixandre MP, Fernández-Pérez J. Selection or adjustment? Explanations of university biology students for natural selection problems. In: Novak, JD. Proceedings of the Second International Seminar on Misconceptions and Educational Strategies in Science and Mathematics, vol II. Ithaca: Department of Education, Cornell University; 1987;224–32.

Jørgensen C, Enberg K, Dunlop ES, Arlinghaus R, Boukal DS, Brander K, et al. Managing evolving fish stocks. Science. 2007;318:1247–8. doi: 10.1126/science.1148089 .

Jungwirth E. The problem of teleology in biology as a problem of biology-teacher education. J Biol Educ. 1975a;9:243–6.

Jungwirth E. Preconceived adaptation and inverted evolution. Aust Sci Teachers J. 1975b;21:95–100.

Jungwirth E. Should natural phenomena be described teleologically or anthropomorphically?—a science educator’s view. J Biol Educ. 1977;11:191–6.

Kampourakis K, Zogza V. Students’ preconceptions about evolution: how accurate is the characterization as “Lamarckian” when considering the history of evolutionary thought? Sci Edu 2007;16:393–422.

Kampourakis K, Zogza V. Students’ intuitive explanations of the causes of homologies and adaptations. Sci Educ. 2008;17:27–47. doi: 10.1007/s11191-007-9075-9 .

Kampourakis K, Zogza V. Preliminary evolutionary explanations: a basic framework for conceptual change and explanatory coherence in evolution. Sci Educ. 2009; in press.

Kardong KV. An introduction to biological evolution. 2nd ed. Boston: McGraw Hill; 2008.

Kargbo DB, Hobbs ED, Erickson GL. Children's beliefs about inherited characteristics. J Biol Educ. 1980;14:137–46.

Kelemen D. Why are rocks pointy? Children's preference for teleological explanations of the natural world. Dev Psychol. 1999a;35:1440–52. doi: 10.1037/0012-1649.35.6.1440 .

Kelemen D. Function, goals and intention: children's teleological reasoning about objects. Trends Cogn Sci. 1999b;3:461–8. doi: 10.1016/S1364-6613(99)01402-3 .

Kelemen D, Rosset E. The human function compunction: teleological explanation in adults. Cognition. 2009;111:138–43. doi: 10.1016/j.cognition.2009.01.001 .

Keown D. Teaching evolution: improved approaches for unprepared students. Am Biol Teach. 1988;50:407–10.

Lawson AE, Thompson LD. Formal reasoning ability and misconceptions concerning genetics and natural selection. J Res Sci Teach. 1988;25:733–46. doi: 10.1002/tea.3660250904 .

MacFadden BJ, Dunckel BA, Ellis S, Dierking LD, Abraham-Silver L, Kisiel J, et al. Natural history museum visitors' understanding of evolution. BioScience. 2007;57:875–82.

Mayr E. The growth of biological thought. Cambridge: Harvard University Press; 1982.

Mayr E. What evolution Is. New York: Basic Books; 2001.

McCloskey M, Caramazza A, Green B. Curvilinear motion in the absence of external forces: naïve beliefs about the motion of objects. Science. 1980;210:1139–41. doi: 10.1126/science.210.4474.1139 .

Moore R, Mitchell G, Bally R, Inglis M, Day J, Jacobs D. Undergraduates' understanding of evolution: ascriptions of agency as a problem for student learning. J Biol Educ. 2002;36:65–71.

Nehm RH, Reilly L. Biology majors' knowledge and misconceptions of natural selection. BioScience. 2007;57:263–72. doi: 10.1641/B570311 .

Nehm RH, Schonfeld IS. Does increasing biology teacher knowledge of evolution and the nature of science lead to greater preference for the teaching of evolution in schools? J Sci Teach Educ. 2007;18:699–723. doi: 10.1007/s10972-007-9062-7 .

Nehm RH, Poole TM, Lyford ME, Hoskins SG, Carruth L, Ewers BE, et al. Does the segregation of evolution in biology textbooks and introductory courses reinforce students' faulty mental models of biology and evolution? Evo Edu Outreach. 2009;2: In press.

Nelson CE. Teaching evolution effectively: a central dilemma and alternative strategies. McGill J Educ. 2007;42:265–83.

Nelson CE. Teaching evolution (and all of biology) more effectively: strategies for engagement, critical reasoning, and confronting misconceptions. Integr Comp Biol. 2008;48:213–25. doi: 10.1093/icb/icn027 .

Packard AS. Lamarck, the founder of evolution: his life and work with translations of his writings on organic evolution. New York: Longmans, Green, and Co; 1901.

Palumbi SR. Humans as the world's greatest evolutionary force. Science. 2001;293:1786–90. doi: 10.1126/science.293.5536.1786 .

Passmore C, Stewart J. A modeling approach to teaching evolutionary biology in high schools. J Res Sci Teach. 2002;39:185–204. doi: 10.1002/tea.10020 .

Pedersen S, Halldén O. Intuitive ideas and scientific explanations as parts of students' developing understanding of biology: the case of evolution. Eur J Psychol Educ. 1992;9:127–37.

Pennock RT. Learning evolution and the nature of science using evolutionary computing and artificial life. McGill J Educ. 2007;42:211–24.

Prinou L, Halkia L, Skordoulis C. What conceptions do Greek school students form about biological evolution. Evo Edu Outreach. 2008;1:312–7. doi: 10.1007/s12052-008-0051-x .

Ridley M. Evolution. 3rd ed. Malden: Blackwell; 2004.

Robbins JR, Roy P. The natural selection: identifying & correcting non-science student preconceptions through an inquiry-based, critical approach to evolution. Am Biol Teach. 2007;69:460–6. doi: 10.1662/0002-7685(2007)69[460:TNSICN]2.0.CO;2 .

Rose MR, Mueller LD. Evolution and ecology of the organism. Upper Saddle River: Prentice Hall; 2006.

Rutledge ML, Mitchell MA. High school biology teachers' knowledge structure, acceptance & teaching of evolution. Am Biol Teach. 2002;64:21–7. doi: 10.1662/0002-7685(2002)064[0021:HSBTKS]2.0.CO;2 .

Scharmann LC. Enhancing an understanding of the premises of evolutionary theory: the influence of a diversified instructional strategy. Sch Sci Math. 1990;90:91–100.

Settlage J. Conceptions of natural selection: a snapshot of the sense-making process. J Res Sci Teach. 1994;31:449–57.

Shtulman A. Qualitative differences between naïve and scientific theories of evolution. Cognit Psychol. 2006;52:170–94. doi: 10.1016/j.cogpsych.2005.10.001 .

Sinatra GM, Southerland SA, McConaughy F, Demastes JW. Intentions and beliefs in students' understanding and acceptance of biological evolution. J Res Sci Teach. 2003;40:510–28. doi: 10.1002/tea.10087 .

Sinatra GM, Brem SK, Evans EM. Changing minds? Implications of conceptual change for teaching and learning about biological evolution. Evo Edu Outreach. 2008;1:189–95. doi: 10.1007/s12052-008-0037-8 .

Southerland SA, Abrams E, Cummins CL, Anzelmo J. Understanding students' explanations of biological phenomena: conceptual frameworks or p-prims? Sci Educ. 2001;85:328–48. doi: 10.1002/sce.1013 .

Spiegel AN, Evans EM, Gram W, Diamond J. Museum visitors' understanding of evolution. Museums Soc Issues. 2006;1:69–86.

Spindler LH, Doherty JH. Assessment of the teaching of evolution by natural selection through a hands-on simulation. Teach Issues Experiments Ecol. 2009;6:1–20.

Stauffer RC (editor). Charles Darwin's natural selection: being the second part of his big species book written from 1856 to 1858. Cambridge, UK: Cambridge University Press; 1975.

Stearns SC, Hoekstra RF. Evolution: an introduction. 2nd ed. Oxford, UK: Oxford University Press; 2005.

Strevens M. The essentialist aspect of naive theories. Cognition. 2000;74:149–75. doi: 10.1016/S0010-0277(99)00071-2 .

Sundberg MD. Strategies to help students change naive alternative conceptions about evolution and natural selection. Rep Natl Cent Sci Educ. 2003;23:1–8.

Sundberg MD, Dini ML. Science majors vs nonmajors: is there a difference? J Coll Sci Teach. 1993;22:299–304.

Tamir P, Zohar A. Anthropomorphism and teleology in reasoning about biological phenomena. Sci Educ. 1991;75:57–67. doi: 10.1002/sce.3730750106 .

Tidon R, Lewontin RC. Teaching evolutionary biology. Genet Mol Biol. 2004;27:124–31. doi: 10.1590/S1415-475720054000100021 .

Vlaardingerbroek B, Roederer CJ. Evolution education in Papua New Guinea: trainee teachers' views. Educ Stud. 1997;23:363–75. doi: 10.1080/0305569970230303 .

Wilson DS. Evolution for everyone: how to increase acceptance of, interest in, and knowledge about evolution. PLoS Biol. 2005;3:e364. doi: 10.1371/journal.pbio.0030364 .

Wood-Robinson C. Young people's ideas about inheritance and evolution. Stud Sci Educ. 1994;24:29–47. doi: 10.1080/03057269408560038 .

Zirkle C. The early history of the idea of the inheritance of acquired characters and of pangenesis. Trans Am Philos Soc. 1946;35:91–151. doi: 10.2307/1005592 .

Zohar A, Ginossar S. Lifting the taboo regarding teleology and anthropomorphism in biology education—heretical suggestions. Sci Educ. 1998;82:679–97. doi: 10.1002/(SICI)1098-237X(199811)82:6<679::AID-SCE3>3.0.CO;2-E .

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5.15: Theory of Evolution by Natural Selection

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How do new species form?

This is the only illustration in Charles Darwin 's 1859 book On the Origin of Species , showing his ideas describing the divergence of species from common ancestors.

Darwin’s Theory of Evolution by Natural Selection

Darwin spent many years thinking about the work of Lamarck, Lyell, and Malthus, what he had seen on his voyage, and artificial selection. What did all this mean? How did it all fit together? It fits together in Darwin’s theory of evolution by natural selection. It’s easy to see how all of these influences helped shape Darwin’s ideas.

For a discussion of the underlying causes of natural selection and evolution see http://www.youtube.com/watch?v=DuArVnT1i-E (19:51).

Evolution of Darwin’s Theory

It took Darwin years to form his theory of evolution by natural selection. His reasoning went like this:

• Like Lamarck, Darwin assumed that species can change over time. The fossils he found helped convince him of that.
• From Lyell, Darwin saw that Earth and its life were very old. Thus, there had been enough time for evolution to produce the great diversity of life Darwin had observed.
• From Malthus, Darwin knew that populations could grow faster than their resources. This “overproduction of offspring” led to a “struggle for existence,” in Darwin’s words.
• From artificial selection, Darwin knew that some offspring have variations that occur by chance, and that can be inherited. In nature, offspring with certain variations might be more likely to survive the “struggle for existence” and reproduce. If so, they would pass their favorable variations to their offspring.
• Darwin coined the term fitness to refer to an organism’s relative ability to survive and produce fertile offspring. Nature selects the variations that are most useful. Therefore, he called this type of selection natural selection .
• Darwin knew artificial selection could change domestic species over time. He inferred that natural selection could also change species over time. In fact, he thought that if a species changed enough, it might evolve into a new species.

Wallace’s paper not only confirmed Darwin’s ideas. It also pushed him to finish his book, On the Origin of Species . Published in 1859, this book changed science forever. It clearly spelled out Darwin’s theory of evolution by natural selection and provided convincing arguments and evidence to support it.

Applying Darwin’s Theory

The following example applies Darwin’s theory. It explains how giraffes came to have such long necks (see Figure below ).

• In the past, giraffes had short necks. But there was chance variation in neck length. Some giraffes had necks a little longer than the average.
• Then, as now, giraffes fed on tree leaves. Perhaps the environment changed, and leaves became scarcer. There would be more giraffes than the trees could support. Thus, there would be a “struggle for existence.”
• Giraffes with longer necks had an advantage. They could reach leaves other giraffes could not. Therefore, the long-necked giraffes were more likely to survive and reproduce. They had greater fitness.
• These giraffes passed the long-neck trait to their offspring. Each generation, the population contained more long-necked giraffes. Eventually, all giraffes had long necks.

Giraffes feed on leaves high in trees. Their long necks allow them to reach leaves that other ground animals cannot.

As this example shows, chance variations may help a species survive if the environment changes. Variation among species helps ensure that at least one will be able to survive environmental change.

A summary of Darwin's ideas are presented in the video ‘‘Natural Selection and the Owl Butterfly’’ : http://www.youtube.com/watch?v=dR_BFmDMRaI (13:29).

KQED: Chasing Beetles, Finding Darwin

It's been over 150 years since Charles Darwin published On the Origin of Species . Yet his ideas remain as central to scientific exploration as ever, and has been called the unifying concept of all biology. Is evolution continuing today? Of course it is.

QUEST follows researchers who are still unlocking the mysteries of evolution, including entomologist David Kavanaugh of the California Academy of Sciences, who predicted that a new beetle species would be found on the Trinity Alps of Northern California. See www.kqed.org/quest/television...inding-darwin2 for more information.

It's rare for a biologist to predict the discovery of a new species. For his prediction, Kavanaugh drew inspiration from Darwin's own 1862 prediction. When Darwin observed an orchid from Madagascar with a foot-long nectar, he predicted that a pollinator would be found with a tongue long enough to reach the nectar inside the orchid's very thin, elongated nectar ‘‘pouch’’, though he had never seen such a bird or insect. Darwin's prediction was based on his finding that all species are related to each other and that some of them evolve together, developing similar adaptations . Darwin's prediction came true in 1903, when a moth was discovered in Madagascar with a long, thin proboscis, which it uncurls to reach the nectar in the orchid's nectar. In the process of feeding from the orchid, the moth serves as its pollinator. The moth was given the scientific name Xanthopan morganii praedicta , in honor of Darwin’s prediction.

As you view Chasing Beetles, Finding Darwin, focus on the following concepts:

• the relationship between studying beetles and evolution,
• the development of new species,
• the relationship between genetic make-up of an organism and evolution,
• the role of beneficial mutations,
• the role of ‘‘habitat islands’’,
• the selection for certain traits among breeders, such as pigeon breeders,
• the importance of identifying new species.

For an additional explanation of natural selection, see Darwin, Mice, and Picky Peacocks at https://www.youtube.com/watch?v=lvfNuz8B1jk .

• Darwin's book On the Origin of Species clearly spells out his theory.
• Darwin's book also provides evidence and logic to support that evolution occurs and that it occurs by natural selection.

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• Charles Darwin & Evolution at darwin200.christs.cam.ac.uk/p...php?page_id=d3.
• What did Darwin mean by "common descent?"
• What did Darwin mean by "gradualism?"
• What is meant by "super fecundity?"
• What did Darwin say would happen to individuals of the same species in an environment of scarce resources?

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• Changes in the Environment at http://www.concord.org/activities/changes-environment .
• Natural Selection
• Define fitness.
• Apply Darwin’s theory of evolution by natural selection to a specific case. For example, explain how Galápagos tortoises could have evolved saddle-shaped shells.
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• Discuss the role artificial selection had on Darwin's theory.

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Natural Selection: Uncovering Mechanisms of Evolutionary Adaptation to Infectious Disease

In the 1940s, J. B. S. Haldane observed that many red blood cell disorders, such as sickle-cell anemia and various thalassemias, were prominent in tropical regions where malaria was endemic (Haldane, 1949; Figure 1). Haldane hypothesized that these disorders had become common in these regions because natural selection had acted to increase the prevalence of traits that protect individuals from malaria. Just a few years later, Haldane's so-called "malaria hypothesis" was confirmed by researcher A. C. Allison, who demonstrated that the geographical distribution of the sickle-cell mutation in the beta hemoglobin gene ( HBB ) was limited to Africa and correlated with malaria endemicity. Allison further noted that individuals who carried the sickle-cell trait were resistant to malaria (Allison, 1954).

Allison's confirmation of Haldane's hypothesis provided the first elucidated example of human adaptation since natural selection had been proposed a century earlier. Today, this and other demonstrations of natural selection help point researchers toward biological mechanisms of resistance to infectious disease . Moreover, such examples also shed light on the ways in which pathogens rapidly evolve to remain agents of human morbidity and mortality.

Selection for Malaria Resistance: A Closer Look

Since Allison and Haldane's work, the action of natural selection on genetic resistance to malaria has been shown in a multitude of contexts (Kwiatkowski, 2005). Indeed, the sickle-cell variant (i.e., the HbS allele ) has been identified in four distinct genetic backgrounds in different African populations, suggesting that the same mutation arose independently several times through convergent evolution . Beyond HbS, other distinct mutations in the HBB gene have generated the HbC and HbE alleles , which arose and spread in Africa and in Southeast Asia, respectively.

The various HBB alleles aren't alone in offering protection against malaria, however. The geographic distributions of several other red blood cell disorders, including a-thalassemia, G6PD deficiency, and ovalocytosis, correlate to malaria endemicity, and the diseases also are linked to malaria resistance. An even more striking worldwide geographical difference exists for a mutation in the Duffy antigen gene ( FY ), which encodes a membrane protein used by the Plasmodium vivax malaria parasite to enter red blood cells. This mutation disrupts the protein, thus conferring protection against P. vivax malaria, and it occurs at a prevalence of 100% throughout most of sub-Saharan Africa yet is virtually absent outside of Africa. Moreover, through convergent evolution , an independent mutation in FY that decreases this gene's expression has also become prevalent in Southeast Asia.

So, why has malaria exerted such strong selective pressure? Scientists now know the answer. Malaria is arguably one of the human population 's oldest diseases and greatest causes of morbidity and mortality. Research indicates that the malaria-causing parasite Plasmodium falciparum has occurred in human populations for approximately 100,000 years, with a large population expansion in the last 10,000 years as human populations began to move into settlements (Hartl, 2004). P. falciparum , together with the other malaria species , P. vivax , P. malariae, and P. ovale , infects hundreds of millions of people worldwide each year, and kills more than 1 million children annually (World Health Organization, 2000). Because this disease is so devastating, humans have had to evolve adaptive traits to survive in the face of this infectious condition over the past few millennia (Kwiatkowski, 2005).

Broader Implications of Natural Selection for Investigating Infectious Disease

While malaria is the best-understood example of an infectious disease that has driven human evolution, numerous other infectious diseases have also acted in human populations over generations, thus allowing resistance alleles to emerge and spread over time (Diamond, 2005). Based on historical records from the last millennium, these diseases might include smallpox in ancient Europe and in Native American populations, as well as cholera, tuberculosis, and bubonic plague in Europe. Many diseases in Africa have likely been endemic for even longer, such as numerous diarrheal diseases, yellow fever, and Lassa hemorrhagic fever.

Today, with access to heretofore unprecedented data sets for the study of human genetic variation , researchers can exploit the genetic signatures of natural selection using novel analytical methods. In this way, they can identify genetic variants conferring resistance to infectious diseases that have spread through human populations over time. These studies will help elucidate natural mechanisms of defense and perhaps uncover novel evolutionary pressures. Moreover, the same tools that have revolutionized the study of natural selection in humans will also make unprecedented studies of pathogens possible.

Investigating the signatures of natural selection can help elucidate the evolutionary adaptations that have allowed humans to withstand some of our most complex and challenging selective agents. In particular, researchers can look for variants that might be readily detected in genetic association studies; for distinctive, detectable patterns of genetic variation in the human genome ; and for clues as to how pathogens themselves evolve so rapidly.

Searching for Variants via Association Studies

By driving highly protective variants to high prevalence, natural selection produces variants that might be readily detected in genetic association studies to help elucidate the biological basis of disease resistance. The classic examples of host genetic factors that play a role in resistance to malaria, such as HbS, are some of the strongest and most robust signals of genetic susceptibility to infectious disease (Hill, 2006). This is because natural selection acts to increase the prevalence of highly advantageous alleles, over time generating common resistance alleles of especially strong effect. For example, a study of genetic susceptibility of HbS in the Gambia detected a significant level of protection using just 315 cases and 583 controls (Ackerman et al. , 2005). By studying other ancient selective pressures in which common resistance alleles of strong effect are acting, scientists may have the power to detect a genetic association even with small sample sizes.

In contrast, no single highly protective variant for emergent diseases like HIV and tuberculosis (in Africa) would have had time to spread. For these diseases, resistance appears to be modulated by many rare genetic variants, most with modest protective effect, and genetic studies require extremely large sample sizes (Hill, 2006). This is likely not a biological but, rather, a historical difference. Indeed, hundreds of structural and regulatory mutations exist in HBB , such as HbS, HbE, or HbC, but in populations under malaria selective pressure , a single highly protective variant will often dominate (Kwiatkowski, 2005). Moreover, many variants nearby on the chromosome will rise in prevalence in the population through genetic hitchhiking , such that other nearby linked alleles can serve as proxies for the underlying causal allele in genetic association studies, further enhancing researchers' ability to detect an association. Thus, natural selection may produce important genetic resistance loci that can more easily be detected in association studies.

Searching for Patterns of Variation

As genetic variants conferring resistance to infectious diseases spread through human populations over time through natural selection, they leave distinctive, detectable patterns of genetic variation in the human genome. These signals of selection can uncover novel resistance alleles or even novel evolutionary pressures. Also, as previously mentioned, as advantageous alleles under positive selection rise in prevalence, variants at nearby locations on the same chromosome (linked alleles) also rise in prevalence. Such genetic hitchhiking leads to a " selective sweep " that alters the typical pattern of genetic variation in the region. Selective sweeps produce numerous detectable signals of selection (Nielsen, 2005; Sabeti et al., 2006). As tests for selection have been applied to newly available genetic variation data across the human genome, many of the top signals of selection that have been identified have been at genes and alleles known to be involved with malaria susceptibility, including HBB , FY , CD36 , and HLA . These signals were identified in just 90 individuals randomly chosen from the population, and they could have been identified without prior knowledge of a specific variant or selective advantage .

Surveys of natural selection can not only identify new resistance variants for known selective pressures, but they can also potentially uncover previously unrecognized selective pressures. For example, in a genome survey of the Yoruba people of Nigeria, two of the top signals of selection were at genes ( LARGE and DMD ) biologically linked to the Lassa hemorrhagic fever virus (Sabeti et al ., 2007). While little studied, Lassa virus in fact infects many millions of West Africans, and based on oral records and epidemiology , it is likely to be an ancient disease (Richmond & Baglole, 2003). Researchers have documented that in several affected West African populations, between 50% and 90% of individuals are resistant to the virus, suggesting that protective alleles emerged at some point (McCormick & Fisher-Hoch, 2002). This finding could open new avenues for research and shine light on other important pathogens in human history.

Searching for Clues about Pathogen Evolution

The same tools that revolutionized the study of natural selection in humans are now making unprecedented studies of pathogens possible, allowing scientists to better understand how these organisms rapidly evolve to remain agents of human morbidity and mortality. Pathogens are perhaps the most intriguing of all the forces shaping humans. They have had a tremendous impact on our evolution, and they, themselves, evolve over time. The great effect that pathogens have exerted on the human genome is demonstrated by positive selection for traits such as sickle-cell hemoglobin (Sabeti et al ., 2006). Natural human defenses have similarly exerted strong pressures on the genomes of pathogens, as has the use of drugs and vaccines (Volkman et al. , 2007). By studying genetic diversity in pathogens, researchers can examine how they have evolved to avoid human immune defenses and therapeutics. Furthermore, scientists can investigate in real time the evolutionary consequences of new vaccines and drugs, with the goal of developing better intervention strategies.

Future Endeavors

Investigation of the links between natural selection and disease resistance has revealed some of the forces that have shaped our species, and the findings of these studies have direct implications for human health. However, research thus far represents just a first glimpse of a vast new landscape. In the years to come, new technologies and analytic methods will enable researchers to learn even more about the genetic basis of evolutionary adaptations that have allowed humans to withstand a wide variety of complex and challenging selective agents.

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January 1, 2009

13 min read

Testing Natural Selection with Genetics

Biologists working with the most sophisticated genetic tools are demonstrating that natural selection plays a greater role in the evolution of genes than even most evolutionists had thought

By H. Allen Orr

Some ideas are discovered late in the history of a scientific discipline because they are subtle, complex or otherwise difficult. Natural selection was not one of these. Although compared with other revolutionary scientific ideas it was discovered fairly recently—Charles Darwin and Alfred Russel Wallace wrote on the subject in 1858, and Darwin’s On the Origin of Species appeared in 1859—the idea of natural selection is simplicity itself. Some kinds of organisms survive better in certain conditions than others do; such organisms leave more progeny and so become more common with time. The environment thus “selects” those organisms best adapted to present conditions. If environmental conditions change, organisms that happen to possess the most adaptive characteristics for those new conditions will come to predominate. Darwinism was revolutionary not because it made arcane claims about biology but because it suggested that nature’s underlying logic might be surprisingly simple.

In spite of this simplicity, the theory of natural selection has suffered a long and tortuous history. Darwin’s claim that species evolve was rapidly accepted by biologists, but his separate claim that natural selection drives most of the change was not. Indeed, natural selection was not accepted as a key evolutionary force until well into the 20th century.

The status of natural selection is now secure, reflecting decades of detailed empirical work. But the study of natural selection is by no means complete. Rather—partly because new experimental techniques have been developed and partly because the genetic mechanisms underlying natural selection are now the subject of meticulous empirical analysis—the study of natural selection is a more active area of biology than it was even two decades ago. Much of the recent experimental work on natural selection has focused on three goals: determining how common it is, identifying the precise genetic changes that give rise to the adaptations produced by natural selection, and assessing just how big a role natural selection plays in a key problem of evolutionary biology—the origin of new species.

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Natural Selection: The Idea The best way to appreciate evolution by natural selection is to consider organisms whose life cycle is short enough that many generations can be observed. Some bacteria can reproduce themselves every half an hour, so imagine a population of bacteria made up of two genetic types that are initially present in equal numbers. Assume, moreover, that both types breed true: type 1 bacteria produce only type 1 offspring, and type 2 bacteria produce only type 2s. Now suppose the environment suddenly changes: an antibiotic is introduced to which type 1s are resistant but to which type 2s are not. In the new environment, type 1s are fitter—that is, better adapted—than type 2s: they survive and so reproduce more often than type 2s do. The result is that type 1s produce more offspring than type 2s do.

“Fitness,” as used in evolutionary biology, is a technical term for this idea: it is the probability of surviving or reproducing in a given environment. The outcome of this selection process, repeated numberless times in different contexts, is what we all see in nature: plants and animals (and bacteria) that fit their environments in intricate ways.

Evolutionary geneticists can flesh out the preceding argument in much richer biological detail. We know, for instance, that genetic types originate in mutations of DNA—random changes in the sequence of nucleotides (or string made up of the letters A, G, C and T) that constitutes the “language” of the genome. We also know a good deal about the rate at which a common kind of mutation—the change of one letter of DNA to another—appears: each nucleotide in each gamete in each generation has about one chance in a billion of mutating to another nucleotide. Most important, we know something about the effects of mutations on fitness. The overwhelming majority of random mutations are harmful—that is, they reduce fitness; only a tiny minority are beneficial, increasing fitness. Most mutations are bad for the same reason that most typos in computer code are bad: in finely tuned systems, random tweaks are far more likely to disrupt function than to improve it.

Adaptive evolution is therefore a two-step process, with a strict division of labor between mutation and selection. In each generation, mutation brings new genetic variants into populations. Natural selection then screens them: the rigors of the environment reduce the frequency of “bad” (relatively unfit) variants and increase the frequency of “good” (relatively fit) ones. (It is worth noting that a population can store many genetic variants at once, and those variants can help it to meet changing conditions as they arise. The gene that protected the type 1 bacteria from the antibiotic may have been useless or even slightly harmful in the earlier, antibiotic-free environment, but its presence enabled the type 1s to survive when conditions changed.)

Population geneticists have also provided insight into natural selection by describing it mathematically. For example, geneticists have shown that the fitter a given type is within a population, the more rapidly it will increase in frequency; indeed, one can calculate just how quickly the increase will occur. Population geneticists have also discovered the surprising fact that natural selection has unimaginably keen “eyes,” which can detect astonishingly small differences in fitness among genetic types. In a population of a million individuals, natural selection can operate on fitness differences as small as one part in a million.

One remarkable feature of the argument for natural selection is that its logic seems valid for any level of biological entity—from gene to species. Biologists since Darwin, of course, have considered differences in fitness between individual organisms, but in principle natural selection could act on differences in survival or reproduction between other entities. For example, one might reason that species with broad geographic ranges will survive—as species—longer than species whose geographic ranges are narrow. After all, broad-ranging species can tolerate the extinctions of a few local populations more readily than species with restricted ranges can. The logic of natural selection might predict, then, that the proportion of broad-ranging species should increase with time.

Yet though this argument is formally sound—and evolutionists do suspect higher-level selection does take place now and then [see “What’s Good for the Group,” on page 51] (need to link) —most biologists agree that natural selection typically occurs at the level of individual organisms or genetic types. One reason is that the lifetimes of organisms are much shorter than the lifetimes of species. Thus, the natural selection of organisms typically overwhelms the natural selection of species.

How Common Is Natural Selection? One of the simplest questions biologists can ask about natural selection has, surprisingly, been one of the hardest to answer: To what degree is it responsible for changes in the overall genetic makeup of a population? No one seriously doubts that natural selection drives the evolution of most physical traits in living creatures—there is no other plausible way to explain such large-scale features as beaks, biceps and brains. But there has been serious doubt about the extent of the role of natural selection in guiding change at the molecular level. Just what proportion of all evolutionary change in DNA is driven, over millions of years, by natural selection—as opposed to some other process?

Until the 1960s biologists had assumed that the answer was “almost all,” but a group of population geneticists led by Japanese investigator Motoo Kimura sharply challenged that view. Kimura argued that molecular evolution is not usually driven by “positive” natural selection—in which the environment increases the frequency of a beneficial type that is initially rare. Rather, he said, nearly all the genetic mutations that persist or reach high frequencies in populations are selectively neutral—they have no appreciable effect on fitness one way or the other. (Of course, harmful mutations continue to appear at a high rate, but they can never reach high frequencies in a population and thus are evolutionary dead ends.) Since neutral mutations are essentially invisible in the present environment, such changes can slip silently through a population, substantially altering its genetic composition over time. The process is called random genetic drift; it is the heart of the neutral theory of molecular evolution.

By the 1980s many evolutionary geneticists had accepted the neutral theory. But the data bearing on it were mostly indirect; more direct, critical tests were lacking. Two developments have helped fix that problem. First, population geneticists have devised simple statistical tests for distinguishing neutral changes in the genome from adaptive ones. Second, new technology has enabled entire genomes from many species to be sequenced, providing voluminous data on which these statistical tests can be applied. The new data suggest that the neutral theory underestimated the importance of natural selection.

In one study a team led by David J. Begun and Charles H. Langley, both at the University of California, Davis, compared the DNA sequences of two species of fruit fly in the genus Drosophila. They analyzed roughly 6,000 genes in each species, noting which genes had diverged since the two species had split off from a common ancestor. By applying a statistical test, they estimated that they could rule out neutral evolution in at least 19 percent of the 6,000 genes; in other words, natural selection drove the evolutionary divergence of a fifth of all genes studied. (Because the statistical test they employed was conservative, the actual proportion could be much larger.) The result does not suggest that neutral evolution is unimportant—after all, some of the remaining 81 percent of genes may have diverged by genetic drift. But it does prove that natural selection plays a bigger role in the divergence of species than most neutral theorists would have guessed. Similar studies have led most evolutionary geneticists to conclude that natural selection is a common driver of evolutionary change even in the sequences of nucleotides in DNA.

The Genetics of Natural Selection Even when biologists turn to ordinary physical traits (“beaks, biceps and brains”) and are confident that natural selection drove evolutionary change, they are often in the dark about just how it happened. Until recently, for instance, little was known about the genetic changes that un­­derlie adaptive evolution. But with the new developments in genetics, biologists have been able to attack this problem head-on, and they are now attempting to answer several fundamen­tal questions about selection. When organisms adapt by natural selection to a new environment, do they do so because of changes in a few genes or many? Can those genes be identified? And are the same genes involved in independent cases of adaptation to the same environment?

Answering those questions is not easy. The main difficulty is that the increase in fitness arising from a beneficial mutation can be very small, making evolutionary change quite slow. One way evolutionary biologists have coped with this problem is to place populations of rapidly reproducing organisms in artificial environments where fitness differences are larger and evolution is, therefore, faster. It also helps if the populations of the organisms are large enough to provide a steady stream of mutations. In microbial experimental evolution, a population of genetically identical microorganisms is typically placed in a novel environment to which they must adapt. Since all the individuals begin by sharing the same DNA sequence, natural selection must operate only on new mutations that arise during the experiment. The experimenter can then plot how the fitness of the population changes with time by measuring the rate of reproduction in the new environment.

Some of the most intriguing research in experimental evolution has been performed with bacteriophages, viruses so small that they infect bacteria. Bacteriophages have commensurately tiny genomes, and so it is practical for biologists to sequence their entire genomes at the beginning and end of experiments as well as at any time in between. That makes it possible to track every genetic change that natural selection “grabs” and then perpetuates over time.

K. Kichler Holder and James J. Bull, both at the University of Texas at Austin, performed such an experiment with two closely related species of bacteriophages: ΦX174 and G4. Both viruses infect the common gut bacterium Escherichia coli. The experimenters subjected the bacteriophages to an unusually high temperature and allowed them to adapt to the new, warm environment. In both species, fitness in the new environment increased dramatically during the experiment. Moreover, in both cases the experimenters saw the same pattern: fitness improved rapidly near the start of the experiment and then leveled off with time. Remarkably, Holder and Bull were able to identify the exact DNA mutations underlying the increased fitness.

Natural Selection “in the Wild” Although research in experimental evolution provides an unprecedented view of natural selection in action, the approach remains limited to simple organisms for which repeated sequencing of entire genomes is feasible. Some workers have also cautioned that experimental evolution might involve unnaturally harsh selective pressures—perhaps much harsher than the ones encountered in the wild. We would like, then, to study selection in higher organisms under more natural conditions—and so we must find another way to investigate the glacial pace of much evolutionary change.

To do so, evolutionists typically turn to populations or species that have been separated long enough that the adaptive differences between them that were crafted by natural selection are readily found. Biologists can then study those differences genetically. For example, Douglas W. Schemske of Michigan State University and H. D. Bradshaw, Jr., of the University of Washington analyzed natural selection in two species of monkeyflower. Though closely related, Mimulus lewisii is pollinated primarily by bumblebees, whereas M. cardinalis is pollinated primarily by hummingbirds. Data from other species show that bird pollination in the genus Mimulus evolved from bee pollination.

Flower color alone—M. lewisii has pink flowers, and M. cardinalis has red [see box at right]—explains much of these differences in pollinator preference. When Schemske and Bradshaw crossed the two species, they showed that this color difference is controlled to a considerable extent by what appears to be a single gene called Yellow Upper, or YUP. On the basis of that finding, they created two kinds of hybrids. In the first kind, the YUP gene came from M. cardinalis, but the rest of the hybrid’s genome derived from M. lewisii. The resulting flowers were orange. The second kind of hybrid was a “mirror image” of the first: the YUP gene came from M. lewisii, but the rest of the genome derived from M. cardinalis. The resulting flowers were pink.

When the hybrids were transplanted into the wild, the investigators noted that YUP had an enormous effect on pollinator visitation: M. lewisii plants, for instance, that carried YUP from M. cardinalis were visited by hummingbirds about 68 times more often than were pure M. lewisii plants; in the reciprocal experiment (M. cardinalis plants with YUP from M. lewisii), the effect was a 74-fold increase in bumblebee visits. There can be no doubt, then, that YUP played a major role in the evolution of bird pollination in M. cardinalis. Schemske and Bradshaw’s work shows that natural selection sometimes builds adaptations from what appear to be fairly simple genetic changes.

The Origin of Species One of Darwin’s boldest claims for natural selection was that it explains how new species arise. (After all, the title of his masterpiece is On the Origin of Species.) But does it? What role does natural selection play in speciation, the splitting of a single lineage into two? To this day, these questions represent an important topic of re­­search in evolutionary biology.

To understand the answers to those questions, one must be clear about what evolutionists mean by “species.” Unlike Darwin, modern biologists generally adhere to the so-called biological species concept. The key idea is that species are reproductively isolated from one another—that is, they have genetically based traits preventing them from exchanging genes. Different species, in other words, have separate gene pools.

It is thought that two populations must be geographically isolated before reproductive isolation can evolve. The finches that inhabit various islands in the Galápagos Archipelago, which Darwin famously describes in Origin of Species, obviously diverged into the distinct species observed today after they became geographically isolated.

Once reproductive isolation does evolve, it can take several forms. For example, during courtship females of one species might refuse to mate with males of another (if the two species ever do come into geographic contact). Females of the butterfly species Pieris occidentalis, for instance, will not mate with males of the related species P. protodice, probably because the males of the two species have different wing patterns. And even if two species do court and mate, the inviability or sterility of any resulting hybrids can represent another form of reproductive isolation: genes cannot move from one species to another if all hybrids between them are dead or sterile. To contemporary biologists, then, the question of whether natural selection drives the origin of species reduces to the question of whether natural selection drives the origin of reproductive isolation.

For much of the 20th century, many evolutionists thought the answer was no. Instead they believed that genetic drift was the critical factor in speciation. One of the most intriguing findings from recent research on the origin of species is that the genetic drift hypothesis about the origin of species is probably wrong. Rather natural selection plays a major role in speciation.

A good example is the evolutionary history of the two monkeyflower species mentioned earlier. Because their pollinators seldom visit the “wrong” species of monkeyflower, the two species are almost completely isolated reproductively. Even though both species sometimes occur in the same locations in North America, a bumblebee that visits M. lewisii almost never visits M. cardinalis, and a hummingbird that visits M. cardinalis almost never visits M. lewisii. Thus, pollen is rarely transferred between the two species. In fact, Schemske and his colleagues showed that pollinator differences alone account for 98 percent of the total blockage in gene flow between the two species. In this case, then, there can be no doubt that natural selection shaped the plants’ adaptations to distinct pollinators and gave rise to strong reproductive isolation.

Other evidence for the role of natural selection in speciation has come from an unexpected quarter. In the past decade or so several evolutionary geneticists (including me) have identified half a dozen genes that cause hybrid sterility or inviability. The genes in question—studied mostly in species of Drosophila fruit flies—play various normal roles within the species: some encode enzymes, others encode structural proteins, and yet others encode proteins that bind to DNA.

These genes exhibit two striking patterns. First, among the genes that cause problems in hybrid offspring, it turns out that many have diverged extremely rapidly. Second, population genetics tests show that their rapid evolution was driven by natural selection.

The studies of the monkeyflower and of hybrid sterility in fruit flies only begin to scratch the surface of a large and growing literature that reveals the hand of natural selection in speciation. Indeed, most biologists now agree that natural selection is the key evolutionary force that drives not only evolutionary change within species but also the origin of new species. Although some laypeople continue to question the cogency or adequacy of natural selection, its status among evolutionary biologists in the past few decades has, perhaps ironically, only grown more secure.

Note: This article was originally printed with the title, "Testing Natural Selection".

Natural Selection

The theory of natural selection was explored by 19th-century naturalist Charles Darwin. Natural selection explains how genetic traits of a species may change over time. This may lead to speciation, the formation of a distinct new species. Select from these resources to teach your classroom about this subfield of evolutionary biology.

All Human Existence May Have Begun in a Black Hole, Some Scientists Believe

There’s an intriguing possibility that the emergence of conscious life is not just a coincidence, but an inevitable outcome of cosmic evolution.

So let’s contemplate something simpler: why does the universe allow us to exist? Yet again, we run into the same problem: if the universe didn’t allow us to exist, we wouldn’t be here to think about it. This is called the “anthropic principle.” For some, it’s the only answer we need to explain, well , everything; but for others, it’s a philosophical thorn in the side. Everything we know about the universe so far—dating back to the 16th-century Polish astronomer Copernicus, who first proposed that Earth travels around the sun rather than the other way around—tells us that we have no special place in the cosmos. We are not at the center. This is the “Copernican principle.”

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The anthropic and Copernican principles are conflicting axioms about the universe’s existence and our place within it. The anthropic principle says the universe depends on our being here. Meanwhile, the Copernican principle says that we are not special, and no law of physics should depend on our existence. Yet, the vast and ancient universe we see in our telescopes appears to balance both principles, like a pin balanced on the edge of a glass.

So why is our universe the way it is, and why do we exist as self-aware beings , tiny in size and minuscule in lifespan, relative to the lonely cosmic vastness mostly devoid of life? If the universe were made just for us, surely it would be small, human sized, perhaps just one planet or solar system or galaxy, not billions. Why should a universe made for us have black holes, for example? They seem to contribute nothing to our welfare.

Some scientists believe the universe wasn’t finely tuned to create intelligent life like us at all. Instead, they say, the universe evolved its own insurance policy by creating as many black holes as possible, which is the universe’s method of reproduction. Following this line of thinking, the universe itself may very well be alive—and the fact that we humans exist at all is just a happy side effect.

A Finely-Tuned Universe

One of the biggest philosophical problems with the universe is that it has to be finely tuned for us to even exist. If the universe were random, things would quickly become messy. If modified only a tiny bit one way or another, physical parameters such as the speed of light ; the mass of the electron, proton, and neutron ; the gravitational constant ; and so on would eliminate all life—possibly all matter itself—and even the universe as a whole would not last long enough to evolve anything. For example, if their masses were slightly different, protons would decay into neutrons instead of the other way around, and as a result, there would be no atoms.

One possible solution to fine tuning is the multiverse . In this speculative theory, our universe is one of many in the same way that the planet Earth is one of many planets. Different universes have different laws of physics and, therefore, that ours supports life is simply a matter of luck. While some theories of the multiverse propose that these universes are essentially random and have no relationship to one another, one particular multiverse theory suggests that universes in fact reproduce like living beings and have ancestors and descendants. This theory is called cosmological natural selection (or CNS for short). First proposed by theoretical physicist Lee Smolin in 1992, the CNS theory is a strong contender for why our universe seems to balance both the Anthropic and Copernican principles.

When we look at the complexity of living things and the sheer number of non-living configurations there are, we’re left to assume that there’s no way species could appear randomly. Hence, some powerful being must have created all types of living creatures individually as a watchmaker builds a watch, the thinking often goes. However, Charles Darwin’s theory of evolution, which he first posited in his 1859 book, On the Origin of Species, provides a mechanism that explains why living things are non-random. Their parameters are not freely chosen; they are the product of natural selection , the process by which members of a species that are better fit to survive and/or reproduce more effectively are more likely to pass on their genes.

The theory of evolution is one of the greatest success stories in the history of science because it provided a mechanism by which a thing that is highly ordered, complex, and finely tuned for its survival could arise from natural processes. The theory was successful not only because it explained how species arise, but also because it generated new predictions that we could then test. For example, the theory of evolution explains why species appear related to one another.

The Beauty in Black Holes

The cosmological natural selection theory solves the pernicious problem of a universe finely tuned for life. That idea may make sense to us, living on a planet full of complex, multicellular organisms, but Earth is surrounded by mostly dead space and, as far as we know, dead planets, and moons and light years of interstellar dust and stray photons.

Earth is finely tuned for life; the universe is not. However, the cosmological natural selection theory says that the universe is finely tuned for something else: its method of reproduction, giving birth to new universes.

Under the CNS theory framework, every black hole becomes a baby universe . Our universe, likewise, started out as a black hole in its mother universe. The theory says that inside every black hole, the central singularity—which is matter highly compressed in space in the mother universe—becomes a highly compressed point in time in the new universe. This point expands, creating new matter and energy. You get a complete universe from even a tiny black hole .

This means that our universe is finely tuned not for life, but for black holes, which typically come from massive stars (although they can have other origins). It turns out that massive star formation depends on an element also important for life on Earth: carbon.

Carbon monoxide is the second-most common molecule in the universe after molecular hydrogen, even more common than water. In the molecular clouds of gas and dust that form from supernovae, massive stars coalesce amid gaseous carbon monoxide molecules, which act as a coolant. This cooling helps matter clump together and form the stars. Carbon is a critical component in all life that we know of. Therefore, life is, in fact, a byproduct of stellar formation, which is itself a byproduct of what the universe evolved to do: create as many black holes as possible.

The cosmological natural selection theory helps explain why our universe is so highly ordered, complex, and self-sustaining like Darwin’s theory explains the same for living things. That leads to the tantalizing, if speculative, conclusion that perhaps, by some definition, our universe itself is alive.

Dr. Tim Andersen is a principal research scientist at Georgia Tech Research Institute. He earned his doctorate in mathematics from Rensselaer Polytechnic Institute in Troy, New York, and his undergraduate degree from the University of Texas at Austin. He has published academic works in statistical mechanics, fluid dynamics (including a monograph on vortex filaments), quantum field theory, and general relativity. He is the author of The Infinite Universe on Medium and Stubstack and a book by the same name. He lives with his wife and two cats, and has a son and daughter at home as well as one grown son.

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