N Natural Selection Introducing natural selection on the first few pages, Darwin emphasized the importance R. Haven Wiley of variation among individuals, in particular Department of Biology, hereditary variation, and a “struggle for exis- University of North Carolina, tence,” in other words, competition, because Chapel Hill, NC, USA “many more individuals of each species are born than can possibly survive”. He thus reasoned, “It follows that any being, if it vary however slightly Definition in any manner profitable to itself ... will have a better chance of surviving and thus be naturally Differences in the propagation of genes in a pop- selected. From the strong principle of inheritance, ulation as a result of survival and reproduction of any selected variety will tend to propagate its new organisms carrying those genes. and modified form.” He also recognized that nat- ural selection is a means of the “coadaptation of organic beings to each other and to the physical Introduction conditions of life” (Darwin 1859,pp.4–5). Inherited differences in reproduction, as well Charles Darwin described natural selection in pri- as survival, can also lead to natural selection. vate essays in 1842 and especially 1844 (Darwin Darwin emphasized this possibility when he pro- 1909; Glik and Kohn 1996, pp. 90–96). He then posed that differences in attracting mates or com- drafted a large manuscript on natural selection, peting for them could lead to the special case of which he left unfinished. It was subsequently sexual selection. Attraction of mates proved to be overlooked until recently (Darwin 1975). especially controversial for Darwin’s successors. Eventually in 1858, the Linnean Society It implied the evolution of behavior, not just mor- published a version of his 1844 essay in conjunc- phology. The idea that natural selection might tion with a communication from Alfred Russell produce preferences for mates, particularly by Wallace. Wallace presented some related ideas, females, was inconceivable to most scientists in but not natural selection as we now understand it the late nineteenth century. (Bulmer 2005). Soon afterward there appeared On Darwin’s original theory thus included all the Origin of Species / By Means of Natural Selec- of the essential elements of our current under- tion, / or the / Preservation of Favoured Races in standing of natural selection (see, for instance, the Struggle for Life (Darwin 1859), which devel- Maynard Smith 1998). Natural selection is a oped the concept in detail. mechanism of evolutionary adaptation that results © Springer Nature Switzerland AG 2020 T. K. Shackelford, V. A. Weekes-Shackelford (eds.), Encyclopedia of Evolutionary Psychological Science, https://doi.org/10.1007/978-3-319-16999-6_2095-1 2 Natural Selection from a combination of heritable variation among between populations, as well as the inherent individuals and differences in their survival randomness in each of these three processes. or reproduction correlated with this variation. Selection is included in equations for changes in Natural selection does not require individuals to allele frequencies by adding a coefficient to adjust change, but it does require new variants to arise the relative survival or reproduction of each allele. occasionally through reproduction. Natural selec- Random changes in allele frequencies become tion requires more than individual differences in more pronounced in smaller populations. Within survival and reproduction. It also requires herita- small populations, rare alleles are more likely to ble variation in these differences. Darwin’s pre- be lost, and one allele is likely to become “fixed” sentation of natural selection included these (universal), so random genetic variation among elements but left many uncertainties, largely individuals is reduced. On the other hand, random because the sciences of genetics and ecology had genetic variation among small populations is yet to come. enhanced. Concurrently with the theoretical advances, experimental studies in laboratories and quantita- Basic Issues tive studies of populations in natural conditions confirmed all of these processes (Dobzhansky When Darwin proposed natural selection, no one 1937). It was found that individuals in a pop- understood the mechanisms of heredity. Darwin ulation often differ in survival or reproductive himself conducted extensive experimental inves- success, these differences are often heritable, tigations of heredity and selection in domestic and genetic variation depends on the sizes pigeons, but he was disturbed by his results. of populations. Hoekstra et al. (2001) and Offspring of differing parents often either com- Kingsolver et al. (2001) provide reviews of the bined the parental features or had intermediate prevalence of selection in natural populations. features. Inheritance in this way, by blending of The theoretical study of natural selection and parental traits, could not produce adaptation by evolution has in recent decades developed great natural selection. Populations would instead con- sophistication in exploring the manifold complex- verge on an overall average and then no longer ities of population size and structure, mating sys- change. Nevertheless it was apparent that pigeons tems, social interactions, migration, and isolation. did sometimes inherit parental features without Empirical studies continue to document the rele- complete blending. Within 50 years, examples of vant processes. These studies, especially in natu- particulate inheritance and its infrequent muta- ral conditions, face challenges in verifying small tions had been thoroughly verified. The inchoate effects of selection and complex contingencies, in field of genetics had provided Darwin’s theory conjunction with randomness in finite with the requisite mechanisms for heredity and populations. These effects are just the sort that variation, in the form of genes, each with variants theoretical investigations tend to explore. (alleles). Despite these theoretical and empirical Furthermore, it became clear that all steps in advances, natural selection still has its perplexities the process of natural selection – differences in and confusions. Natural selection, those who survival and reproduction, heritabilities, and rates study it agree, results from the correlated conse- of mutation – were measurable and thus open to quences of individual variation, heredity, repro- mathematical analysis. Within the first decades of duction, survival, and competition and produces the twentieth century, the mathematical theory of adapted change in the composition of a popula- evolution by natural selection had established tion. In various contexts, these components have its basic principles. Evolution, that is, a change raised many contentious issues. Is natural selec- in the frequencies of alleles in a population of tion the result or the cause of adaptations? What organisms, depends quantitatively on a balance kinds of variation and heredity are affected between selection, mutation, and migration by natural selection? How do survival and Natural Selection 3 reproduction interact? What about cooperation as become the subject of rapidly expanding research. well as competition? Even more important, is Fifty years later, a century after the Origin, the natural selection fundamentally misleading? On molecular structure of a gene had been discov- the one hand, is it so simple that it reduces to a ered. Now, some 60 years later, many complexi- tautology and explains nothing? On the other ties of molecular genetics have been investigated, hand, is there enough complexity to explain the although challenges remain. emergence of cooperation, culture, language? Is it The basis for heredity is an organism’s even specifically a biological process? genome, strands of DNA of enormous length First, a clear definition is needed. Natural duplicated in each of its cells. Segments of this selection, along with mutation, migration, and DNA encode the amino acid sequences for several drift (randomness), produces evolution. Evolution tens of thousands of different proteins, which is a change in the genetic structure of a population compose much of each cell’s structure and regu- of organisms. In the simplest case, it is a change late its vital functions. Other segments encode a in the frequencies of alleles in the population. large variety of RNA molecules, which them- Natural selection then occurs when individuals selves (without translation into proteins) provide differ in their survival or reproduction in ways a diverse array of regulatory actions. An organ- associated with differences in their alleles. It is ism’s DNA also includes parasitic components, important to point out that natural selection does which hitchhike on the mechanisms for duplica- not result merely from differences in survival or tion or which subvert the mechanisms for transla- reproduction of individuals. It also requires heri- tion for its own purposes. All of these direct and tability of those differences. Natural selection is indirect effects of DNA provide mechanisms for thus a change in the frequencies of alleles in a heredity and opportunities for variation. population as a result of differences in the survival The genome, we now know, is not the only and reproduction of individuals that carry those way that parents can transmit their features to their alleles. It is a matter of arithmetic: in any popula- progeny (Jablonka and Lamb 2014; Robert 2009). tion, genetic variants spread when they leave more The cytoplasmic