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Department of Biology, Washington University, St. Louis, Missouri ISRAEL JOURNAL OF ECOLOGY & EVOLUTION, Vol. 55, 2009, pp. 207–214 DOI: 10.1560/IJEE.55.3.207 NATURAL SELECTION FROM DARWIN TO THE 21ST CENTURY ALAN R. TEMPLETON Department of Biology, Washington University, St. Louis, Missouri 63130, USA and Institute of Evolution and Department of Evolutionary and Environmental Biology, University of Haifa, Haifa 31905, Israel The publication of Darwin’s On The Origin of Species by Means of Natural Selection in 1859 shook the biological world not only because Darwin argued effectively for evolu- tion, an idea that many others had advocated before, but also because Darwin proposed a compelling mechanism for adaptive evolution called natural selection. Darwin argued that organisms vary in many traits. If these traits influence the ability of an individual to survive and reproduce in an environment, and if the variation can be transmitted to subsequent generations, then those traits that increase the chances for survival and re- production in the context of the environment should increase in frequency in the popula- tion, leading to adaptive evolutionary change. Concerning heritable variation, Darwin initially accepted the theory of blending inheritance, in which traits represent a mixture or average of the traits of the two parents. The underlying mechanism of blending inheri- tance, or any mode of inheritance, was unknown to Darwin, but he did argue that “Some have even imagined that natural selection induces variability, whereas it implies only the preservation of such variations as arise and are beneficial to the being under its condi- tions of life” (Darwin, 1872). Hence, Darwin initially made a strict separation between the creation of heritable variation that was random with respect to the conditions of life and the subsequent operation of natural selection upon this variation. Indeed, this was the principle distinction between Darwin’s theory of natural selection and Lamarck’s earlier proposals for adaptive evolution through acquired characteristics. Darwin’s ideas were attacked by many, and Darwin addressed these criticisms in subsequent editions of On The Origin of Species after 1859. Darwin was particularly disturbed by the attack of the Scottish engineer Fleeming Jenkin who pointed out in 1867 that when heritable variation arose in the environmentally independent fashion envisioned by Darwin, it would quickly be homogenized back to the population aver- age under blending inheritance (half of the variation would be lost every generation). Natural selection would have to be extremely strong to overcome this strong tendency towards homogenization. Darwin admitted (Darwin, 1872) that “until reading an able and valuable article in the North British Review (1867), I did not appreciate how rarely single variations, whether slight or strongly-marked, could be perpetuated.” To solve this problem, Darwin invoked in Chapter IV of his 6th edition of The Origin the idea of directed variation in addition to natural selection: “There can also be little doubt that the tendency to vary in the same manner has often been so strong that all the individu- als of the same species have been similarly modified without the aid of any form of E-mail: [email protected] Received 10 August 2009, accepted 11 September 2009. 208 A.R. TEMPleton Isr. J. Ecol. Evol. selection.” Darwin further retreated from the idea that natural selection was the sole mechanism of adaptive evolution in his 1868 work The Variation of Animals and Plants Under Domestication. In this book, Darwin presented his hypothesis of pangenesis, which would explain this tendency to vary in the same manner under a Lamarckian mode of inheritance. Thus, Darwin adopted a more pluralistic view of adaptive evolu- tion in which natural selection was the major but not exclusive mechanism of adaptive change, unlike his defenders Huxley and Wallace (Larkum, 2009). As the 19th century progressed, these alternative mechanisms of adaptive evolution gained support, lead- ing to what was described as an “eclipse of Darwinism” (i.e., strict natural selection on random variation) (Huxley, 1942) and with few biologists accepting Darwin’s views on natural selection (Mayr, 1991). Natural SELECTION IN THE 20TH Century As the criticism by Jenkin illustrates, the ability of natural selection to serve as a plausi- ble explanation for adaptive evolution depends critically upon the mechanism of inheri- tance. The beginning of the 20th century was marked by the rediscovery of Mendelian inheritance, but initially Mendelism was perceived by many as incompatible with Dar- winian natural selection (Huxley, 1942). Darwin had emphasized continuous variation and had dismissed as unimportant discontinuous “sports”, but Mendelian inheritance focused on discontinuous genetic variation. Hence, the early part of the 20th century was marked by the continued “eclipse of Darwinism” (Huxley, 1942) and in particular the rejection of Darwin’s theory of natural selection (Mayr, 1991). This eclipse ended with the publication of Fisher’s (1918) classic paper showing that continuous variation was compatible with Mendelian inheritance. Fisher, along with Wright and Haldane, soon thereafter developed the new field of theoretical population genetics, which clearly invoked natural selection working under Mendelian inheritance. However, the meaning of natural selection was now fundamentally altered by this synthesis of Darwin’s ideas with Mendelian genetics. Darwin had emphasized natural selection operating upon populations of individuals, but in the new field of population genetics natural selection was envisioned as operating upon populations of genes (gene pools). Indeed, the fundamental unit of evolutionary change became a change in the frequency of an allele or gamete type in the gene pool (Templeton, 1982, 2006). The sole determinant of the direction of evolutionary change under this genetical theory of natural selection is the difference between the average fitness of all individuals bearing a particular gamete type minus the average fitness of the entire population (Templeton, 1982, 2006). This 20th century insight represents a profound difference from Darwinian thinking. For example, Darwin fully embraced Spencer’s phrase “survival of the fittest” as a synonym for natural selection, changing the title of Chapter IV in The Origin of Species from “Natural Selection” in the first edi- tion to “Natural Selection; or The Survival of The Fittest” in his 6th edition (Darwin, 1872). “The Fittest” refers to the most fit individuals, but natural selection favors those gametes with genetic variants that result in higher-than-average fitness in their bearers. .
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