Modern Unified Theory of Evolution

Ernst Mayr's early interests in evolution and genetics (pp. 26-29, 45) led to his decisive contributions to the modern synthesis of the late 1930s and 1940s, when a largely unified evolutionary theory emerged. Biological evolution comprises two components-adaptive development of populations during long geological periods (anagenesis, evolution as such) and multiplication of species during relatively short periods (speciation, cladogenesis). Evolution as such and the theory of common descent were accepted by biologists within a few years of the publication of the Origin in 1859 during the first Darwinian revolution. A synthesis of modern genetics and evolution as such was accomplished when, by 1932, the mathematical geneticists R. A. Fisher, J. B. S. Haldane and S. Wright had convincingly shown that small mutations and natural selection play the main roles in the gradual process of adaptive evolution of populations through time, thereby solving one of the two major problems of evolutionary biology, the problem of anagenesis, a historical accomplishment which Mayr (1999k, 2004a) called the "Fisherian synthesis." During the so-called Evolutionary Synthesis of the period 1937-1950 (the second Darwinian revolution) the other main problems of evolutionary biology were solved or generally accepted (gradualism, speciation, and natural selection) and the processes of speciation were combined with those of adaptive evolution (Mayr 1993a). None of the mathematical geneticists had discussed the phenomenon of speciation or did so only superficially. In general, the period of the modern synthesis, when a new unified theory of evolution originated, saw a synthesis (1) between the thinking in three major biological disciplines-genetics, systematics and paleontology, (2) between an experimental-reductionist approach (genetics) and an observational-holistic approach (naturalists-systematists) and (3) between an anglophone tradition with an emphasis on mathematics and adaptation and a continental European tradition with an emphasis on populations, species, and higher taxa. The three genetical aspects (then not new insights) that were firmly and universally adopted during the evolutionary synthesis were (1) that inheritance is hard, there is no inheritance of acquired characters, (2) that inheritance is particulate, that is, the genetic contributions of the parents do not blend but remain separate, to be differently recombined in future generations, and (3) that most mutations are very small and evolution therefore is gradual. The evolutionary synthesis also led to a refutation of the three anti-Darwinian paradigms (a) the typological-saltational, (b) the teleological-orthogenetic and (c) the transformationist-lamarckian theories.

Based on his background as a naturalist-systematist in Russia during the 1920s, Dobzhansky (1937) produced a first synthesis between the views of the naturalists-systematists and the geneticists. The other "architects" of the evolutionary synthesis widened the path which Dobzhansky had blazed-Ernst Mayr (1942e, species and speciation), Julian Huxley (1942, general evolution), George Gaylord Simpson (1944, paleontology), Bernhard Rensch (1947, macroevolution), and Ledyard Stebbins (1950, botany).1 Because most mutations are very small, Th. Dobzhansky (1937) and E. Mayr (1942e) were able to demonstrate that there was no conflict between the results of the population geneticists and those of the systematists who had discussed gradual differentiation of populations and geographical speciation for a long time (although, until the early 1930s, based largely on the assumption of the Lamarckian inheritance of acquired characters). The original Darwinian paradigm of variation and selection was confirmed during the evolutionary synthesis. Between 1937 and 1950, the "architects" combined in synthetic publications the results of their own research with those of population genetics. The process of this unification of evolutionary biology is referred to as the evolutionary synthesis and the product as the synthetic evolutionary theory (Synthetic Darwinism, Junker 2004). Mayr's specific contributions to the evolutionary synthesis were his analyses of the nature of biological species and of the origin of organic diversity (speciation) making "the species problem" a central concern of evolutionary biology. Speciation and other processes in evolution are not simply a matter of genes but of populations and of species. His 1942 volume explained a large part of evolutionary theory well-known to naturalists-systematists but not to geneticists, particularly species and speciation and the role of geography in the evolution of populations and species. He discussed populations at various intermediate stages between variously differentiated subspecies and variously differentiated biological species in line with gradual Darwinian change. Mayr's work demonstrated the importance of taxonomic research for evolutionary theory.

At an international conference in Princeton, New Jersey, in January 1947 there was general agreement among the participating geneticists and naturalists-system-ists on the nature of species, the gradualness of evolution, the importance of natural selection, and the populational aspect of the gradual origin of species. A synthesis indeed had taken place, but it goes without saying that, by present standards, this synthesis was still incomplete. It did not include molecular evolution, comparative genomics, evolutionary developmental biology or phylogenetics, i.e., the actual, detailed history of life on Earth (Wilkins 2007). Some differences that remained at that time included the problem of the target of selective demands which, for the population geneticists, continued to be the gene, whereas the naturalists-systematists insisted, as had Darwin, that it was the individual organism as a whole. The individual either survives or it does not; it either reproduces successfully or it

1 It should be noted that the "Evolutionary Synthesis" was an international research program to which also several other European biologists contributed, especially E. Baur, N. Timofeeff-Ressovsky, and W. Zimmermann in Germany, S. Chetverikov and N. Dubinin in the Soviet Union, G. Teissier in France, and A. Buzzati-Traverso in Italy (Mayr 1999a; Reif et al. 2000; Junker 2004); see p. 357.

does not. The gene is never isolated and can never be selected by itself, as Mayr (e.g., 1959f) argued in his attack on "beanbag" genetics showing that it is individuals that really count. In general, there are 2 or 3 possible targets of selection. One is the gametes which are directly selected. The next one is the individual and the third one are certain types of social groups, those consisting of cooperating individuals (e.g., in early humans).

In general, a unification of biology "was not an objective in the minds of any of the architects of the synthesis during the 1930-1940 period. They were busy enough straightening out their own differences and refuting the anti-Darwinians to have time for such a far-reaching objective. It wasn't until the 1950s that most of the previous difficulties had been resolved that one could begin to think seriously about the role of evolutionary biology in the whole of biology and about the capacity of evolutionary biology to achieve a unification of the previously badly splintered biology" (Mayr 1993a: 33; see also Smocovitis 1997: 202).

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