- Darwin and the first evolutionary synthesis: Its grandeur, constraints, and difficulties
- Genetics and the "black day" of Darwinism
- Population genetics, Fisher's theorem, fitness landscapes, drift, and draft
- Positive and purifying (negative) selection: Classifying the forms of selection
- Modern Synthesis
- Recommended further reading
The unification of Darwinian evolution and genetics achieved primarily in the seminal studies of Fisher, Wright, and Haldane prepared the grounds for the Modern Synthesis of evolutionary biology. The phrase itself comes from the eponymous 1942 book by Julian Huxley (Huxley, 2010), but the conceptual framework of Modern Synthesis is considered to have solidified only in 1959, during the centennial celebration of Origin. The new synthesis itself was the work of many outstanding scientists. The chief architects of Modern Synthesis were arguably experimental geneticist Theodosius Dobzhansky, zoologist Ernst Mayr, and paleontologist George Gaylord Simpson. Dobzhansky's experimental and field work with the fruit fly Drosophila melanogaster provided the vital material support to the theory of population genetics and was the first large-scale experimental validation of the concept of natural selection. Dobzhansky's book Genetics and the Origin of Species (Dobzhansky, 1951) is the principal manifesto of Modern Synthesis, in which he narrowly defined evolution as "change in the frequency of an allele within a gene pool." Dobzhansky also famously declared that nothing in biology makes sense except in the light of evolution 9 (see more about "making sense" in Appendix A). Ernst Mayr, more than any other scientist, is to be credited with an earnest and extremely influential attempt at a theoretical framework for Darwin's quest, the origin of species. Mayr formulated the so-called biological concept of species, according to which speciation occurs when two (sexual) populations are isolated from each other for a sufficiently long time to ensure irreversible genetic incompatibility (Mayr, 1963). Simpson reconstructed the most comprehensive (in his time) picture of the evolution of life based on the fossil record (Simpson, 1983). Strikingly, Simpson recognized the prevalence of stasis in the evolution of most species and the abrupt replacement of dominant species. He also introduced the concept of quantum evolution, which presaged the punctuated equilibrium concept of Stephen Jay Gould and Niles Eldredge (see Chapter 2).
The consolidation of Modern Synthesis in the 1950s was a somewhat strange process that included remarkable "hardening" (Gould's word) of the principal ideas of Darwin (Gould, 2002). Thus, the doctrine of Modern Synthesis effectively left out Wright's concept of random genetic drift and its evolutionary importance, and remains uncompromisingly pan-adaptationist. Rather similarly, Simpson himself gave up the idea of quantum evolution, so gradualism remained one of the undisputed pillars of Modern Synthesis. This "hardening" shaped Modern Synthesis as a relatively narrow, in some ways dogmatic conceptual framework.
To proceed with the further discussion of the evolution of evolutionary biology and its transformation in the age of genomics, it seems necessary to succinctly recapitulate the fundamental principles of evolution that Darwin first formulated, the first generation of evolutionary biologists then amended, and Modern Synthesis finally codified. We return to each of these crucial points throughout the book.
- Undirected, random variation is the main process that provides the material for evolution. Darwin was the first to allow chance as a major factor into the history of life, and this was arguably one of his greatest insights. Darwin also allowed a subsidiary role for directed, Lamarckian-type variation, and he tended to give these mechanisms more weight in later editions of Origin. Modern Synthesis, however, is adamant in its insistence on random mutations being the only source of evolutionarily relevant variability.
- Evolution proceeds by fixation of rare beneficial variations and elimination of deleterious variations: This is the process of natural selection that, along with random variation, is the principal driving force of evolution, according to Darwin and Modern Synthesis. Natural selection, which is obviously akin to and inspired by the "invisible hand" of the market that ruled economy according to Adam Smith, was the first mechanism of evolution ever proposed that was simple and plausible and that did not require any mysterious innate trends. As such, this was Darwin's second key insight. Sewall Wright emphasized that chance could play a substantial role in the fixation of changes during evolution rather than only in their emergence, via genetic drift that entails random fixation of neutral or even moderately deleterious changes. Population-genetic theory indicates that drift is particularly important in small populations that go through bottlenecks. Genetic draft (hitchhiking) is another form of stochastic fixation of nonbeneficial mutations. However, Modern Synthesis in its "hardened" form effectively rejected the role of stochastic processes in evolution beyond the origin of variation and adhered to a purely adaptationist (pan-adaptationist) view of evolution. This model inevitably leads to the concept of "progress," gradual improvement of "organs" during evolution. Darwin endorsed this idea as a general trend, despite his clear understanding that organisms are less than perfectly adapted, as strikingly exemplified by rudimentary organs, and despite his abhorrence of any semblance of an innate strive for perfection of the Lamarckian ilk. Modern Synthesis shuns progress as an anthropomorphic concept but nevertheless maintains that evolution, in general, proceeds from simple to complex forms.
- The beneficial changes that are fixed by natural selection are infinitesimally small (in modern parlance, the evolutionarily relevant mutations are supposed to have infinitesimally small fitness effects), so evolution occurs via the gradual accumulation of these tiny modifications. Darwin insisted on strict gradualism as an essential staple of his theory: "Natural selection can act only by the preservation and accumulation of infinitesimally small inherited modifications, each profitable to the preserved being. ... If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down." (Origin of Species, Chapter 6). Even some contemporaries of Darwin believed that was an unnecessary stricture on the theory. In particular, the early objections of Thomas Huxley are well known: Even before the publication of Origin, Huxley wrote to Darwin, "You have loaded yourself with an unnecessary difficulty in adopting Natura non facit saltum so unreservedly" (http://aleph0.clarku.edu/huxley/). Disregarding these early warnings and even Simpson's concept of quantum evolution, Modern Synthesis uncompromisingly embraced gradualism.
- An aspect of the classic evolutionary biology that is related to but also distinct from the principled gradualism is uniformitarianism (absorbed by Darwin from Lyell's geology). This is the belief that the evolutionary processes have remained essentially the same throughout the history of life.
- This key principle is logically linked to gradualism and uniformitarianism: Macroevolution (the origin of species and higher taxa), is governed by the same mechanisms as microevolution (evolution within species). Dobzhansky, with his definition of evolution as the change of allele frequencies in populations, was the chief proponent of this principle. Darwin did not use the terms microevolution and macroevolution; nevertheless, the sufficiency of intraspecies processes to explain the origin of species and, more broadly, the entire evolution of life can be considered the central Darwinian axiom (or perhaps a fundamental theorem, but one for which Darwin did not have even an inkling of the proof). It seems reasonable to speak of this principle as "generalized uniformitarianism": The processes of evolution are the same not only throughout the history of life, but also at different levels of evolutionary transformation, including major transitions. The conundrum of microevolution versus macroevolution is, in some ways, the fulcrum of evolutionary biology, so we revisit it repeatedly throughout this book.
- Evolution of life can be accurately represented by a "great tree," as emphasized by the only illustration in Origin (in Chapter 4). Darwin introduced the Tree of Life (TOL) only as a general concept and did not attempt to investigate its actual branching order. The tree was populated with actual life forms, to the best of the knowledge at the time, by the chief German follower of Darwin, Ernst Haeckel. The founders of Modern Synthesis were not particularly interested in the TOL, but they certainly embraced it as a depiction of the evolution of animals and plants that the fossil record amply supported in the twentieth century. By contrast, microbes that were increasingly recognized as major ecological agents remained effectively outside the scope of evolutionary biology.
- A corollary of the single TOL concept deserves the status of a separate principle: All extant diversity of life forms evolved from a single common ancestor (or very few ancestral forms, under Darwin's cautious formula in Chapter 14 of Origin; see Darwin, 1859). Many years later, this has been dubbed the Last Universal Common (Cellular) Ancestor (LUCA). For the architects of Modern Synthesis, the existence of LUCA was hardly in doubt, but they did not seem to consider elucidation of its nature a realistic or important scientific goal.