Complexity_ A Guided Tour - Melanie Mitchell [41]
Wright believed that random genetic drift played a significant role in evolutionary change and the origin of new species, whereas in Fisher’s view, drift played only an insignificant role at best.
These are both reasonable and interesting speculations. One would think that Fisher and Wright would have had lots of heated but friendly discussions about it over beer (that is, when the Briton, Fisher, and the American, Wright, were on the same continent). However, what started as a very productive and stimulating interchange between them ended up with Fisher and Wright each publishing papers that offended the other, to the point that communication between them basically ended by 1934. The debate over the respective roles of natural selection versus drift was almost as bitter as the earlier one between the Mendelians and the Darwinists—ironic, since it was largely the work of Fisher and Wright that showed that these two sides actually need not have disagreed.
The Modern Synthesis was further developed in the 1930s and 1940s and was solidified into a set of principles of evolution that were almost universally accepted by biologists for the following fifty years:
Natural selection is the major mechanism of evolutionary change and adaptation.
Evolution is a gradual process, occurring via natural selection on very small random variations in individuals. Variation of this sort is highly abundant in populations and is not biased in any direction (e.g., it does not intrinsically lead to “improvement,” as believed by Lamarck). The source of individual variation is random genetic mutations and recombinations.
Macroscale phenomena, such as the origin of new species, can be explained by the microscopic process of gene variation and natural selection.
The original architects of the Modern Synthesis believed they had solved the major problems of explaining evolution, even though they still did not know the molecular basis of genes or by what mechanism variation arises. As the evolutionist Ian Tattersall relates, “Nobody could ever again look at the evolutionary process without very consciously standing on the edifice of the Synthesis. And this edifice was not only one of magnificent elegance and persuasiveness; it had also brought together practitioners of all the major branches of organismic biology, ending decades of infighting, mutual incomprehension, and wasted energies.”
Challenges to the Modern Synthesis
Serious challenges to the validity of the Modern Synthesis began brewing in the 1960s and 1970s. Perhaps the most prominent of the challengers were paleontologists Stephen Jay Gould and Niles Eldredge, who pointed out some discrepancies between what the Modern Synthesis predicted and what the actual fossil record showed. Gould went on to be simultaneously the best-known proponent and expositor of Darwinian evolution (through his many books and articles for nonscientists) and the most vociferous critic of the tenets of the Synthesis.
One major discrepancy is the prediction of the Modern Synthesis for gradual change in the morphology of organisms as compared with what Gould, Eldredge, and others claimed was the actual pattern in the fossil record: long periods of no change in the morphology of organisms (and no new species emerging) punctuated by (relatively) short periods of large change in morphology, resulting in the emergence of new species. This pattern became labeled punctuated equilibria. Others defended the Modern Synthesis, asserting that the fossil record was too incomplete for scientists to make