Complexity_ A Guided Tour - Melanie Mitchell [40]
The Modern Synthesis
You would think that the dissemination of Mendel’s results would be a big boost for Darwinism, since it provided Darwin’s theory with an experimentally tested mechanism of inheritance. But for decades, Mendel’s ideas were considered to be opposed to Darwin’s. Darwin’s theory asserted that evolution, and therefore variation, is continuous (i.e., organisms can differ from one another in arbitrarily minute ways) and Mendel’s theory proposed that variation is discrete (a pea plant is either tall or dwarf, but nothing in between). Many early adherents to Mendel’s theories believed in mutation theory—a proposal that variation in organisms is due to mutations in offspring, possibly very large, which themselves drive evolution, with natural selection only a secondary mechanism for preserving (or deleting) such mutations in a population. Darwin and his early followers were completely against this idea; the cornerstones of Darwin’s theory were that individual variations must be very small, natural selection on these tiny variations is what drives evolution, and evolution is gradual. “Natura non facit saltum” (Nature does not make leaps) was Darwin’s famous dismissal of mutation theory.
After many bitter arguments between the early Darwinists and Mendelians, this false opposition was cleared up by the 1920s when it was discovered that, unlike the traits of Mendel’s pea plants, most traits in organisms are determined by many genes, each with several different alleles. The huge number of possible combinations of these many different alleles can result in seemingly continuous variation in an organism. Discrete variation in the genes of an organism can result in continuous-seeming variation in the organ-ism’s phenotype—the physical traits (e.g., height, skin color, etc.) resulting from these genes. Darwinism and Mendelism were finally recognized as being complementary, not opposed.
One reason the early Darwinists and Mendelians disagreed so strongly is that, although both sides had experimental evidence supporting their position, neither side had the appropriate conceptual framework (i.e., multiple genes controlling traits) or mathematics to understand how their respective theories fit together. A whole new set of mathematical tools had to be developed to analyze the results of Mendelian inheritance with many interacting genes operating under natural selection in a mating population. The necessary tools were developed in the 1920s and 1930s, largely as a result of the work of the mathematical biologist Ronald Fisher.
Fisher, along with Francis Galton, was a founder of the field of modern statistics. He was originally spurred by real-world problems in agriculture and animal breeding. Fisher’s work, along with that of J.B.S. Haldane and Sewall Wright, showed that Darwin’s theories were indeed compatible with Mendel’s. Moreover, the combined work of Fisher, Haldane, and Wright provided a mathematical framework—population genetics—for understanding the dynamics of alleles in an evolving population undergoing Mendelian inheritance and natural selection. This unification of Darwinism and Mendelism, along with the framework of population genetics, was later called “the Modern Synthesis.”
Fisher, Wright, and Haldane are known as the three founders of the Modern Synthesis. There were many strong disagreements among the three, particularly a bitter fight between Fisher and Wright over the relative roles of natural selection and “random genetic drift.” In the latter process, certain alleles become dominant in a population merely as a chance event. For instance, suppose that in a population of pea plants, neither the dwarf nor tall alleles really affect the fitness of the plants as a whole. Also suppose that at some point the dwarf allele, just by chance, appears in a higher fraction of plants than the tall allele. Then, if each dwarf and tall plant has about