Evolution

By the 1920s, Mendel's theory of heredity had been rediscovered, but the early Mendelians, such as William Bateson and Hugo de Vries, opposed Darwin's theory of gradual evolutionary change. These geneticists favored the position that evolution proceeds in large jumps, or macro-mutations, rather than the gradual, incremental changes proposed by Darwin. The dispute between the Mendelians and the Darwinians was reconciled by important theoretical work published in the early 1930s by independent population geneticists Ronald A. Fisher (1890–1962), J. B. S. Haldane (1892–1964), and Sewall Wright (1889–1988). Their classic work showed that natural selection was compatible with the laws of Mendelian inheritance.

This reconciliation gave rise to a period of prolific research in genetics as the basis of evolution. Theodosius Dobzhansky (1900–1975), a Russian emigrant to the United States, began his classic investigations of evolution in fruit fly populations, and in 1937 published a book entitled Genetics and the Origin of Species, which has been among the most influential books in modern genetics. Others, like E. B. Ford (1901–1988) and H. B. D. Kettlewell (1901–1979), helped pioneer the subject Ford called "ecological genetics." This was a fundamental concept because it brought together two formerly separate areas of investigation: ecological variation (the influence of different environmental conditions) and genetic variation (differences in genetic make-up) in natural populations.