The starting point was the rediscovery of Mendelian particulate genetics to account for Darwinian variation, the identification of the egrm plasm of August Weismann (1834-1914) with Mendelian heredity factors and the evolvement of the chromosome theory of the gene by Thomas H. Morgan (1866-1945). Building on Neo-Darwinism, the synthesis itself began in the 1920s. It involved the replacement of Mendelian genetics by the mathematical formulations of population genetics that deals with gene ratios in groups of organisms, their integration with Darwin’s original theory of natural selection and an emphasis on phyletic gradualism. Key figures in this respect were Sergei S. Chetverikov (1880-1959), Ronald A. Fisher (1890-1962), John B.S. Haldane (1892-1964), Sewall Wright (1889-1988) and Theodosius Dobzhansky (1900-1975), whose book Genetics and the origin of species published in 1937 can be taken as the first comprehensive outline of the synthesis. It differed from Neo-Darwinism in no longer holding to a one-to-one correspondence between genes and heritable characters, but rather many-to-one and one-to-many correspondences. Furthermore, it defined natural selection in terms of differential reproductive success without reference to phenotypic fitness ensuring individual fitness. In other respects, it left Neo-Darwinism unchanged. Like Neo-Darwinism, it treated ontogenetic development as being largely irrelevant for understanding biological evolution. This neglect of ontogenetic development led to criticisms of the Modern synthesis over the years and to the present attempt by evolutionary developmental biology to forge a new synthesis that includes it.
See Adaptation, Biological evolution, Central dogma of molecular biology, Darwinism, Developmental biology, Evolutionary developmental biology, Fisher’s theory of evolutionary mimicry, Founder effect, Genetic assimilation, Germ plasm, ‘Hopeful’ monsters, Lamarckism, Macroevolution (or horizontal evolution) and microevolution (or vertical evolution), Mendelian genetics, Natural selection, Neo-Darwinism, Phyletic gradualism and punctuated equilibrium, Population genetics, Recapitulation theory (or biogenetic law), Tautology, Theory of natural selection