Modern evolutionary synthesis

The modern evolutionary synthesis crystallized in the 1930s and 1940s because three previously isolated intellectual currents finally reached confluence. For nearly forty years, biologists had lived with an uncomfortable paradox: Darwin's theory of natural selection seemed incompatible with Mendel's laws of inheritance. The synthesis resolved this tension not through new data, but by recognizing that pieces had been waiting to fit together.

By 1900, two foundational discoveries sat in separate warehouses. Darwin's Origin of Species (1859) established natural selection, but he struggled to explain how variation persisted—blending inheritance predicted variation would rapidly disappear. Mendel's 1866 experiments demonstrated hereditary factors remain discrete, but his work languished until rediscovery in 1900. Early geneticists like Hugo de Vries saw Mendelian mutations as large discontinuous jumps—saltationism contradicting Darwin's gradualism.

The adjacent possible opened when three mathematically-minded biologists—Ronald Fisher, J.B.S. Haldane, and Sewall Wright—independently developed population genetics between 1918 and 1932. Their models demonstrated Darwinian gradualism and Mendelian genetics were not just compatible but complementary. Fisher's "Fundamental Theorem of Natural Selection" (1930) showed selection acts efficiently on additive genetic variance. Wright's "adaptive landscape" metaphor visualized how populations navigate fitness peaks.

Theodosius Dobzhansky provided the crucial bridge: his 1937 Genetics and the Origin of Species united laboratory genetics with field natural history. Ernst Mayr (1942) redefined "species" as reproductively isolated populations. George Gaylord Simpson (1944) demonstrated fossil record patterns matched predictions. Julian Huxley coined "modern synthesis" in 1942. The synthesis later accommodated molecular evolution when DNA's structure was discovered.