Motoo Kimura

Motoo Kimura (1924–1994) was a Japanese population geneticist who developed the neutral theory of molecular evolution, the claim that most evolutionary change at the molecular level is driven by genetic drift acting on neutral or nearly neutral mutations, not by natural selection.

This was not a philosophical position. It was a response to data. Early molecular biologists expected protein sequences to show signs of adaptive evolution — rapid change in functionally important regions, constraint in unimportant ones. Instead, Kimura observed that synonymous substitutions (which do not change amino acids) and nonsynonymous substitutions (which do) both occurred at rates too constant to be explained by fluctuating selection. The molecular clock ticked too steadily to be driven by adaptation.

Kimura's insight: if most observed substitutions are neutral, their rate is determined by mutation rate and effective population size, not fitness. The math is simple: the rate of neutral substitution equals the mutation rate, independent of population size. This explained the data. It also implied that most of molecular evolution is not adaptive.

The neutral theory does not claim that selection is unimportant — only that most substitutions are invisible to it. Adaptations exist, but they are rare events against a background of neutral drift.