Motoo Kimura

Motoo Kimura (1924–1994) was a Japanese theoretical population geneticist whose Neutral Theory of Molecular Evolution (1968) argued that the vast majority of evolutionary change at the molecular level is driven by the random fixation of selectively neutral mutations through genetic drift, not by natural selection. The theory was initially rejected by adaptationist biologists as inconsistent with the evidence, then gradually accepted as the dominant explanation for molecular evolution — a scientific reversal that itself illustrates how uncomfortable findings about randomness are to communities invested in purposive narratives.

Kimura's work established the molecular clock hypothesis as a testable consequence of neutral theory: if most molecular evolution is drift-driven, then substitution rates should be roughly constant over time, enabling dating of evolutionary divergences from sequence differences. The hypothesis holds well enough to be useful in practice, and deviations from it are themselves informative about where selection acts.