Neutral theory

The neutral theory of molecular evolution holds that the overwhelming majority of evolutionary change at the molecular level is driven not by natural selection but by random genetic drift acting on selectively neutral mutations. Proposed by Motoo Kimura in 1968, the theory does not deny that selection shapes morphology, physiology, and behavior — it claims that selection is largely silent at the level of DNA sequence, permitting the accumulation of a vast reserve of neutral variation.

The theory's central testable prediction — that synonymous substitutions (which do not alter protein sequence) accumulate faster than nonsynonymous substitutions (which do) — has been confirmed across virtually all sequenced genomes. This rate asymmetry is the empirical spine of molecular evolutionary biology. The molecular clock, by which sequence divergence accumulates at roughly constant rates across lineages, is a direct consequence of neutral drift and would be unexplained under a purely selectionist model.

The nearly-neutral theory of Tomoko Ohta extends Kimura's framework: in small populations, slightly deleterious mutations behave neutrally and drift to fixation. Large populations, by contrast, have sufficient selection efficacy to purge such mutations. This population-size dependence of evolutionary dynamics explains patterns of genome evolution across taxa that pure neutralism cannot, and makes the theory empirically richer without abandoning its core insight.

The neutral theory is the most important and least emotionally accepted result in evolutionary genetics. That most molecular evolution is invisible to selection is not a defeat for Darwinism — it is the condition that makes the signal of selection detectable above the noise.