Motoo Kimura developed mathematical approaches to population genetics and the neutral theory of molecular evolution. His 1968 paper argued that observed molecular substitution rates would be difficult to explain if every substitution represented favorable natural selection. Many changes could instead have little effect on biological fitness and spread through genetic drift.[ref]
Evolutionary Rate at the Molecular Level by Motoo Kimura. Nature 217, 624–626, molecular substitution-rate argument
A simple neutral calculation makes the idea concrete. In a diploid biological population of individuals, neutral copies arise per generation when the mutation rate is , and each has fixation probability . Their product is : under these assumptions, the substitution rate equals the neutral mutation rate. This distinguishes molecular change from inevitable adaptive improvement.
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Motoo Kimura was a prominent Japanese evolutionary biologist known for his contributions to the field of population genetics. He is best known for proposing the **neutral theory of molecular evolution** in the 1960s. This theory suggests that the majority of genetic mutations that occur in a population are neutral, meaning they do not confer any significant advantage or disadvantage to an organism's survival or reproduction.