= Motoo Kimura
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<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>.<Evolutionary Rate at the Molecular Level by Motoo Kimura>[Nature 217, 624–626, molecular substitution-rate argument]{ref}
A simple neutral calculation makes the idea concrete. In a diploid <biological population> of $N$ individuals, $2N\mu$ neutral copies arise per generation when the mutation rate is $\mu$, and each has fixation probability $1/(2N)$. Their product is $\mu$: under these assumptions, the substitution rate equals the neutral mutation rate. This distinguishes molecular change from inevitable adaptive improvement.
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