= Neutral evolution
<Neutral evolution> is change in inherited variants whose differences do not affect <biological fitness> in the population and environment being modeled. Their <allele frequencies> can change through <genetic drift> even without a selective advantage. Neutrality concerns effects on <reproductive success>, rather than requiring that the underlying <DNA> sequences are chemically identical.
An idealized <diploid> <population (biology)> with $N$ individuals makes a useful prediction. If the neutral <mutation rate> per copy per generation is $\mu$, the <expected value> of the number of new copies is $2N\mu$. Each has <fixation probability> $1/(2N)$, so the neutral substitution rate is
$$
(2N\mu)\frac{1}{2N}=\mu.
$$
The cancellation explains why a <molecular clock> can reflect a <mutation rate> rather than population size under these assumptions. <Motoo Kimura>'s molecular argument motivated a major role for <neutral evolution>; it does not assert that every <mutation> is neutral.<Evolutionary Rate at the Molecular Level by Motoo Kimura>[Nature 217, pp. 624–626; neutral molecular substitutions and substitutional load]{ref}
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