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 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 isThe 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.^[ref]