 Neutral theory of molecular evolution

The neutral theory of molecular evolution proposes that much molecular evolutionary change arises from mutations with little or no effect on biological fitness, whose fates are governed largely by genetic drift. It concerns molecular substitutions and variation; it does not require adaptive traits or natural selection to be absent. Motoo Kimura argued in 1968 that observed molecular substitution rates were difficult to explain if all such changes were advantageous.^[ref]

The basic neutral calculation is especially revealing. In an idealized diploid biological population of size $N$, a neutral mutation rate $\mu$ per gene copy produces $2N\mu$ new copies per generation. Each has fixation probability $1/(2N)$, so the substitution rate isMore new mutations in a larger population are offset by the lower fixation chance of each one. This is a model prediction with explicit neutrality and population assumptions, not a claim that every mutation is neutral.