= 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.<Evolutionary Rate at the Molecular Level by Motoo Kimura>[Nature 217, 624–626, substitution rates and neutrality argument]{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 is
$$
k=(2N\mu)\frac{1}{2N}=\mu.
$$
More 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.
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