Is adaptation limited by mutation? A timescale dependent effect of genetic diversity on the adaptive substitution rate in animals.

Abstract

Whether adaptation is limited by the beneficial mutation supply is a long-standing question of evolutionary genetics, which is more generally related to the determination of the adaptive substitution rate and its relationship with the effective population size Ne. Empirical evidence reported so far is equivocal, with some but not all studies supporting a higher adaptive substitution rate in large-Ne than in small-Ne species.\n\nWe gathered coding sequence polymorphism data and estimated the adaptive amino-acid substitution rate {omega}a, in 50 species from ten distant groups of animals with markedly different population mutation rate {theta}. We reveal the existence of a complex, timescale dependent relationship between species adaptive substitution rate and genetic diversity. We find a positive relationship between {omega}a and {theta} among closely related species, indicating that adaptation is indeed limited by the mutation supply, but this was only true in relatively low-{theta} taxa. In contrast, we uncover a weak negative correlation between {omega}a and {theta} at a larger taxonomic scale. This result is consistent with Fishers geometrical model predictions and suggests that the proportion of beneficial mutations scales negatively with species long-term Ne.