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

Whether adaptation is limited by the supply of beneficialmutations is a long-standing question of evolutionary genetics, which more generally relates to the determination of the adaptive substitution rate and its relationship with the effective population sizeNe. The empirical evidence so far is equivocal, with some but not all studies supporting a higher adaptive substitution rate in large-Ne than in small-Ne species. We gathered coding sequence polymorphism data and estimated the adaptive amino-acid substitution rate ω a , in 50 species from ten distant groups of animals with markedly different population mutation rateθ. We reveal the existence of a complex, timescale dependent relationship between species adaptive substitution rate and genetic diversity. On the one hand, we report a positive relationship between ω a and θ among closely related species, indicating that adaptation is indeed limited by the supply of beneficialmutations, but this was only true in relatively low-θ taxa. On the other hand, we uncovered a weak negative correlation between ω a and θ at a larger taxonomic scale. This result is consistent with predictions of Fisher9s geometrical model and suggests that the proportion of mutations that are beneficialscales negatively with species long-term Ne.