Molecular adaptation reflects taxon-specific mutational biases

A fundamental question in molecular evolution is the extent to which patterns of adaptive change are shaped by mutational biases that make some variants more likely than others to arise. Past studies provide support for important effects of mutation bias on adaptive change, but leave open the empirical question of how strongly and how broadly evolutionary patterns depend on taxon-specific mutational tendencies. To characterize this effect quantitatively, we aggregated frequency spectra of adaptive amino acid changes from 14 species, comprising over 5000 total adaptive events, primarily from adaptive laboratory evolution studies. We then paired each species-specific spectrum of adaptive changes with independent measurements of the mutation spectrum of that same species. Across the 14 species, we find considerable heterogeneity in the relative frequencies of the six possible types of single-nucleotide changes, for both the mutational and adaptive spectra. Comparing these spectra across species, we find that, for any given mutation type, the stronger the bias toward or against that type in a species’ mutation spectrum, the more enriched or depleted it tends to be in that species’ adaptive spectrum. We conclude that, for the adaptation of proteins via amino acid changes, taxon-specific evolutionary preferences are strongly responsive to taxon-specific mutational preferences over their observed range.