Mutation interactions and adaptability
Populations evolving in constant environments exhibit declining adaptability. Understanding the basis of this pattern could reveal underlying processes determining the repeatability of evolutionary outcomes. In principle, declining adaptability can be due to a decrease in the effect size of beneficial mutations, a decrease in the rate at which they occur, or some combination of both. By evolving replay populations started from different steps along a single evolutionary trajectory we show that declining adaptability is best explained by a decrease in the size of available beneficial mutations. This pattern reflected the dominant influence of diminishing-returns genetic interactions that cause new beneficial mutations to confer smaller benefits in fitter genotypes. Genome sequencing of replay populations found that starting genotypes that were more similar to one another did not exhibit greater similarity of new beneficial mutations, supporting the view that epistasis acts globally, changing the effect but not the identity of available mutations along an adaptive trajectory. Our findings provide support for a general mechanism leading to predictability of phenotypic evolutionary trajectories.