Impact of Epistasis on Evolutionary Adaptation

Background Evolutionary adaptation is a process where a population increases its “fit” to the world it inhabits, and is often likened to climbing a hill or peak. While this process is trivial for fitness landscapes where each mutation provides an advantage (or disadvantage) to the organism that is independent of the fitness effect of a mutation on a different locus, the interaction between mutations (epistasis) is crucial in complex and realistic fitness landscapes. Methodology/principal findings We investigate the impact of epistasis on adaptive evolution in a multiplicative NK model, by evolving a population of asexual haploid organisms with circular binary genomes of length N , where each locus interacts with K neighbors. We use a quantitative measure of epistasis and find that epistasis is a monotonically increasing function of K. At high mutation rates selection favors epistatic interactions between mutations, whereas at low mutation rates epistatic interactions hinder adaptation, and are selected against. Higher fitness is attained when more epistatic interactions are possible, with an optimum at an intermediate amount of epistasis. Increasing epistasis transforms the fitness landscape from smooth to rugged, and it is this ruggedness that enables mutations to have a larger fitness effect compared to non-epistatic landscapes. Conclusions/Significance When the environment changes and the population finds itself at a suboptimal location of an epistatic landscape, the interactions between loci enables the population to increase fitness drastically with only a few mutational steps. Crossing valleys between peaks in the fitness landscape requires deleterious mutations, which then become essential for adaptation by interacting with subsequent mutations. Because of this sign epistasis, both deleterious and beneficial mutations contribute to adaptation, underscoring the benefit of initially deleterious mutations and epistasis in adaptive evolution.