Epigenetic variation can promote adaptation by smoothing rugged fitness landscapes

Heritable non-genetic phenotypic variation-broadly, epigenetics-can potentially influence evolutionary outcomes as direct targets of selection or through interactions with genetic variation. While their evolutionary benefits in generating phenotypic diversity in changing environments are well-characterized, there has been relatively little consideration of how the joint influence of epigenetic changes and mutations would affect traversal of multi-peak adaptive landscapes. Here, we discover general principles for how epigenetics, by generating an epigenetic quasispecies (clusters of semi-stable phenotypes mapped to a single genotype), tends to improve adaptive outcomes of an asexual population on rugged fitness landscapes even in a constant environment. In particular, rapid epigenetic changes tend to smooth out suboptimal fitness peaks through incorporating fitness contributions of epimutations, allowing access to better adaptive outcomes. Remarkably, the average impact of epigenetics is more strongly influenced by an approximate balance between switching rates rather than the absolute rate at which switching occurs. These findings demonstrate that epigenetic changes can be influential even without having strong heritability and have a striking, yet generally invisible, beneficial role in shaping a population's adaptive trajectory.