Epigenetic mutations can both help and hinder adaptive evolution.

Epigenetic variation is being integrated into our understanding of adaptation, yet we lack models on how epigenetic mutations affect evolution that includes de novo genetic change. We model the effects of epigenetic mutations on the dynamics and endpoints of adaptive walks-a process where a series of beneficial mutations move a population towards a fitness optimum. We use an individual-based model of an asexual population, where mutational effects are drawn from Fisher's geometric model. We find cases where epigenetic mutations speed adaptation or result in populations with higher fitness. However, we also find cases where they slow adaptation or result in populations with lower fitness. The effect of epigenetic mutations on adaptive walks depends crucially on their stability and fitness effects relative to genetic mutations, with small-effect epigenetic mutations generally speeding adaptation, and epigenetic mutations with the same fitness effects as genetic mutations slowing adaptation. Our work reveals a complex relationship between epigenetic mutations and natural selection and highlights the need for empirical data.