On the accumulation of deleterious mutations during range expansions

TLDR

The study investigates how spatial range expansions affect fitness evolution when beneficial and deleterious mutations cosegregate. The authors use individual‑based simulations in one‑ and two‑dimensional expansions, supplemented by analytical approximations, to model mean fitness dynamics at the expansion front. Deleterious mutations accumulate steadily at the wave front, creating an expansion load that spreads across newly colonized habitats and can persist for thousands of generations, potentially explaining the excess of deleterious mutations observed in recently expanded populations such as humans; functional genetic diversity analyses in humans support these predictions.

Abstract

Abstract We investigate the effect of spatial range expansions on the evolution of fitness when beneficial and deleterious mutations cosegregate. We perform individual‐based simulations of 1 D and 2 D range expansions and complement them with analytical approximations for the evolution of mean fitness at the edge of the expansion. We find that deleterious mutations accumulate steadily on the wave front during range expansions, thus creating an expansion load . Reduced fitness due to the expansion load is not restricted to the wave front, but occurs over a large proportion of newly colonized habitats. The expansion load can persist and represent a major fraction of the total mutation load for thousands of generations after the expansion. The phenomenon of expansion load may explain growing evidence that populations that have recently expanded, including humans, show an excess of deleterious mutations. To test the predictions of our model, we analyse functional genetic diversity in humans and find patterns that are consistent with our model.