On the accumulation of deleterious mutations during range expansions

TL;DR

This paper studies how range expansions lead to the accumulation of harmful mutations, causing a persistent reduction in fitness across newly colonized areas, supported by simulations, analytical models, and human genetic data.

Contribution

It introduces a model combining simulations and analytical approximations to explain the accumulation of deleterious mutations during range expansions, extending to two-dimensional scenarios.

Findings

Deleterious mutations steadily accumulate at the expansion front.

Expansion load causes reduced fitness over large areas, not just at the front.

Model predictions align well with human genetic diversity data.

Abstract

We investigate the effect of spatial range expansions on the evolution of fitness when beneficial and deleterious mutations co-segregate. We perform individual-based simulations of a uniform linear habitat 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 thousands of generations after the expansion. Our results extend qualitatively and quantitatively to two-dimensional expansions. 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 analyze patterns of neutral and non-neutral genetic diversity in humans and find an excellent fit between theory and data.