Relaxed selection during a recent human expansion
Humans have colonized the planet through a series of range expansions, which deeply impacted genetic diversity in newly settled areas and potentially increased the frequency of deleterious mutations on expanding wave fronts. To test this prediction, we studied the genomic diversity of French Canadians who colonized Quebec in the 17th century. We used historical information and records from [~]4000 ascending genealogies to select individuals whose ancestors lived mostly on the colonizing wave front and individuals whose ancestors remained in the core of the settlement. Comparison of exomic diversity reveals that i) both new and low frequency variants are significantly more deleterious in front than in core individuals, ii) equally deleterious mutations are at higher frequencies in front individuals, and iii) front individuals are two times more likely to be homozygous for rare very deleterious mutations present in Europeans. These differences have emerged in the past 6-9 generations and cannot be explained by differential inbreeding, but are consistent with relaxed selection on the wave front. Modeling the evolution of rare variants allowed us to estimate their associated selection coefficients as well as front and core effective sizes. Even though range expansions had a limited impact on the overall fitness of French Canadians, they could explain the higher prevalence of recessive genetic diseases in recently settled regions. Since we show that modern human populations are experiencing differential strength of purifying selection, similar processes might have happened throughout human history, contributing to a higher mutation load in populations that have undergone spatial expansions.