The demographic history and mutational load of African hunter-gatherers and farmers

Interesting new article (behind paywall), The demographic history and mutational load of African hunter-gatherers and farmers, Nat Ecol Evol (2018)

Abstract (emphasis mine):

Understanding how deleterious genetic variation is distributed across human populations is of key importance in evolutionary biology and medical genetics. However, the impact of population size changes and gene flow on the corresponding mutational load remains a controversial topic. Here, we report high-coverage exomes from 300 rainforest hunter-gatherers and farmers of central Africa, whose distinct subsistence strategies are expected to have impacted their demographic pasts. Detailed demographic inference indicates that hunter-gatherers and farmers recently experienced population collapses and expansions, respectively, accompanied by increased gene flow. We show that the distribution of deleterious alleles across these populations is compatible with a similar efficacy of selection to remove deleterious variants with additive effects, and predict with simulations that their present-day additive mutation load is almost identical. For recessive mutations, although an increased load is predicted for hunter-gatherers, this increase has probably been partially counteracted by strong gene flow from expanding farmers. Collectively, our predicted and empirical observations suggest that the impact of the recent population decline of African hunter-gatherers on their mutation load has been modest and more restrained than would be expected under a fully recessive model of dominance.

“Inferred demographic models of the studied populations. a, EUR-first branching model, in which ancestors of EUR (aEUR) diverged from African populations before the divergence of the ancestors of RHG (aRHG) and AGR (aAGR). b, RHG-first branching model, in which aRHG were the first to diverge from the other groups. c, AGR-first branching model, in which aAGR were the first to diverge from the other groups. We assumed an ancient change in the size of the ancestral population of all humans (ANC). We assumed that each subsequent divergence of populations was followed by an instantaneous change in the effective population size (Ne). We also assumed that there were two epochs of migration between the following population pairs: wAGR/aAGR and wRHG/aRHG, eAGR/aAGR and eRHG/aRHG, and EUR and eAGR/aAGR. The figure labels correspond to the parameters of the model estimated by maximum likelihood and the 95% confidence intervals assessed by bootstrapping by site 100 times (Supplementary Table 4). Vertical arrow corresponds to the direction of time, from past to present, with divergence times given on the left and expressed in thousand years ago(ka). Effective population sizes (N) are given within the diagram and expressed in thousands of individuals. Bold horizontal arrows indicate an estimated parameter for the effective strength of migration 2Nm > 1, while thin horizontal arrows indicate 2Nm ≤ 1.”

See also: