Unlinked background selection reduces neutral diversity more than linked background selection

Abstract

ABSTRACTBackground selection describes the reduction in neutral diversity caused by selection against deleterious alleles at other loci. It is typically assumed that the purging of deleterious alleles affects linked neutral variants, and indeed simulations typically only treat a genomic window. However, background selection at unlinked loci also depresses neutral diversity. We find that for a realistically high genome-wide deleterious mutation rate, the effects of unlinked background selection exceed those of linked background selection. This can be derived from previous approximate analytical approximations of the effects of both kinds of background selection, and we confirm it with simulations that treat multi-locus complexities in a human-like genome. Background selection reduces neutral genetic diversity by a factor that is independent of census population size. Outside of genic regions, the strength of background selection increases with the mean selection coefficient, contradicting the linked theory but in agreement with the unlinked theory. Neutral diversity within genic regions is fairly independent of the strength of selection.