Background selection

Background selection describes the loss of genetic diversity at a non-deleterious locus due to negative selection against linked deleterious alleles. It is one form of linked selection, where the maintenance or removal of an allele from a population is dependent upon the alleles in its linkage group. The name emphasizes the fact that the genetic background, or genomic environment, of a neutral mutation has a significant impact on whether it will be preserved (genetic hitchhiking) or purged (background selection) from a population. In some cases, the term background selection is used broadly to refer to all forms of linked selection, but most often it is used only when neutral variation is reduced due to negative selection against deleterious mutations. Background selection and all forms of linked selection contradict the assumption of the neutral theory of molecular evolution that the fixation or loss of neutral alleles is entirely stochastic, the result of genetic drift. Instead, these models predict that neutral variation is correlated with the selective pressures acting on linked non-neutral genes, that neutral traits are not necessarily oblivious to selection. Because they segregate together, non-neutral mutations linked to neutral polymorphisms result in decreased levels of genetic variation relative to predictions of neutral evolution. The reduction in neutral variation due to background selection can be modeled by an exponential function of the total mutation rate at the deleterious regions of the section of genome involved. The overall effect of background selection on genetic diversity resembles a reduction in effective population size. As a consequence, background selection has been used to explain many of the inconsistencies between classical models of neutral variation and observed studies of genetic diversity. For instance, the observation that genetic diversity is weakly correlated with population size, or not correlated at all, is called the "paradox of variation".