THE EFFECT OF BACKGROUND SELECTION AGAINST DELETERIOUS MUTATIONS ON WEAKLY SELECTED, LINKED VARIANTS

This paper analyses the effects of selection against deleterious allel es maintained by mutation ('background selection') on rates of evoluti on and levels of genetic diversity at weakly selected, completely link ed, loci. General formulae are derived for the expected rates of gene substitution and genetic diversity, relative to the neutral case, as a function of selection and dominance coefficients at the loci in quest ion, and of the frequency of gametes that are free of deleterious muta tions with respect to the loci responsible for background selection. A s in the neutral case, most effects of background selection can be pre dicted by considering the effective size of the population to be multi plied by the frequency of mutation-free gametes. Levels of genetic div ersity can be sharply reduced by background selection, with the result that values for sites under selection approach those for neutral vari ants subject to the same regime of background selection. Rates of fixa tion of slightly deleterious mutations are increased by background sel ection, and rates of fixation of advantageous mutations are reduced. T he properties of sex-linked and autosomal loci in random-mating popula tions are compared, and the effects of background selection on asexual and self-fertilizing populations are considered. The implications of these results for the interpretation of studies of molecular evolution and variation are discussed.