THE EFFECT OF RECOMBINATION ON BACKGROUND SELECTION

An approximate equation is derived, which predicts the effect on viabi lity at a neutral locus of background selection due to a set of partly linked deleterious mutations. Random mating, multiplicative fitnesses , and sufficiently large population size that the selected loci are in mutation/selection equilibrium are assumed. Given these assumptions, the equation is valid for an arbitrary genetic map, and for an arbitra ry distribution of selection coefficients across loci. Monte Carlo com puter simulations show that the formula performs well for small popula tion sizes under a wide range of conditions, and even seems to apply w hen there are epistatic fitness interactions among the selected loci. Failure occurred only with very weak selection and tight linkage. The formula is shown to imply that weakly selected mutations are more like ly than strongly selected mutations to produce regional patterning of variability along a chromosome in response to local variation in recom bination rates. Loci at the extreme tip of a chromosome experience a s maller effect of background selection than loci closer to the centre. It is shown that background selection can produce a considerable overa ll reduction in variation in organisms with small numbers of chromosom es and short maps, such as Drosophila. Large overall effects are less likely in species with higher levels of genetic recombination, such as mammals, although local reductions in regions of reduced recombinatio n might be detectable.