Genetic hitchhiking

Selection on one or more genes inevitably perturbs other genes, even when t hose genes have no direct effect on fitness. This article reviews the theor y of such genetic hitchhiking, concentrating on effects on neutral loci. Ma ynard Smith and Haigh introduced the classical case where the perturbation is due to a single favourable mutation. This is contrasted with the apparen tly distinct effects of inherited variation in fitness due to loosely linke d loci. A model of fluctuating selection is analysed which bridges these al ternative treatments. When alleles sweep between extreme frequencies at a r ate lambda, the rate of drift is increased by a factor (1 + E[1/pq]lambda/( 2(2 lambda + r))), where the recombination rate r is much smaller than the strength of selection. In spatially structured populations, the effects of any one substitution are weaker, and only cause a local increase in the fre quency of a neutral allele. This increase depends primarily on the rate of recombination relative to selection (rls), and more weakly, on the neighbou rhood size, Nb=4 pi rho sigma (2). Spatial subdivision may allow local sele ctive sweeps to occur more frequently than is indicated by the overall rate of molecular evolution. However, it seems unlikely that such sweeps can be sufficiently frequent to increase significantly the drift of neutral allel es.