Figure 1.—

Illustration of the hitchhiking effect on the ancestral lines of a neutral locus. The shaded area corresponds to individuals with the advantageous allele B at the selected locus in the population. Close to the selected locus, most lines are identical by descent to the originator of the sweep (line iii). Recombination (shown as dashed lines) can cause a line to escape the sweep; i.e., the originator does not belong to the ancestral line, because at some stage a recombination event causes the allele at the neutral locus to be inherited from an ancestral line that has not yet been caught by the sweep (line i). Much less likely, but still possible, is for the line to first escape but later recombine back into the path of the sweep (line ii) (after Durrett and Schweinsberg 2004).