Figure 1.

Model. The effect of an old selective sweep at time T_a on a sample of six lineages $\{a,b,c,d,e,f\}$ at a nearby neutral site. Tracing the genealogy pastward, we first observe a neutral coalescence of the b and c lineages. The second event is the selective sweep, which occurs quickly on the time scale of coalescent events. This induces what appears to be a multiple-merger coalescence of d, e, and f (as in the star-like approximation). On closer inspection, we see the stochastic frequency trajectory of the adaptive mutation (shown in red) that structures the coalescent during the sweep. Here, the a and bc lineages recombine out of the sweep, and although the events occur in rapid succession, the remaining lineages do indeed coalesce pairwise. Prior to the sweep, neutral coalescence of the remaining lineages continues until a common ancestor is found.