FIG. 2.

Approximation to the coalescent process in a pair of populations. (a) A random genealogy under the standard coalescent process. (b) An approximation to the coalescent process in which the number of lineages at time t is the expected number of lineages. Although the number of lineages remaining from a given taxon is deterministic in our approximation, the number of interspecific coalescences that occur in some time interval Δt is random, and it depends on the approximate numbers of lineages in the two taxa.