Figure 3. The benefits of studying polymorphisms in many populations, rather than within a single population. Three populations (A, B, and C) diverge from an ancestral population, D. The genealogy of a single region is shown (slanted lines) with mutations in the region denoted by orange slashes. Additional lineages sampled in population A are likely to coalesce recently with other lineages (for example, the red clade in population A ) and, therefore, carry few mutations that have not already been observed in the sample. In comparison, the same number of lineages sampled from a second population are likely to carry additional independent polymorphisms (for example, the red lineages in population B). If the selective pressures at the locus in populations A and B are similar, then the SFS in the two populations should be similar, and the additional lineages in B can provide additional information about the SFS. For example, if the demographic histories and selective pressures at the locus are identical in populations A and B, and if the samples from populations A and B are sufficiently diverged, then a sample of K lineages from each population, A and B, will contain double the number of independent polymorphisms that are observed in a sample of K lineages from population A alone, providing double the number of mutations that can be used to estimate the SFS.