Speciation as a sieve for ancestral polymorphism.

Because they are considered rare, balanced polymorphisms are often discounted as crucial constituents of genome-wide variation in sequence diversity. Despite its perceived rarity, however, long-term balancing selection can elevate genetic diversity and significantly affect observed divergence between species. Here, we discuss how ancestral balanced polymorphisms can be "sieved" by the speciation process, which sorts them unequally across descendant lineages. After speciation, ancestral balancing selection is revealed by genomic regions of high divergence between species. This signature, which resembles that of other evolutionary processes, can potentially confound genomic studies of population divergence and inferences of "islands of speciation."