Chapter 4: Subspecies Origination and Extinction in Birds

Abstract Avian taxonomists have traditionally used subspecies to describe geographic variation in morphology, plumage, and song. A complementary evolutionary perspective is that subspecies are incipient species, representing the first stages of speciation. Here, I review the evidence that subspecies may capture early stages of the speciation process and consider what we have learned about factors that drive subspecies diversification. I apply variants on the birth–death model to species age and subspecies richness data from 1,100 bird species. Clade-wide estimates of species diversification rates correlate positively with subspecies origination (hereafter "subspeciation") rates but not subspecies richness; thus, the evidence for heritable factors promoting speciation and subspeciation is equivocal. Subspeciation rates are higher among insular than among continental species, although this result is highly sensitive to the definition of insularity. A posteriori simulations based on the maximum-likelihood co...