Latitudinal variation in subspecific diversification of birds

Patterns of evolution are believed to vary latitudinally, but our understanding of this variation remains limited. Here we examine how patterns of subspecific diversification vary within species of birds, specifically addressing three questions: (1) Are subspecies more numerous at lower latitudes within species, consistent with greater phenotypic differentiation at lower latitudes? (2) If there are more subspecies at lower latitudes within species, can area of breeding range explain this relationship? and (3) how do latitudinal differences in subspecies within species vary geographically across the globe? Using all species with five or more subspecies from 12 of the most diverse families of birds in the world, we found consistently more subspecies at lower latitudes across all families, both hemispheres, and all continents examined. Despite the positive influence of area on the number of subspecies within species, area did not explain the greater number of subspecies at lower latitudes within species. Global patterns of subspecies support the idea that phenotypic differentiation of populations is greater at lower latitudes within species. If subspecies density provides an index of rates of incipient speciation, then our results support evolutionary hypotheses for the latitudinal diversity gradient that invoke higher tropical speciation rates.