The latitudinal gradient in species richness is a nearly universal ecological phenomenon.
Similarly, conspecific genetic diversity often increases towards the equator – usually
explained as the consequence of post-glacial range expansion or due to the shared
response of genetic diversity to processes that promote species richness. However, no
study has yet examined the relationship between latitude and within-population genetic
diversity in exclusively tropical species. We surveyed genetic variation in nine resident
bird species co-occurring in tropical lowlands between southern Mexico and western
Ecuador, where avian species richness increases with decreasing latitude. Withinpopulation
genetic variation was always highest at mid-range latitudes, and not in the
most equatorial populations. Differences in demography and gene flow across species
ranges may explain some of our observations; however, much of the pattern may be due
simply to geometric constraints. Our findings have implications for conservation
planning and for understanding how biodiversity scales from genes to communities.