Neotropical birds show a humped distribution of within-population genetic diversity along a latitudinal transect.

TLDR: Within-population genetic variation was always highest at mid-range latitudes, and not in the most equatorial populations, which has implications for conservation planning and for understanding how biodiversity scales from genes to communities.

Abstract: 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.