Department of Biological Sciences, Louisiana State University, Baton Rouge, LA 70803, UNITED STATES OF AMERICA.
The causes and implications of differences in geographic variation across species are generally poorly understood, but comparative studies have the potential to provide better understanding of what factors predispose species to undergo population divergence and whether population divergence has lasting evolutionary impacts. Here, I examined geographic variation in birds using molecular data from across the genome. I characterized genetic diversity, estimated population history, and tested for impacts of landscape history as well as ecological traits on genetic parameters. I found evidence that diverse historical processes have led to present-day genetic variation in Neotropical bird species, including divergence, population expansion, migration, and gene flow. Genetic diversity and historical processes differed across species, and some of these differences were associated with habitat. Birds of upland forest had greater genetic diversity, higher divergence between populations, and deeper population histories than birds of floodplain forest in the Amazon. This may result from higher dispersal in floodplain species, recent population expansion in or colonization of floodplain habitats, or persistent demographic differences between habitats. I also found that rates of population divergence within species predicted rates of speciation in their ancestral lineages. This result suggests that traits that predict population divergence within species, such as their habitat associations, will impact their diversification over long evolutionary timescales.