Department of Biological Sciences, Louisiana State University, Baton Rouge, LA 70803, UNITED STATES OF AMERICA.
Despite the theoretical link between the ecology and the population genetics of species, little empirical evidence is available that corroborates the association. Here, I examined genetic variation in 40 co-distributed species of lowland Neotropical rainforest birds that have populations isolated on either side of the Andes, Amazon River, and Madeira River. I found widely varying levels of genetic divergence among these taxa between the same biogeographic barriers. My investigation of the extent to which ecological traits predicted the level of cross-barrier divergence revealed a significant relationship between the forest stratum at which a species forages and the level of within-population and cross-barrier genetic differentiation. Canopy species had statistically lower divergence values across the Andes and two riverine barriers than did understory birds. I hypothesize that the association reflects an effect of dispersal propensity on the geographic structuring of genetic variation, and, consequently, on the ancestral and extant effective population sizes of each species. This is the first large-scale avian comparative study to document a significant association between ecological traits of a species and its level of genetic differentiation. I examined further the contrasting genetic patterns revealed previously by comparing the range-wide mitochondrial (mtDNA) phylogeography of two canopy and two understory species of lowland Neotropical rainforest birds. All species exhibited divergence between cross-Andean populations. Unlike canopy species, understory birds were structured at smaller spatial scales, particularly across riverine barriers of the Amazon basin. Surprisingly, estimates of isolation-by-distance, a proxy for dispersal propensity, are similar within areas of endemism for all taxa suggesting levels of gene flow are comparable through contiguous habitat in canopy and understory species. Lastly, I examined the multilocus phylogeography of three previously studied species with contrasting mtDNA patterns to investigate the role of historical demography in cross-Andean divergence. Demographic estimates using an isolation-with-migration model suggest among-taxa variance in cross-Andean divergences reflects a history of staggered isolation versus a simultaneous isolating event. Nuclear sequence data reveal asymmetrical gene flow in two species marked by relatively shallow cross-Andean divergence, further evidence of differential effectiveness of the Andes as a barrier to gene flow among co-distributed taxa.