Ecology predicts levels of genetic differentiation in neotropical birds.

Despite the theoretical link between the ecology and the population genetics of species, little empirical evidence is available that corroborates the association. Here, we examined genetic variation in 40 codistributed species of lowland Neotropical rain forest birds that have populations isolated on either side of the Andes, the Amazon River, and the Madeira River. We found widely varying levels of genetic divergence among these taxa across the same biogeographic barriers. Our investigation of the extent to which ecological traits predicted the amount of cross-barrier divergence revealed a strongly significant relationship between the forest stratum at which a species forages and the level of cross-barrier genetic differentiation. Canopy species had statistically lower genetic divergence values across the Andes and the two Amazonian rivers than did understory birds. We hypothesize that the association reflects an effect of dispersal propensity, which is greater in canopy birds, on the movement of alleles among demes (i.e., migration) and, consequently, on the interdemic proportion of the genetic variance. Differences in dispersal propensity may also explain the observation that understory species contain a significantly greater number of subspecies than do canopy species. This result indicates that higher rates of diversification may occur in lineages with lower dispersal propensity.