River Permeability Predicts Population Structure in Birds

Abstract

Intra-specific population genetic differentiation is an important predictor of speciation. Understanding how geographic barriers impact the cohesion of populations is therefore critical in explaining diversification. However, quantifying barrier permeability at broad scales is difficult, as permeability is determined by a host of factors including the attributes of the barrier, ecological constraints on dispersal, and species-level differences in dispersal ability; as a result, large scale comparative empirical studies that examine the relationship between barrier permeability and population divergence are limited. We test whether river and mountain permeability predict intra-specific genetic divergence for 30 species of birds spanning 17 families across both passerines (90%) and non-passerines (10%). Using 141 cross-barrier populations, we model mitochondrial divergence as a function of barrier permeability (river width, and least-cost-distance across mountains), dispersal ability, average population size, diet, habitat, and territoriality. Surprisingly, we find no relationship between dispersal ability and divergence, and no evidence that dispersal ability mediates the effects of barrier permeability on divergence. Similarly, we find no evidence of a strong role for any of our ecological variables on divergence. Variation in mountain permeability does not predict differentiation, suggesting mountains can largely be considered similar in their effects on population genetic structure. We do find that variation in river permeability predicts genetic differentiation, suggesting that rivers vary in their impacts on gene flow, and that width may be a good indicator of river permeability despite its dynamic nature.