What drives genetic and phenotypic divergence in the Red-crowned Ant tanager (Habia rubica, Aves: Cardinalidae), a polytypic species?

Aim The effects of geographic and environmental variables on patterns of genetic and phenotypic differentiation have been thoroughly studied. Ecological speciation involves reproductive isolation due to divergent natural selection that can result in a positive correlation between genetic divergence and adaptive phenotypic divergence (isolation by adaptation, IBA). If the phenotypic target of selection is unknown or not easily measured, environmental variation can be used as a proxy, expecting positive correlation between genetic and environmental distances, independent of geographic distances (isolation by environment, IBE). The null model is that the amount of gene flow between populations decreases as the geographic distance between them increases, and genetic divergence is due simply to the neutral effects of genetic drift (isolation by distance, IBD). However, since phenotypic differentiation in natural populations may be autocorrelated with geographic distance, it is often difficult to distinguish IBA from the neutral expectation of IBD. In this work, we test hypotheses of IBA, IBE, and IBD in the Red-crowned Ant tanager (Habia rubica). Location Mesoamerica (Mexico-Central America) and South America. Taxon Habia rubica (Aves: Cardinalidae). Methods We compiled genetic data, coloration, and morphometric data from specimens from collections in Mexico and the United States. We used the Multiple Matrix Regression with Randomization (MMRR) approach to evaluate the influence of geographic and environmental distances on genetic and phenotypic differentiation of H. rubica at both phylogroup and population levels. Results Our results provide strong evidence that geographic distance is the main driver of genetic variation in H. rubica. We did not find evidence that climate variation is driving population differentiation in this species across a widespread geographic region. Main conclusions Our data point to geographic isolation as the main factor structuring genetic variation within populations of H. rubica and suggest that climate is not playing a major role in genetic differentiation within this species.