Testing Simultaneous Divergence of Isolated Avian Populations in the Caucasus

Island populations, whether isolated by water or inhospitable terrestrial habitats, have been important in revealing many aspects of the evolutionary process. In this proposal, we investigate the role of ecological, behavioral, and demographic characters in divergence of isolated forest bird populations in the Caucasus; independent estimates place the origin of the Caucasus forest environment at 430,000 ybp. Specifically, we address the question of why some of our 35 widespread Palearctic forest species exhibit differentiation whereas others do not. We have found mtDNA divergences of 18 population pairs ranging from 0 to ca. 5%. We adopt a novel Bayesian approach to first test whether these populations diverged simultaneously. 

Our initial analysis suggests that we cannot reject a single diversification event. This guides our interpretation of the roles of ecological niche breadth and migratory status on population divergence. In particular, it suggests that these factors influence the degree of population differentiation but not the initial divergence itself. However, we will test this conclusion with 17 other species and 10 nuclear loci per species. If the preliminary results are upheld, it will indicate a contemporaneous origin of the forest avifauna and underscore its historical continuity. We will document the range of mtDNA and nuclear gene divergences across a common, well-dated biogeographic boundary; this will lead to confidence limits on calibrations. 

If there are multiple events, we will associate species with particular times of isolation. In either model, we will still evaluate the role of ecological breadth and migratory status on population divergence across a common boundary. Early analyses suggest that habitat specialists are less differentiated than generalists, as are sedentary (vs migratory) populations. Our study also provides 35 tests of the relationship between mtDNA and nuclear gene divergence. In all, we expect that our study will cast new light on the evolutionary divergence of populations, taxonomic status of each population, as well as characterizing the avifauna of the Caucasus, which is an area of conservation interest because of its high level of endemism, which our studies show to be greater than previously appreciated.