Variable phylogeographic histories of five forest birds with populations in Upper and Lower Guinea: implications for taxonomy and evolutionary conservation

This study evaluates phylogeographic structure in selected forest bird species distributed on either side of the Dahomey Gap in West Africa to extend analyses of avian diversification across the Guineo-Congolean forest. Mitochondrial DNA sequence data were collected for five species, including four with a break in distribution across the Dahomey Gap and one with a more continuous distribution. Data for the five focal species were included in a single analysis along with appropriate outgroup taxa, allowing estimates of divergence time on a common timescale. Both phylogeographic patterns and estimated divergence times were highly variable. In two cases (Green Hylia Hylia prasina and Yellow-whiskered Greenbul Eurillas latirostris), samples from east and west of the Dahomey Gap belong to divergent mitochondrial lineages, but with substantially older divergence in Eurillas (similar to 4.1 MY). In Western Bluebill Spermophaga haematina, relatively divergent eastern and western lineages (similar to 1.1 MY) were sampled in the same locality in eastern Ghana, suggesting east to west gene flow across the gap following earlier vicariance. In Forest Robin Stiphrornis erythrothorax, birds sampled within Ghana were from two highly divergent lineages (similar to 1.9 MY), one of which is more closely related to birds from Cameroon and Equatorial Guinea, such that divergence across the Dahomey Gap was comparatively recent (similar to 0.36 MY). For both Hylia and Stiphrornis, we corroborate recent studies documenting multiple divergent lineages within each genus, some of which may represent distinct species. Consistent with previous studies, our analysis shows that the Dahomey Gap can be a significant barrier to gene flow for species restricted to forest habitats. At the same time, individual species exhibit a broad range of divergence times and phylogeographic histories of variable complexity. We discuss the taxonomic implications of our results for each focal species.