The Great Rift Valley is a biogeographic barrier to montane birds.
In all six species examined here, we were able to differentiate populations on either side of the GRV using both genetic clustering and phylogenetic methods (Fig. 2). Based on these results we may infer that the GRV has at least acted as a moderate barrier to gene flow in montane birds, even if they have a generally higher dispersal ability than non-flying species. We found that genetic differentiation (as measured by FST) varies by an order of magnitude across species (Table 1), and isolation with migration models with varied connectivity patterns over the past ~350,000 years indicate population differentiation across the GRV was at least partly species-specific. But which species-specific factors may have driven these differences in the magnitude of population differentiation?
Dispersal ability shows a significant negative association with a measure of population genetic differentiation (Fig. 5A, Fig. 5C). It is interesting to find this significant relationship even with a small number of focal taxa. It is intuitive that dispersal ability would in general have a negative association with the ability for fragmented populations to differentiate, as increased dispersal among populations would generally lead to increased gene flow between populations. As such, different modes of dispersal show different tempos of population differentiation (Medina, Cooke, & Ord, 2018), and dispersal ability has been implicated in (1) differential rates of population differentiation in closely-related taxa (e.g., Peterman et al., 2015), and (2) differential macroevolutionary trends of speciation rates (e.g., Claramunt et al., 2011; Weeks & Claramunt, 2014). Our results in Ethiopian montane forest birds are suggestive of a relationship between dispersal ability and geographic population structure and is an intriguing line of inquiry for future study.
Population sizes show a weak negative association with population genetic differentiation (not statistically significant; Fig. 5B, Fig. 5D). This is somewhat expected, as smaller and more fragmented populations will naturally exhibit less gene flow and undergo relatively faster genetic drift. Additionally, the calculations for genetic differentiation (as measured by FST) intrinsically incorporate within-population genetic diversity, which is generally linked with long-term effective population sizes (Fig. 3A).