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).