Comparative patterns of population structure.
Montane Ethiopian species share at least four biogeographic barriers
(briefly reviewed by Manthey et al., 2017), with the GRV being the most
studied to date. A diversity of taxa exhibit population or species level
differentiation on either side of the GRV, including brush-furred rats,
groove-toothed rats, Ethiopian wolf, gelada, frogs of several genera,
giant lobelia, coffee, and the species studied here (Belay & Mori,
2006; Evans et al., 2011; Freilich et al., 2016; Gottelli et al., 2004;
Kebede et al., 2007; Komarova et al., 2021; Manthey et al., 2017;
Reyes-Velasco et al., 2018; Reyes‐Velasco et al., 2018; Silvestrini et
al., 2007).
While the GRV has impacted a plethora of taxa, species’ evolutionary
responses have varied in timing and/or magnitude. For example, the
Ethiopian wolf (Canis simensis ) is a relatively good disperser
and exhibits weakly differentiated populations on either side of the
GRV; this differentiation is hypothesized to have occurred over the last
20,000 years since the Last Glacial Maximum of the Pleistocene Epoch
(Gottelli et al., 2004). In contrast, semi-arboreal frogs (GenusLeptopelis ) are relatively poor dispersers and exhibit many
distinct lineages across the Ethiopian Highlands with divergence dates
predating the Pleistocene (Reyes‐Velasco et al., 2018). Based on this
study, songbirds have diversified during Pleistocene glacial cycles,
with population differentiation and cessation of connectivity dating to
the past 50 kya to 350 kya (Fig. 4). Interestingly, population
differentiation in most taxa appears to have occurred during a pulse
Great Rift Valley volcanism that took place between 170 kya and 320 kya
(Hutchison et al., 2016).
The asynchronous and varied patterns of diversification in Ethiopian
montane taxa exemplify the complex geological and climatic history of
the region. The formation of the GRV is old (~ 20 Mya)
and ongoing (Frisch et al., 2010; Sepulchre et al., 2006), which has
provided growing and continuous elevational heterogeneity in the central
portion of the Ethiopian Highlands. During the last few million years,
the Pleistocene climatic cycles have added climatic heterogeneity to the
topographic variation; during glacial minima (e.g., contemporary
periods), the GRV was likely relatively arid and hot compared to glacial
maxima (e.g., the Last Glacial Maximum). These oscillations would have
generally resulted in elevational shifts and greater potential
geographic range in montane taxa during glacial maxima. Montane species
inhabiting the Ethiopian Highlands would have experienced these combined
geological and climatic forces; their population differentiation
throughout these periods would have likely been shaped by the age of the
lineage inhabiting the highlands (i.e., time of colonization) and
species-specific dispersal ability, niche breadth, and interpopulation
reproductive compatibility shaping whether populations underwent gene
flow during periods of relatively higher habitat connectivity.
Drivers of genomic diversity. Genomic diversity is
expected to be influenced by mutation rates, patterns of selection and
linked selection, and effective population sizes (Amos & Harwood, 1998;
Ellegren & Galtier, 2016). The largest effectors of mutation rates and
effective population sizes are life history characteristics, namely
fecundity rates (Romiguier et al., 2014). When focusing on species with
similar life history strategies—as we do here with passerine
birds—we may expect most of the variation in genomic diversity to be
shaped by patterns of long-term linked selection across the genome and
the demographic histories of populations that in turn shape effective
population sizes. We find a negative correlation between genome wide
estimates of FST and DXY, indicative of
the effects of widespread linked selection in all six taxa (Fig. S5)
(Cruickshank & Hahn, 2014), although with our limited sample sizes it
wouldn’t be practical to identify direct effects of linked selection on
genomic diversity. Overall, we find that the strongest predictors of
genomic diversity across Ethiopian montane bird populations are recent
population sizes and harmonic mean of population sizes over the past
200,000 years (Fig. 3).