Population differentiation and connectivity histories
We used MSMC-IM to interpret general patterns of population differentiation and connectivity for populations separated by the GRV in each of the six focal taxa. We found that a majority of the taxa exhibited a single period of population differentiation in the past 250,000 years followed by isolation (Fig. 4). Two species showed variations of this pattern: the Abyssinian Catbird and the Abyssinian Thrush. The isolation with migration model for the Abyssinian Catbird indicated a prolonged period of population differentiation with ongoing connectivity between populations (Fig. 4). For the Abyssinian Thrush, the model implied an initial period of differentiation followed by several bouts of connectivity (i.e., gene flow) through time (Fig. 4). This pattern in the Abyssinian Thrush is consistent with very low FST estimates (Table 1), a very shallow MRCA in the phylogenomic results (Fig. 2), and shared mtDNA haplotypes among populations on either side of the GRV (Fig. 2).
Because of caveats associated with a lack of knowledge about and estimating generation times in tropical montane birds (see next section of RESULTS), we refrain from interpreting exact divergence times, but rather interpret the general trends of population differentiation and connectivity through time. For example, four of the species (Rüppell’s Robin-chat, Brown-rumped Seedeater, Abyssinian Slaty Flycatcher, Ethiopian White-eye) exhibited a single period of population differentiation followed by isolation (Fig. 4). If we strictly interpret the time periods of these differentiation events with assumed generation times, we would interpret that these species generally exhibit asynchronous diversification. However, with any error in our generation time assumptions, these periods of differentiation could potentially by synchronous. Because of these issues, we only discuss the general trends of population differentiation and connectivity through time.