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.