Comparative phylogeography of West African amphibians and reptiles

Comparative phylogeographic studies often support shared divergence times
for co-distributed species with similar life histories and habitat
specializations. During the late Holocene, West Africa experienced
aridification and the turnover of rain forest habitats into savannas.
These fragmented rain forests harbor impressive numbers of endemic and
threatened species. In this setting, populations of co-distributed rain
forest species are expected to have diverged simultaneously, whereas
divergence events for species adapted to savanna and forest-edge habitats
should be absent or idiosyncratic. We conducted a Bayesian analysis of
shared evolutionary events to test models of population divergence for 20
species of anurans (frogs) and squamates (lizards and snakes) that are
distributed across the Dahomey Gap, a climate change induced savannah
barrier responsible for fragmenting previously contiguous rain forests of
Ghana into two regions: the Togo-Volta Hills and the Southwestern Forests.
A model of asynchronous diversification is supported for anurans and
squamates, suggesting that drivers of diversification are not specifically
related to ecological and life history associations with habitat types.
Instead, the wide variability of genetic divergence histories exhibited by
these species suggests that biodiversity in this region has been shaped by
diversification events that extend beyond the Holocene. Comparisons of the
genealogical divergence index (gdi), a measure of the genetic divergence
between populations due to the combined effects of genetic isolation and
gene flow, illustrate that these populations represent a broad sampling of
the speciation continuum.