They probably diverged in the Pliocene, about 3.6 million years ago.
The origin of new species is mostly linked to particular environmental conditions in the past. In my own research on the evolutionary history of geese, for example, I showed that the diversification within two genera (Anser and Branta) happened about 2 million years ago, coinciding with the glacial cycles during the Pleistocene. In fact, conditions during the Pleistocene – commonly known as the ice ages – have been called upon to explain the origin of numerous species around the world.
Let’s have a look at the situation in Central America. Here, the Isthmus of Tehuantepec represents an important geographical barrier. In 2010, Brian Barber and John Klicka found that several avian species pairs diversified across the Isthmus during the Pleistocene. They linked this diversification to the habitat fragmentation that occurred during the ice ages. Does this scenario hold for all birds in this area?
A recent study in the journal Molecular Phylogenetics and Evolution explored the evolutionary history of the Dendrortyx Wood-partridges. These birds occur in the highlands of Central America. Three species have been described: the Bearded Wood-partridge (D. barbatus), the Buffy-crowned Wood-partridge (D. leucophrys) and the Long-tailed Wood-partridge (D. macroura). Whitney Tsai and her colleagues extracted DNA from several specimens of these species and included Elegant Quail (Callipepla douglasii) as an outgroup.
The genetic analyses – based on 1516 SNPs – showed that these three species started diverging about 3.6 million years ago. This estimate predates the Pleistocene ice ages and suggests that other environmental factors have driven the evolution of Wood-partridges in Central America. The researchers think that tectonic activity in the late Miocene and early Pliocene might have played a role. But it is difficult to exclude other influences, such as climate change.
This study also highlights the importance of museum specimens. In some cases, it is impossible or unethical to collect samples, because the species under investigation are endangered or extinct. Museum specimens provide an excellent solution. The authors nicely summarize this implication in the conclusions-section:
In these particular cases, DNA from museum specimens offers the only way to assess biodiversity by leveraging the efforts of collectors over the last several hundred years. In this study, the legacy of these collectors reveals previously undescribed phylogenetic diversity in the Mesoamerican Highlands and shows that both the Pleistocene ice ages and events in the Pliocene were important to diversification of cloud forest birds.
Barber, B. R., & Klicka, J. (2010). Two pulses of diversification across the Isthmus of Tehuantepec in a montane Mexican bird fauna. Proceedings of the Royal Society B: Biological Sciences, 277(1694), 2675-2681.
Tsai, W. L., Mota-Vargas, C., Rojas-Soto, O., Bhowmik, R., Liang, E. Y., Maley, J. M., Zarza, E. & McCormack, J. E. (2019). Museum genomics reveals the speciation history of Dendrortyx wood-partridges in the Mesoamerican highlands. Molecular Phylogenetics and Evolution, 136, 29-34.