Genetic study of two neotropical bird species reveals different evolutionary responses to a common barrier.
South America is known for its diversity in bird species. But how did this multitude of species evolve? The classical explanation is geographic isolation. Several landscape features, such as Amazonian rivers and Andean mountains, break up species in different populations which consequently diverge into separate species. In reality, however, things are more complex. This is illustrated by a recent study in Molecular Ecology.
Open and dry environments can form open vegetation corridors between rainforests. In South America, for example, Amazonia and the Andean forests are isolated from the Atlantic Forest by such an open vegetation corridor. Several populations of birds are distributed east and west of this corridor. Pablo Lavinia and his colleagues investigated the influence of this corridor on the evolution of two bird species: the Large-headed Flatbill (Ramphotrigon megacephalum) and the Fawn-breasted Tanager (Pipraeidea melanonota).
The Large-headed Flatbill is a lowland, forest specialist in the bamboo understory where it forages on insects. It comprises four subspecies: one in the Atlantic Forest (megacephalum) and three in Amazonia and at the base of Andes (bolivianium, pectorale and venezuelense).
Genetic analyses of two subspecies (megacephalum and bolivianium) revealed a deep split, indicating that these populations diverged about 3.4 million years ago. How this divergence happened is not clear. Perhaps the Atlantic Forest was colonized when it was connected with the Amazonian region. A likely colonization route would have been the Chapare Buttress in present-day Bolivia which connected the Andes to the Brazilian shields. Alternatively, the continuous distribution of the Large-headed Flatbill was disrupted by the formation of the open vegetation corridor during the Plio-Pleistocene (beginning about 5 million years ago).
In any case, there is clear genetic divergence between the subspecies, which is accompanied by differences in song. The researchers found that “the disyllabic whistle of R. m. megacephalum sounds more high-pitched and slowly paced than that of R. m. bolivianum.” Moreover, a second type of vocalization – the dawn song – is only sung by bolivianum.
In contrast to the Large-headed Flatbill, the Fawn-breasted Tanager is a generalist that occurs in a variety of closed and semi-open environments where it feeds on fruits and insects. Although its movement patterns are poorly documented, it is considered a seasonal migrant. One population (subspecies melanonota) is restricted to the Atlantic Forest while another population (subspecies venezuelensis) is found in the Andes region.
Genetically, these subspecies are very similar. Demographic modelling of these populations suggest a complex history of divergence and secondary contact. Indeed, the analyses pointed to gene flow from the Andes region into the Atlantic Forest. The subspecies also differ in plumage: the brightness of the chest and rump is higher in venezuelensis than in melanonota. This difference could have been driven by divergent sexual selection.
Reconstructing the evolutionary histories of Large-headed Flatbill and Fawn-breasted Tanager indicated that these species have been affected differently by the open vegetation corridor. The researchers nicely summarize their findings:
The shallow genetic divergence and signs of gene flow between its subspecies as a result of secondary contacts, together with differentiation in plumage coloration but not in vocalizations, might be reflecting the early stages of the speciation process in P. melanonota . On the contrary, the deep genetic divergence and the consistent song differentiation between R. m. megacephalum and R. m. bolivianum suggest that these geographically isolated subspecies could actually constitute two different species.
This study illustrates that the diversification of birds in South America is more complicated than simple geographic isolation. The ecology of the species should also be taken into account (similar to this study on islands colonization).
Lavinia, P.D., Barreira, A.S., Campagna, L., Tubaro, P.L. & Lijtmaer D.A. (2019) Contrasting evolutionary histories in Neotropical birds: divergence across an environmental barrier in South America. Molecular Ecology.
This paper has been added to the Thraupidae page.