Did they originate by vicariant events or due to extinction of related lineages?
Hybrid hummingbirds are relatively common. Gary Graves has described numerous hybrid species combinations (see here for an overview). The widespread occurrence of hybridization – and possibly gene flow – complicates the taxonomic work on hummingbirds. In some cases, such as the Visorbearers (genus Augastes), the taxonomy is even complicated without hybridization. This genus houses two species: the Hyacinth Visorbearer (A. scutatus) and the Hooded Visorbearer (A. lumachella). They are probably closely related to the genus Schistes, but this remains to be confirmed. A recent study in the journal Ibis reconstructed the evolutionary history of the Visorbearers to settle this taxonomic issue.
In addition to the mystery of the closest relative of the Visorbearers, their evolutionary history is still unclear. Two hypotheses have been put forward to explain the origin of these hummingbirds. Silva (1995) proposed that genus used to be widespread across South America, but that extinction of several ancestral lineages during the Pleistocene (less than 2 million years ago) resulted in the current two species. An alternative scenario was put forward by Vasconcelos and colleagues (2012): they envisioned that a barrier arose in Brazil, separating the two Augastes species.
To figure out which scenario is more likely, Anderson Chaves and his colleagues sequenced several mitochondrial and nuclear genes to reconstruct the evolutionary history of the Visorbearers. The analyses confirmed the idea that these hummingbirds are closely related to the genus Schistes. Moreover, the researchers managed to date the divergence events: the genera Schistes and Augastes split about 7.34 million years ago, and the two Visorbearer species went their separate ways around 3.2 million years ago.
The latter divergence time is more consistent with the vicariance-hypothesis of Vasconcelos and colleagues. The timing corresponds to the uplift of the Brazilian plateau which may have resulted in distinct climatic conditions. The ancestral lineages probably adapted to the different climates and diverged into different species.
Chaves, A. V., Vasconcelos, M. F., Freitas, G. H., & Santos, F. R. (2019). Vicariant events in the montane hummingbird genera Augastes and Schistes in South America. Ibis. Early View.
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2012. First record of Augastes scutatus for Bahia refines the location of a purported barrier promoting speciation in the Espinhaço Range. Brazil. Rev. Bras. Ornitol. 20: 443– 446., &