Genetic study unravels the evolutionary history of an Amazilia species complex.
Hummingbirds hybridize a lot. The Avian Hybrids page of this bird order (Apodiformes) lists numerous hybrids, mostly described by Gary Graves. This high incidence of interbreeding can complicate the reconstruction of the evolutionary histories. But that doesn’t stop ornithologists from trying to figure it out. A recent study in Journal of Avian Biology describes this challenge for a Mexican species complex.
Before I introduce the hummingbirds in the species complex, you need to know more about the geographical setting of this study, namely the Mexican Transition Zone (MTZ). Several avian hybrid zones have been studied in this area, which lies between the Isthmus of Tehuantepec and a belt of volcanoes running through central Mexico. This belt – known as the Trans-Mexican Volcanic Belt – cuts Mexico into northern and southern halves (see map below).
Now for the hummingbirds that flutter around in this Mexican Transition Zone. The white-chested hummingbird complex consists of two species, each comprising two subspecies. The violet-crowned hummingbird (Amazilia violiceps) is widely distributed from the Southern USA to Southern Mexico, and holds two subspecies: violiceps and elloiti. The green-fronted hummingbird (A. viridifrons) is endemic to Mexico and also holds two subspecies: viridifrons and villadai.
A previous study reported three clusters in this species complex: a population of violiceps north of the volcanic belt, a mixture of violiceps and viridifrons south of the volcanic belt, and a population of villadai east of the Isthmus of Tehuantepec. But how did we get to this situation? That is what Flor Rodríguez-Gómez and Juan Francisco Ornelas attempted to figure out using 10 microsatellites.
The genetic analyses confirmed the results from the previous study and indicated a complicated history of ancient gene flow. The researchers compared several scenarios of divergence and gene flow. Here is the most likely scenario: first, there was a split that gave rise to the violet-crowned hummingbird (A. violiceps) and the green-fronted hummingbird (A. viridifrons). This split was probably driven by the Isthmus of Tehuantepec. Later, the green-fronted hummingbird split into the two subspecies viridifrons and villadai.
And now it gets a bit uncertain. The mixed population of viridifrons and violiceps can be the result of violiceps expanding its range and establishing a contact zone with viridifrons. Alternatively, viridifrons is a hybrid population between violiceps and villadai (if this sounds confusing, check the figure below). Either way, hybridization has been a key process in the evolutionary history of this species complex.
Rodríguez-Gómez, F. & Ornelas, J.F. (2018) Genetic structuring and secondary contact in the white-chested Amazilia hummingbird species complex. Journal of Avian Biology 49(4), jav-01536.
The paper has been added to the Apodiformes page.