The Red-eyed Vireo complex houses more than the five species that are currently recognized. And of course, there has been some hybridization.
The taxonomic world of birds is full of species complexes, groups of closely related species that are so similar in appearance that the boundaries between them are fuzzy. And often these ‘species’ are also interbreeding, rendering the situation even more complex. Some examples include Redpolls (genus Acanthis) and Bean Geese (Anser fabalis, see here). These species complexes provide great opportunities to study hybridization and speciation in birds.
In a recent study, published in Molecular Phylogenetics and Evolution, C.J. Battey and John Klicka focus on the Red-eyed Vireo (Vireo olivaceus) species complex. This group of passerine birds is currently divided into five species:
- Red-eyed Vireo (V. olivaceus)
- Noronha Vireo (V. gracilirostris)
- Yellow-green Vireo (V. flavoviridis)
- Black-whiskered Vireo (V. altiloquus)
- Yucatan Vireo (V. magister)
The researchers obtained material of all species, except for the Noronha Vireo, a species endemic to the Noronha Island (Brazil). A DNA analysis of the other four species led to some interesting findings. Let’s have look!
The genetic data revealed that there are more species than meets the eye. The Red-eyed Vireo consists of Northern and Southern hemisphere populations, whereas the Yellow-green Vireo is structured into Eastern and Western populations. For the Red-eyed Vireo this distinction is quite obvious and there does not seem to be any gene flow between the populations. The situation for the Yellow-green Vireo, however, is less clear. It appears that this species is currently stuck in the “species/subspecies conundrum.” More data are warranted here.
Ancient Gene Flow
Two species pairs show evidence for gene flow. The Northern population of the Red-eyed Vireo has exchanged genes with the Western population of the Yellow-green Vireo. And the Southern population of the Red-eyed Vireo has interbred with the Black-whiskered Vireo. The very low levels of introgression suggest that it probably concerns historic gene flow.
Interestingly, populations of Red-eyed Vireo on the island of Trinidad seem to be hybrids between Red-eyed Vireo and Black-whiskered Vireo. However, more extensive sampling is needed to check whether there is a hybrid zone on this island.
Speciation via Migration Loss
Finally, the Yucatan Vireo might represent an example of speciation via loss of migration. This speciation model states that a group of individuals ‘decides’ to stop migrating and settle down, while the remainder of the population continues its migratory habits. Over time, the resident and migratory populations diverge until they can be considered two separate species. Two observations support this scenario for the Yucatan Vireo: (1) its ancestors are mainly migratory and (2) its current range is close to a stopover site for the Black-whiskered Vireo, which is its closest relative.
Battey, C. J. and J. Klicka (2017). Cryptic speciation and gene flow in a migratory songbird Species Complex: Insights from the Red-Eyed Vireo (Vireo olivaceus). Molecular Phylogenetics and Evolution 113: 67-75
This paper has been added to the brand-new Vireonidae page