Most white-eye species have been found on islands, but what about the diversity on the mainland?
When I say white-eyes, you say islands (if you are an ornithologist). About 90 percent of described white-eye species – the bird family Zosteropidae – occurs on islands. This bias is also apparent on the Avian Hybrids blog: all the papers about white-eyes that I covered took place on islands, such as the interactions between Solomons white-eye (Zosterops kulambangrae) and Kolombangara white-eye (Z. murphyi) on Kolombangara Island (see here) and the evolution of the Reunion grey white-eye (Z. borbonicus) on the small island of Reunion (see here). Could this focus on islands distort our perspective on these small passerines? What about the species diversity on the mainland? A recent study in the journal Molecular Phylogenetics and Evolution explored the diversity of white-eye species on the African mainland.
A Single Colonization Event
Frederico Martins and his colleagues collected genetic material from the 14 white-eye species and 18 subspecies that are currently recognized on the African mainland. Comparing these specimens with species from Asia revealed that the African mainland was colonized about 1.3 million years ago. After this single colonization event, the white-eyes spread to different African oceanic islands (for example, in the Gulf of Guinea) and several ecological sky-islands in the mountains. There, they diversified into a range of new species and subspecies. This begs the question: how many species are there on the African mainland?
A species delimitation analysis based on mitochondrial DNA indicated that several taxa should be elevated to species level, resulting in 27 African white-eye species (remember, we started with 14). However, the researchers realize that this analysis relies on just one molecular marker. Clearly, there is more to a species than mitochondrial DNA (see this blog post on species concepts), indicating that more detailed studies are needed to describe all the white-eye species on the African continent. Nonetheless, this study shows that we are probably underestimating the diversity of white-eye species on the mainland.
Martins, F. C., Cox, S. C., Irestedt, M., Prŷs-Jones, R. P., & Day, J. J. (2020). A comprehensive molecular phylogeny of Afrotropical white-eyes (Aves: Zosteropidae) highlights prior underestimation of mainland diversity and complex colonisation history. Molecular Phylogenetics and Evolution, 149, 106843.