A phylogenomic study reconstructs the evolution of the genus Turdus.
The genus Turdus contains some of the most familiar birds in the world. European readers will certainly know the Blackbird (T. merula), while most readers from North America have seen an American Robin (T. migratorius). In total, there are about 86 different Turdus species worldwide that show a variety of plumage patterns and ecological peculiarities. Despite their commonness, the evolutionary relationships between these species remain controversial. As a consequence of this phylogenetic instability, ornithologists are still not sure how these thrushes managed to spread across the globe. A recent study in the journal Proceedings of the Royal Society B resorted to genomic data in order to solve this biogeographic riddle.
Previous genetic studies attempted to unravel the evolutionary history of this genus. Johan Nylander and his colleagues, for example, found one Eurasian, three American, and three African groups. They suggested that several dispersal events occurred between Africa and South America. However, the evolutionary tree in this study contained several relationships with low statistical support, making some biogeographic inferences uncertain.
What can you do when phylogenies have low statistical support? Add more data. That is exactly what Romina Batista and her colleagues did. Based on a large genomic dataset (about 2 million base pairs), they managed to reconstruct the evolutionary history of the genus Turdus with more confidence than previous studies.
Arriving on the Antilles
The phylogenomic analyses suggest that the genus Turdus originated in Eurasia. The most basal species are two Western Palearctic species: the song thrush (T. philomelos) and mistle thrush (T. viscivorus). From Europe, ancestral thrush species colonized Africa and spread eastwards into Asia.
In contrast to previous studies, this phylogeny points to a single colonization event from Eurasia (and thus not Africa) into the New World. A closer look at the species in the evolutionary tree shows that the Antilles were colonized first. Indeed, two Caribbean species – the white-chinned thrush (T. aurantius) and the forest thrush (T. lherminieri) – are located at the base of the New World group.
From the Caribbean Islands, these thrushes consequently colonized Panama (perhaps over a land bridge) and South America. Interestingly, the genus Turdus is not the only bird group that reached South America via the Caribbean: New World orioles (genus Icterus) and screech-owls (genus Megascops) probably took the same route. And who could blame them. I wouldn’t mind a stop-over on a tropical island…
Batista, R., Olsson, U., Andermann, T., Aleixo, A., Ribas, C. C., & Antonelli, A. (2020). Phylogenomics and biogeography of the world’s thrushes (Aves, Turdus): new evidence for a more parsimonious evolutionary history. Proceedings of the Royal Society B, 287(1919), 20192400.