A phylogenetic perspective on the relationships between Wagtail species and subspecies.
Ornithologists love to delineate subspecies. One differently colored feather can already trigger a response in the most extreme splitters. But are subspecific divisions always supported by genetic data? Rebecca Harris and her colleagues test this idea for a bird group that has its fair share of subspecies: the Wagtails (genus Motacilla). The paper was published in Molecular Phylogenetics and Evolution.
There are 12 species of Wagtails, distributed across the Old World. Two species complexes have fallen prey to subspecific splitters: the Western Yellow Wagtail (M. flava) consists of 13 subspecies, while the White Wagtail (M. alba) “only” comprises 9 subspecies. Some time ago I wrote about hybridization between two White Wagtail subspecies (alba and personata, read all about it here).
An African Assembly
The researchers managed to collect samples from all 12 species and constructed the first time-calibrated species tree for this genus. Impressive work.
Let’s have a look at the phylogeny. In line with previous studies, they find an African and a Eurasian group. In the African group, we encounter Cape Wagtail (M. capensis), Mountain Wagtail (M. clara), Madagascan Wagtail (M. flaviventris) and Sao Tome Shorttail (M. bocagii). The latter species was previously thought to belong to the superfamily Sylvioidea, but turned out to be a Wagtail (members of the superfamily Passeroidea). This misclassification can be forgiven: who would have thought a short-tailed bird would be a wagtail?!
Now for the Eurasian group, which is subdivided according to plumage color: a black-and-white and a yellow clade. Alongside the White Wagtail, the black-and-white group contains African Wagtail (M. aguimp), Japanese Wagtail (M. grandis) and Mekong Wagtail (M. samveasnae). We can probably add Large Pied Wagtail (M. maderaspatensis) to this group, but the researchers did not manage to obtain SNP data for this species. The yellow clade houses three species: Western Yellow Wagtail , Grey Wagtail (M. cinerea) and Citrine Wagtail (M. citreola).
The phylogenetic tree I described above is based nuclear DNA. They also constructed a tree based solely on mitochondrial DNA, which is strongly incongruent and probably does not reflect the actual species tree. For example, Western Yellow Wagtail and Citrine Wagtail form a mixed group. This could be due to rapid speciation or hybridization. More research is needed to clarify this.
Let’s return to the question I posed in the beginning: are the subspecific divisions of White Wagtail and Western Yellow Wagtail supported by genetic data? The short answer: no. The authors write that “the pronounced plumage differences among the many subspecies are not at all reflected in any of our genetic datasets. Instead, the only indications of genetic divergence are between geographical regions, each of which is home to two or more different-looking subspecies.”
So, we might be dealing with parallel evolution: different populations that develop similar plumage patterns independently. However, recent studies have shown that plumage differences can evolve rapidly in a few genomic regions without corresponding divergence in the rest of the genome (see here for a recent Avian Hybrids story on warbler coloration). Perhaps the same has happened in Wagtails.
Harris, R. B., Alström, P., Ödeen, A., & Leaché, A. D. (2018). Discordance between genomic divergence and phenotypic variation in a rapidly evolving avian genus (Motacilla). Molecular phylogenetics and evolution, 120: 183-195.