Study describes how two species are at different stages of diversification.
A few months ago, I wrote about the “Paradox of the Great Speciator”, which applies to species that diversify quickly while colonizing a vast geographical range (you can read the blog post or my digest for Evolution). Incipient species are expected to come into secondary contact during their rapid colonization. Next, hybridization and consequent gene flow prevent further divergence and effectively reverse the speciation process. So, how can you reconcile widespread distributions with diversification?
One possible solution to this paradox concerns the taxon cycle concept. First, a taxon colonizes a new area. As time progresses, the taxon shows a decrease in vagility (i.e. the ability to spread across the environment), eventually culminating in an endemic taxon. A recent study in the journal Ibis provides evidence for this scenario for a particular great speciator: the Kingfisher genus Todiramphus.
Darren O’Connell and his colleagues studied two species of Todiramphus Kingfisher in southern Sulawesi (Indonesia). The first species – the Collared Kingfisher (T. chloris) – is actually a species complex that comprises numerous lineages (five of which have recently been elevated to species rank). For this story, we can forget about most of these lineages and just focus on one subspecies: chloris. The second species is the Sacred Kingfisher (T. sanctus). Both species occur on the mainland of Sulawesi and the neighboring Wakatobi Islands. The main difference between these species is their migratory behavior: the Collared Kingfisher is sedentary while the Sacred Kingfisher migrates.
Analyses of mitochondrial DNA and morphology of these species revealed some interesting patterns. The Collared Kingfishers from mainland Sulawesi and from the Wakatobi Islands were genetically distinct (about 0.4% difference for the ND2-gene). Moreover, birds from the islands were significantly larger. The situation for the Sacred Kingfisher was completely different: there were no genetic or morphological differences between the mainland and the island birds.
These results could be explained by the taxon cycle outlined above. The Sacred Kingfisher represents a species early in the cycle. It still has a high level of vagility (as evidenced by its migratory behavior) and it probably colonized the Wakatobi Islands recently. The Collared Kingfisher, on the other hand, has completed the first part of the taxon cycle and consists of several diverging populations. The genetic differences between the mainland and island birds can be due to genetic drift, a founder effect or different selection pressures. The authors suspect that the latter process plays an important role here: the environment on the islands is quite different from the mainland, forcing the Collared Kingfishers to change their habitat use to a more generalist niche. Indeed, in other species, a generalist lifestyle has led to increases in body and bill size.
It would be interesting to expand this line of research to more members of the Todiramphus genus. Can we find representatives for each part of the taxon cycle?
O’Connell, D. P., Kelly, D. J., Lawless, N., Karya, A., Analuddin, K., & Marples, N. M. (2019). Diversification of a ‘great speciator’in the Wallacea region: differing responses of closely related resident and migratory kingfisher species (Aves: Alcedinidae: Todiramphus). Ibis, 161(4), 806-823.