The island form of a widespread Asian babbler has now been declared a full species, and a Chinese endemic.
Black-throated Laughingthrush Garrulax chinensis is a common and widespread timaliid babbler in South-East Asia, generally held to consist of five subspecies, inhabiting moist forests and parkland with trees from sea level to mountain treelines.
The five subspecies vary in their hues of brown, olive and black; however, the form found on the Chinese island of Hainan, G c monachus, also has black spots on its ear coverts as well as more subtle variations in plumage tone.
Clearly, its relationship with the mainland forms deserved investigation, particularly with regard to how Pleistocene glacial oscillations – the advance and retreat of ice fields, which raised and lowered sea levels to create isolated refuges for large numbers of species; this in turn frequently resulted in the continuing evolution in situ before populations were reunited when conditions changed.
A team mostly from China sequenced two mitochondrial and two nuclear genes from two mainland subspecies and the Hainan form to reveal the latter's likely evolutionary history. They found that G c monachus formed its own separate monophyletic clade – that is, it consists of one taxonomic entity and all its descendants, sharing the same characteristics. It forms a greater clade with its sister form, the nominate subspecies, on the adjacent mainland, and both are a similar distance genetically from the next subspecies along geographically, G c lochmius of Burma and Thailand.
Despite this similarity in genetic distance, the authors make a claim for full species status for the Hainan endemic form "under the general lineage concept of species". They say that chinensis satisfies the criteria of the concept, citing the absence of gene flow produced by its long-term separation from the mainland, as well as the differences in morphology; the bird is also five mutational steps from its sister form and shares none of its definitive inherited genes or haplotypes with the other currently recognised subspecies.
This lack of mixing has remained in place despite the island being joined by land bridges during its recent history. Consequently, the authors declare the Hainan Black-throated Laughingthrush a full species G monachus. No official English name has yet been put forward.
Reference
Wu, Y, Huang, J, Zhang, M, Luo, S, Zhang, Y, Lei, F, Sheldon, F H, and Zpi, F. 2012. Genetic divergence and population demography of the Hainan endemic Black-throated Laughingthrush (Aves: Timalidae, Garrulax chinensis monachus) and adjacent mainland subspecies. Molecular Phylogenetics and Evolution, In press. doi.10.1016/j.ympev.2012.07.005.
