How does speciation unfold in these mountain ranges?
Allopatric speciation seems to be the most common model for the origin of new bird species. However, given the widespread occurrence of hybridization (as exemplified by this website), it is possible that some species arose in the face of extensive gene flow. A recent study in the journal Molecular Phylogenetics and Evolution explores this question in the Himalayan-Hengduan Mountains where four pairs of closely related species co-occur. Did they originate in allopatry or not?
Fend Dong and colleagues examined the genetic make-up of eight babbler species:
- Grey-cheeked Fulvetta (Alcippe morrisonia)
- Nepal Fulvetta (Alcippe nipalensis)
- White-browed Fulvetta (Fulvetta vinipectus)
- Grey-hooded Fulvetta (Fulvetta cinereiceps)
- Red-billed Leiothrix (Leiothrix lutea)
- Silver-eared Mesia (Leiothrix argentauris)
- Yellow-throated Fulvetta (Schoeniparus cinereus)
- Rufous-winged fulvetta (Schoeniparus castaneceps)
For each species pair – you can match the species pairs by genus name – the researchers constructed phylogenetic networks and performed Isolation-with-Migration analyses.
Both approaches pointed to an allopatric scenario for all species pairs. The networks clearly separated the species and in the Isolation-with-Migration analyses allopatric models were favored over models with gene flow. So, avian speciation in the Himalayan-Hengduan Mountains seems to be mainly allopatric.
An interesting, but not very surprising finding… But wait, there is more! The researchers also uncovered a positive relationship between divergence time and level of sympatry. In other words, species that diverged longer ago showed a higher degree of overlap in distribution. This could be because these older species have had more time to develop behavioral and ecological differences that allow sympatric coexistence. This observation fits the so-called speciation cycle as described in a recent Nature Ecology & Evolution paper by Jay McEntee and colleagues: “From onset to completion, the process is often viewed as a cycle with three stages, beginning with geographic isolation (allopatry), followed by secondary contact initiated at range edges and finally prolonged spatial coexistence in overlapping geographical ranges (sympatry).” These speciation cycles result in local accumulation of species numbers, possibly explaining some of the biodiversity hotspots on Earth.
Dong, F., Hung, C. M., & Yang, X. J. (2020). Secondary contact after allopatric divergence explains avian speciation and high species diversity in the Himalayan-Hengduan Mountains. Molecular Phylogenetics and Evolution, 143, 106671.
McEntee, J. P., Tobias, J. A., Sheard, C., & Burleigh, J. G. (2018). Tempo and timing of ecological trait divergence in bird speciation. Nature Ecology & Evolution, 2(7), 1120-1127.