Multiple lines of evidence point to separately evolving lineages.
Evolutionary biologists often describe islands as “natural laboratories”. Textbook examples include the evolution of Darwin’s Finches on the Galapagos Islands and honeycreepers on Hawaii. These two cases played out on oceanic islands, which are separated from continents by deep seas and have never been connected to them. Continental land-bridge islands, however, originate as part of the continental shelf but are later isolated due to rising sea levels. These types of islands generally receive less attention than their oceanic counterparts.
In a recent Zoologischer Anzeiger study, Fionn Ó Marcaigh and his colleagues studied the evolution of the Sulawesi Babbler (Pellorneum celebense) on a series of land-bridge islands associated with Sulawesi. These islands only recently separated from the mainland, so it is uncertain whether enough time has passed for populations to have diverged. The researchers used a combination of genetic, acoustic and morphological data to find out.
Four Lineages
Genetic analyses of two mitochondrial genes (ND2 and ND3) pointed to four distinct populations that correspond to birds from Central Sulawesi, Southeast Sulawesi, and two land-bridge islands (Wawonii and Kabaena). Acoustic comparisons of these populations corroborated this pattern. The researchers noted that “analysis on the seven acoustic traits had a 100% success rate in classifying birds according to our population divisions.” The morphological analyses only included individuals from Southeast Sulawesi and the Kabaena population. Nonetheless, the results revealed significant differences in several traits (see also this previous study).
Taken together, the “genetic, acoustic, and morphological data agree on multiple isolated populations, likely representing independently evolving lineages.”
Understory Birds
This study nicely shows that babblers on the land-bridge islands Kabaena and Wawonii have diverged from their mainland relatives. They are not only genetically distinct, but also show acoustic and morphological differences. These patterns are rather surprising given that these islands recently split from the mainland (within the last 10,000 years) and are still relatively close to Sulawesi (Kabaena 18 km and Wawonii 7 km).
Why was there no gene flow between the mainland and these nearby island populations? The researchers point to the understory lifestyle of the Sulawesi Babbler.
This supports the hypothesis that these understory birds are as disinclined to disperse as are non-volant organisms, with corresponding impacts on evolutionary trajectories. This is in keeping with findings from other understory babbler species, which have also diverged genetically in areas that were recently connected by land bridges
As I mentioned in another blog post on land-bridges in Southeast Asia: ecology matters!
References
Marcaigh, F. O., Kelly, D. J., O’Connell, D. P., Dunleavy, D., Clark, A., Lawless, N., Karya, A., Analuddin, K. & Marples, N. M. (2021). Evolution in the understorey: the Sulawesi babbler Pellorneum celebense (Passeriformes: Pellorneidae) has diverged rapidly on land-bridge islands in the Wallacean biodiversity hotspot. Zoologischer Anzeiger, 293, 314-325.
Featured image: Sulawesi Babbler (Pellorneum celebense) © Trinity College Dublin
Avian Hybrids 