What is the first image that comes to mind when you think of evolution? Possibly a line of cartoon primates marching, slouching monkeys at one end and naked men with spears at the other. Or a branching tree diagram where each twig represents an organism, maybe with a tentative “I think” scribbled above it. Alternatively, you may have pictured an illustration of related birds from isolated islands, each showing a dramatically different bill shape adapted to a different diet. Darwin’s Galápagos finches represent a foundational influence in terms of where we tend to look for signs of evolution and what we expect these signs to look like. Our new paper, just published Open Access in Zoologischer Anzeiger: A Journal of Comparative Zoology, provides a contrasting image. We looked at the Sulawesi babbler (Pellorneum celebense), a dull brown bird that spends its time hiding in bushes on less isolated islands in Indonesia, looking pretty similar from one island to the next. Nevertheless, we found that several of its populations are quite different from one another in mitochondrial DNA, in morphology, and in song.
Sulawesi is a geologically complex island made up of different peninsulae, as seen in the above map. Off the southeast peninsula are several islands of a type known as continental land-bridge islands, surrounded by shallow seas which, over geological time, sometimes disappear and are replaced by land connections. This is in contrast to oceanic islands like the Galápagos, which have been separated from other landmasses for as long as they have existed and so have provided ample time for evolution to work in isolation. Historic naturalists named different Sulawesi babbler subspecies from Central and Southeast Sulawesi, but later work merged these together as they had similar plumage. This means that, officially, one subspecies (P. c. rufofuscum) covers a range of hundreds of kilometres over Sulawesi and the land-bridge islands, over mountains and past and present seas. To maintain a population so uniform that it all forms one subspecies, animals would need to be moving around that range quite frequently and exchanging genes. While the Sulawesi babbler would be physically able to fly these distances, it seems less taken with wanderlust than most birds. This is because babblers are understorey birds, spending so much time in bushes near the forest floor that they have been compared to mice.
This lack of dispersal, and the resulting lack of gene flow, has resulted in marked variation. Even though they all look similar to the eye, our study finds that the babblers in this one P. c. rufofuscum subspecies are up to 1/3 as genetically different from each other as they are from a totally different species, Pellorneum rostratum from Borneo. This comparison is remarkable, as the sea between Borneo and Sulawesi is so deep that the two islands have never been connected for the millions of years that Sulawesi has existed above water. The land-bridge islands, in contrast, were connected just tens of thousands of years ago, so the divergence between them must have evolved in this short amount of time. As previously discussed on EcoEvo@TCD, another paper from our research group found that babblers are more sexually dimorphic on these young islands than the mainland, hinting at changes that must have evolved quite rapidly. To this we have now added differences in DNA, song acoustics, and the morphology of the babblers’ wings, bills, and skulls. Other birds our group has looked at in the past, such as white-eyes and sunbirds, are perfectly willing to fly between Sulawesi and the land-bridge islands, and so unlike the babblers they present one uniform population across these islands.
Based on the differences we found, we have recommended that the Southeast Sulawesi babbler subspecies be reinstated, a new subspecies be named for Kabaena, and the Wawonii babblers be examined in more detail as we only had access to one DNA sequence from that island (though it did appear quite distinct). The Southeast Sulawesi subspecies was called P. c. improbatum, formerly P. c. sordidum until it was realised that there was already a subspecies named sordidum in the same genus. How babblers managed to be so offensive that multiple taxonomists named them as sordid and lacking in honesty and morals, I’m not sure.
So many years after Darwin made his pivotal observations on oceanic islands, these most isolated islands continue to draw the most attention in biogeography. But the Sulawesi babbler shows us that continental land-bridge islands can still harbour unique populations that are important if we are to understand the evolution of species. Meanwhile, on Sulawesi’s land-bridge islands the habitats that survived agriculture are soon to be strip mined, and whatever biodiversity they hold may be lost before it is ever even noticed. It’s easy to overlook brown birds that hide in the understorey, as it has been easy for biologists to overlook land-bridge islands while studying the evolution of species. But our understanding of evolution and biodiversity can only be complete if it includes these species and habitats.
It’s all a matter of where you look.
To find out more, check out our full article in Zoologischer Anzeiger, ‘Evolution in the understorey: The Sulawesi babbler Pellorneum celebense (Passeriformes: Pellorneidae) has diverged rapidly on land-bridge islands in the Wallacean biodiversity hotspot’.
Reference:
Ó Marcaigh F, Kelly DJ, O’Connell DP, Dunleavy D, Clark A, Lawless N, Karya A, Analuddin K, Marples NM (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 2021; 293: 314–325. https://doi.org/10.1016/j.jcz.2021.07.006
