Sheryl Lee Tian Tong – Picking out the fluty whistle of a Sulawesi babbler (Pellorneum celebense) is easy. Spotting it is far more difficult: these shy and diminutive birds, endemic to the forests of the Indonesian island of Sulawesi, spend their days flitting about the understory, where their brown plumages allow them to blend discreetly into the environment.
The babbler's unassuming appearance poses a challenge to more than bird-watchers: taxonomists have long had difficulty separating the species, commonly found across Sulawesi and its nearby isles, into different subspecies based on visual cues. Today, there are officially four subspecies (a fifth, P. c. improbatum for populations in Southeast Sulawesi, was recognized and later abolished). But pay attention to the DNA, body size and song of a particular population on Kabaena Island, and you will find a sixth, according to a new study.
Researchers from Trinity College, Dublin, whose study was published in Zoologischer Anzeiger last month, sequenced DNA and took measurements and song recordings from dozens of babblers from Southeast Sulawesi and the surrounding land-bridge islands of Kabaena, Muna, Buton and Wawonii.
The team found that, while current taxonomy assigns all babbler populations from Central and Southeast Sulawesi, as well as the land-bridge islands, into one P. c. rufofuscum subspecies, the subspecies itself contains four independently evolving lineages. Two of these lineages, from Southeast Sulawesi and Kabaena, are even genetically distinct enough to be classified as separate subspecies, they wrote.
Tens of thousands of years ago, when Sulawesi's land-bridge islands were first separated from the mainland, they took with them existing populations of P. c. rufofuscum. The birds' understory lifestyles and reluctance to cross water reinforced their geographic isolation, and the populations began evolving independently.
"Sometimes evolution can occur on much smaller scales of time and space and can be harder to detect just by looking at the animals in question," first author Fionn O Marcaigh said. "That's especially true for babblers. It's not like they're one color in one place and a different color in another place. They're all just sort of dull brown."
Rather than rely on visual differences to distinguish new babbler subspecies, the researchers collected and analyzed genetic, morphological and acoustic data, looking for patterns consistently reflected in all three.
According to their analysis, babblers from Southeast Sulawesi, Buton and Muna are genetically and acoustically divergent from the P. c. rufofuscum population of Central Sulawesi. "We thus recommend that the subspecies [P. c. improbatum] be reinstated for babblers from Southeast Sulawesi, Buton and Muna," they wrote.
The team is also proposing a new subspecies, P. c. kabaena, to be recognized for the Kabaena population's "genetic, acoustic and morphological divergence from Southeast Sulawesi, Buton, and Muna."
Taxonomic revisions aside, the study is shedding light on the phenomenon of rapid evolution. Genetic divergence within the P. c. rufofuscum subspecies was up to one-third as much as the divergence between the Sulawesi babbler species and other distantly related species that had separated millions of years ago, the researchers found.
"The islands have only been separated for tens of thousands of years, so to get one-third as much evolution in such a short period of time was surprising," O Marcaigh said.
Apart from behavioral factors, the babblers' rapid evolution could be a result of the islands' unique geology, according to the study. "The islands with the most distinct populations were those made of a particular rock type. This ultramafic rock is full of minerals like nickel, which get into the soil and change which plants can grow, to which the birds have to adapt," O Marcaigh said.
Kabaena and Wawonii's mineral-rich soils aren't just speeding up evolution. They're also accelerating the pace of resource extraction, with companies rushing to mine nickel in these biodiversity hotspots.
Though the Sulawesi babbler is on the whole "quite common and adaptable" and is rated a species of least concern on the IUCN Red List, resource extraction could prove disastrous to distinct babbler populations concentrated on these islands, O Marcaigh said. Rarer populations of less accessible and charismatic animals, such as insects, amphibians, reptiles and more – the product of the islands' mineral-rich soils – could also vanish before scientists manage to get proper records, he said.
"The more we study biodiversity, the more we realize is out there, as species and islands that have never been examined closely can turn out to be full of surprises," O Marcaigh said. "[But] time is running out for the islands' biodiversity before we've even captured a full picture of it or understood how it's evolved."
O Marcaigh, F., Kelly, D. J., O'Connell, D. P., Dunleavy, D., Clark, A., Lawless, N., ... 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. doi:10.1016/j.jcz.2021.07.006