Rewriting The (Under)story Of Rapid Bird Evolution

Contrary to typically long evolutionary timeframes, Sulawesi babblers have diversified surprisingly quickly on Indonesia’s islands.

AsianScientist (Sep. 6, 2021) – Tucked away beneath the tree tops in Indonesia’s forests, native birds called Sulawesi babblers have diversified unusually rapidly despite their similar appearances. The findings were published in Zoologischer Anzeiger—A Journal of Comparative Zoology.

Diverse species have come and gone in the planet’s history, with new ones diverging from existing lineages, often with surprisingly changed features. Take the case of the famed Galapagos finches, which Charles Darwin observed to come up with his theory of evolution.

With distinct beak shapes and sizes, the birds adapted to the different types of food available on the separate islands they inhabited. Such speciation happened over millions of years, driven by varying environments and the geographic distance that prevented the finches from interbreeding.

This prolonged evolutionary timescale is often the norm, but the Sulawesi babblers or Pellorneum celebense proved to be an exception. Together with international collaborators, scientists from Universitas Halu Oleo in Indonesia discovered that the birds had diversified after just a few thousand years.

Small, plump and with brown plumage, the Sulawesi babbler subspecies that inhabit five separate islands in Indonesia can hardly be distinguished. However, the birds’ DNA and songs had changed far more rapidly than expected given their short time apart.

By comparing the mitochondrial genes of Sulawesi babblers with a more distant bird species, the researchers found a seven percent difference, arising from millions of years of evolution. In just 12,000 years, however, the subspecies from the five islands had already developed about a third of the genetic variation recorded in the distant relative.

It should seem unlikely that the Sulawesi babblers would diverge so quickly. After all, the islands were near each other, separated only by shallow seas. If they chose to, the birds could easily fly and cross that short distance, with the islands even connected by land-bridges in the last 20,000 years.

Yet the diversification was likely driven less by geographic distance and more so by the naturally isolated lifestyle of the shy Sulawesi babblers. As they lived in the understory, or the layer of small trees and shrubs just above the forest floor, the birds were less inclined to travel about, preferring to forage and skulk between the foliage.

Besides highlighting rapid evolution, the results also call attention to threatened biodiversity hotspots, including Sulawesi. Some islands bore certain rocks full of nickel, which can seep into the soil and force the birds to adapt the way Darwin’s finches did. But the team noted that these minerals could attract mining operations, potentially jeopardizing these species’ homes.

“There is an urgent need to consider which areas are most important in generating this biodiversity through speciation,” the authors wrote. “Time is running out to build a full picture of the biodiversity of these islands and their evolutionary dynamics.”

The article can be found at: Ó Marcaigh et al. (2021) Evolution in the Understorey: the Sulawesi Babbler Pellorneum celebense (Passeriformes: Pellorneidae) Has Diverged Rapidly on Land-bridge Islands in the Wallacean Biodiversity Hotspot.


Source: Trinity College Dublin; Photo: Shutterstock.
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