Two new, fast-evolving bird species have been identified on a small island in Indonesia. One is the Wakatobi White-eye, which was previously thought to be a sub-species of Zosterops White-eye, but turns out to be a species in its own right. The second is the Wangi-wangi white-eye, previously unknown to science.
Weirdly, the closest relative to the newly-found Wangi-wangi lives 3,000 kilometers away. The Wangi-wangi may thus well be the sole surviving pocket of what had once been a large, widespread population.
The discovery of the two birds was made by a team headed by Prof. Nicola Marples and was revealed Wednesday in the Zoological Journal of the Linnean Society.
Like their relatives in the lineage, the Wakatobi and the Wangi-wangi white-eyes feature a white ring around the eye, black “eyeliner” beneath the bottom eyelid and green back and wing feathers. The wangi-wangi has a green tummy, while the Wakatobi has a white one.
The Wakatobi and Wangi-Wangi belong to the Zosterops white-eye lineage, which has the distinction of being among the fastest-evolving vertebrate known, together with cichlid fish.
“The Wangi-wangi white-eye is only found on Wangi-wangi Island,” lead author Darren O’Connor told Haaretz. “The Wakatobi white-eye is found throughout the Wakatobi Islands, an archipelago of oceanic islands found off the coast of South-east Sulawesi.”
One might wonder how any new species could be lurking, given the state of global overpopulation and the advances in observation technology. Some have been known all along, just not recognized as an undescribed species.
A classic in that in-plain-sight genre is a 100-foot-tall tree in the Andes that turned out to belong to a new genus, and won the name “Incadendron.” It was less unknown than unnoticed. Another example of unnoticed creatures are little frogs so effectively camouflaged that they might as well be invisible, or animals so shy and rare that finding any of them was quite the miracle, like the naked-faced Lesula monkey discovered in the Democratic Republic of Congo.
The Sulawesi archipelago was a natural place to seek insight into the Zosterops lineage, it being “a particularly peculiar island known for its weird and wonderful plants and animals," as the researchers, from Trinity College Dublin, Halu Oleo University and Operation Wallacea, put it.
Its islands are located right on the (imaginary) boundary between Asian and Australian species. Much like Crete, for instance, Sulawesi is surrounded by deep ocean trenches and had remained isolated even when ocean levels dropped during ice ages.
Although Sulawesi has been populated since prehistory, by archaic hominins who occupied the island possibly as much as 200,000 years ago, apparently its zoology still has surprises in store.
Evolution on speed
It is an evolutionary conundrum how certain birds, which can easily flap across deep waters, colonize islands in proximity to one another and then diverge into discrete species. The Zosterops white-eyes in particular had previously been realized to have experienced extraordinarily rapid speciation. They diversified so extensively that they won the soubriquet of “the great speciator”, exhibiting some of the fastest rates of species diversification among vertebrates.
The Zosterops all evolved in the last two million years, in the Indo-Pacific region, O’Connell tells Haaretz. Today, 96 species of white-eye have been identified, spanning a vast area from Africa to the Indo-Pacific.
The Wakatobi white-eye split off from the Lemon-bellied white-eye population from the nearby continental islands of Sulawesi around half a million years ago. The origin of the Wangi-Wangi white-eye is more mysterious.
“Its closest relatives are found around 3,000 kilometers to the east in the Solomon Islands, which are relatively distantly related to the Wangi-wangi white-eye. It is probably part of an older branch of the white-eye lineage which colonized the Sulawesi region in the past, which subsequently went extinct everywhere except on Wangi-wangi Island,” O’Connell says.
Chirping to a different tune
Defining “species” is one of those things that put scientists at each other's throats. According to the prevalent definition, two species shouldn’t be able to interbreed and have viable offspring. The horse and donkey for instance are considered separate species because even though they can and will have sex, their offspring is the sterile mule.
But by that narrow definition, Neanderthals, Denisovans, us humans and heavens knows who else, are just different types within the same species. “Recent research has found that many distinct species inter-breed to some extent, blurring the lines further,” the team notes.
Prof. Nicola Marples and the team of Irish and Indonesian zoologists, who have been cataloging and analyzing animals on Sulawesi since 1999, identified the two new species using modern techniques to narrow down distinctions: they examined differences in genetics, morphology, and song.
“Our research emphasizes the important link between mating signals and speciation in birds,” O’Connell explains. “While researching the white-eyes in Sulawesi, we found that one of the new species we outline, the Wakatobi White-eye, sings significantly differently from its mainland relatives [the Lemon-bellied White-eyes]. Differences in song are particularly important as this is how birds find their mates, so if populations are singing differently they won’t breed and so stay separate.”
Marples also explains the differences in song tallied with the genetic differences they found between populations, which may help solve the great mystery of how birds can evolve so rapidly.
“If a bird population becomes isolated on an island it often evolves physically while adapting to local resources. In the case of the Wakatobi white-eye, it became smaller on the Wakatobi Islands than its mainland relatives. Smaller birds tend to sing higher pitched songs with more notes, which is what we found with the Wakatobi white-eye. Therefore the physical adaptation to the Wakatobi Islands likely led to a change in song in this population, ensuring that they would not breed with mainland relatives even if they flew across.”
Since the Wangi-wangi lives only on one tiny and heavily touristed island of 85 square kilometers, it is keenly vulnerable to habitat loss.