Research on two species of a small and delicate bird found in Sri Lanka have opened up questions with far reaching implications on genes and evolution not just in the world of fauna but also the human species. Kumudini Hettiarachchi reports
The highland Ceylon White-eye
A surprisingly exciting ‘revelation’ surrounds a common bird, most often heard rather than seen, rocketing Sri Lanka to a ‘unique’ branch with regard to genes and evolution.
Small and delicate is the bird with a distinctive white ring around its eye, aptly named White-eyes of the genus Zosterops who gave Zoologist Nelum Wickramasinghe and her supervisor, evolutionary biologist and geneticist Dr. Sampath Seneviratne, an “unexpected” but “pleasant” surprise when they attempted to establish whether the two species in Sri Lanka were sisters.
The endemic highland Ceylon White-eye (Zosterops ceylonensis) flying in the mountains is a distinct species, with no sisterly links (relationship), while Sri Lanka’s lowland kind, the Oriental White-eye (Z. palpebrosus), is on the same branch as India’s Oriental White-eye, inhabiting both the highlands of the Western Ghats (Z. palpebrosus nilgiriensis) and the lowlands.
The bigger surprise that caught them unaware is – the Ceylon White-eye in the central hills is a ‘root species’ and has come from the ancestor of all White-eyes of the world. These findings have now become the basis of questions being asked about the origins of the White-eyes, whether they evolved in Southeast Asia as thought before or in South Asia on our very own soil.
The lowland Oriental White-eye. (Pix by Moditha Kodikara-arachchi)
This study also determines without doubt that the Ceylon White-eye is the oldest in the world as at present, as we know of. It is spotted flitting around at the Nuwara Eliya golf course, the Victoria Park or tea estates of the central highlands. “It is difficult not to see it,” says Dr. Seneviratne.
These passerine birds (songbirds or perching birds) are canopy-dwelling, with their upper bodies covered in olive-green feathers, yellow throats and yellow or grayish-white bellies. They hover around flowers, piercing them, tasting the sweet nectar and also feeding on buds, petals and the hapless tiny insects within them.
Sri Lanka’s lowland White-eye, commonly found here as well as in the South Asian region, is more often than not heard through its cicada-like call rather than seen, says Dr. Seneviratne.
During the study, the team debated and discussed whether their findings could be due to an odd data point? Did the Ceylon White-eye which may have flown in from Southeast Asia become isolated in the central hills while all the others of the same species became extinct or are yet to be discovered in some hidden pockets elsewhere?
Also how far apart are the Ceylon White-eye and the lowland White-eye? The time-axis was an eye-opener. The highland Ceylon White-eye appeared 1.8 million years ago and the lowland White-eye 0.5 million years ago. So the Ceylon White-eye is “very old”, more than a million years, while the lowland White-eye is “very young”.
Another interesting trace was that the lowland White-eyes of Sri Lanka are close relatives of those in Senegal in Africa which is a pointer that they would have been colonized by those from Sri Lanka and southern India. The highland Ceylon White-eye, however, is related to the Southeast Asian White-eyes.
This reveals that the two species of White-eyes in strategically-placed Sri Lanka have links with those in two different directions — Africa and Southeast Asia respectively.
In the study, birds along two transects in Sri Lanka – Colombo to Pidurutalagala in the Wet Zone and Namunukula to Bundala in the Dry Zone – and one transect in India – Kerala coast to Palani hills – had been studied.
Both Pidurutalagala at 2,400 metres above sea level and Palani hills at 2,500 metres are similar elevation-wise but the gradient of the former is 100km and the latter much smaller at 700km.
What the researchers set out to find was:
Researcher Nelum Wickramasinghe
Are the two species of White-eyes in Sri Lanka sisters or totally different? Is one or the other a sub-species of the Indian White-eye?
Did the gradient of the two mountains, Pidurutalagala which is steeper and Palani which is not so steep, have an impact on speciation?
Was speciation impacted by the smaller numbers of these birds in Sri Lanka?
The first year of study revolved around morphology (colour, shape, size and plumage); vocalization (calls and singing); and genetics through mitochondrial and nucleus DNA testing bringing about the unexpected results.
Funded by the National Research Council, the project was an “ambitious” one, and Dr. Seneviratne pays tribute to his ‘golaya’ Nelum as one of few who could have handled it.
These important findings have been published in the open-access peer-reviewed PLOS (Public Library of Science) 1 on August 9, 2017. Considered the largest journal by its number of publications, it is of high-volume as well as of high-impact and is used as a teaching tool.
Dr. Sampath Seneviratne
White-eyes, from the Old World itself, are spread across the tropical belt and were earlier believed to have had Southeast Asia as their centre of divergence, from where they took wing to South Asia including Sri Lanka and onto Africa.
This is one notion that has been dispelled with confirmation that the root species is still stuck in the hills of Sri Lanka.
“We began studying one thing but ended by stumbling onto a much bigger find which not only throws light on these birds and other animals’ evolution but also on our own (human) evolution,” says Dr. Seneviratne of the Colombo University’s Avian Evolution Node under the Faculty of Science, Department of Zoology and Environment Sciences.
The study team was “really interested” and picked the White-eyes because they are a “special” group in studies on evolution as they have the highest rate of evolution among the current vertebrates across the world. While Ms. Wickramasinghe is also from the Avian Evolution Node and India’s National Centre for Biological Sciences, Tata Institute of Fundamental Research, Bangalore, the others involved are V.V. Robin and Uma Ramakrishnan from the National Centre for Biological Sciences and Sushma Reddy from America’s Biology Department, Loyola University Chicago, Illinois.
Pointing out that White-eyes are dubbed the ‘Great Speciators’, Dr Seneviratne explains that they are the fastest evolving group in current times. To the uninitiated, he introduces ‘speciation’ as a process in evolution where new species are formed from an ancestor. The main facet of evolution or origins of any animals is speciation.
While studying speciation may be an academic exercise, it is the “very root” of all biodiversity, he says. For, biodiversity comes in the wake of many speciations and as such speciation is central to the very existence of both fauna (animals) and flora (plants).
Dealing with why the White-eyes are undergoing rapid speciation, Dr. Seneviratne refers to the “special network of genes” that they have – a mix of hydrophilic (water-loving) and hydrophobic (water-hating) genes.
“Wherever they may be when in the hydrophilic phase they freely roam all over, across seas and large water bodies to colonize the lands beyond. However, when in the hydrophobic phase, they will not stir even over a river,” he says, adding that this causes both isolation and speciation. This is also a major factor for some of them to live in isolation, stuck in tight pockets but all the while undergoing a rapid increase in speciation.
Citing the old view, Dr. Seneviratne says that the thinking is that the ancestor of the White-eyes was in Southeast Asia and in the hydrophilic phase, flew over the Pacific, colonizing along the way the hundreds of islands dotting the area. Thereafter, the White-eyes on those islands evolved into their own different species and in the hydrophobic phase dared not cross even a small river.
Humans have also followed a similar pattern of hydrophilia and hydrophobia, it is learnt, with Polynesians, in the south, travelling from New Zealand to countries as far Peru and Chile in South America, colonizing the islands along the way. The Vikings from Scandinavia did the same in the world’s north. While this would be the hydrophilic phase, the hydrophobic phase would see them establish in those countries, becoming set in the way they live within their very own cultures and speaking their very own languages.
“Humans have been great voyagers, sailing across the high seas but later becoming more sedentary, growing in population to 700 billion with much diversity,” says Dr. Seneviratne, adding that hominoids have passed that rapid phase of divergence over 5 million years. This is while the White-eye has evolved over 2 million years.
Source – 27/08/2017, The Sunday Times, See more at – http://www.sundaytimes.lk/170827/plus/white-eyes-create-quite-a-flutter-256157.html
