Washington: The rise from extinction of a flightless bird species in the Indian Ocean was possible due to a rare evolutionary process called 'Iterative Evolution' says a new report. The white-throated rail, a chicken-sized bird, indigenous to Madagascar in the south-western Indian Ocean is presently found in the isolated island of Aldabra.
Available fossil evidence suggests that the bird lived on the island thousands of years ago. In the absence of predators and just like the Dodo of Mauritius, the rails evolved so that they lost the ability to fly and they could not fly to higher ground. They became extinct as the island was inundated.
A study published in the latest issue of the Zoological Journal of the Linnean Society researched the effects of changing sea levels on extinction and re-colonisation events. The research found that on two occasions, separated by tens of thousands of years, the rail species were able to successfully colonise on Aldabra and subsequently became flightless on both occasions.
The last surviving colony of flightless rails is still found on the island today. They are persistent colonisers of isolated islands, who would have frequent population explosions and migrate in great numbers from Madagascar. This is the first time that iterative evolution (the repeated evolution of similar or parallel structures from the same ancestor but at different times) has been seen in rails and is one of the most significant in bird records.
Many of those that went north or south drowned in the expanse of ocean and those that went west landed in Africa, where predators ate them. Of those that went east, some landed on the many ocean islands such as Mauritius, Reunion and Aldabra, the last-named is a ring-shaped coral atoll that formed around 400,000 years ago. With the absence of predators on the atoll, and just like the Dodo of Mauritius, the rails evolved so that they lost the ability to fly.
However, Aldabra disappeared when it was completely covered by the sea during a major inundation event around 136,000 years ago, wiping out all fauna and flora including the flightless rail. Researchers studied fossil evidence from 100,000 years ago when sea-levels fell during the subsequent Ice Age and the atoll was re-colonised by flightless rails. They compared bones of a fossilised rail from before the inundation event with bones from a rail after the inundation event.
They found that the wing bone showed an advanced state of flightlessness and the ankle bones showed distinct properties that it was evolving toward flightlessness. This indicates that one species from Madagascar gave rise to two different species of flightless rail on Aldabra in the space of a few thousand years.
Lead researcher Dr Julian Hume, said, "These unique fossils provide irrefutable evidence that a member of the rail family colonised the atoll, most likely from Madagascar, and became flightless independently on each occasion. Fossil evidence presented here is unique for rails, and epitomises the ability of these birds to successfully colonise isolated islands and evolve flightlessness on multiple occasions."
Co-author Professor David Martill, said, "We know of no other example in rails, or of birds in general, that demonstrates this phenomenon so evidently. Only on Aldabra, which has the oldest palaeontological record of any oceanic island within the Indian Ocean region, is fossil evidence available that demonstrates the effects of changing sea levels on extinction and re-colonisation events.
