Today the Aldabra Atoll in the Indian Ocean is home to hundreds of species, including the Aldabra giant tortoise. But 136,000 years ago, a huge flood engulfed its islands, destroying all life in its path, including a chicken-sized bird called the Aldabra rail. Then, like a phoenix, this bird was reborn. It is now the last species of flightless bird left in the Indian Ocean.
The cause of this bizarre phenomenon was a very rare natural process called iterative evolution. Thousands of years ago, Madagascan white-throated rails (Dryolimnas cuvieri) migrated to Mauritius, Reunion, and the coral limestone islands of the Aldabra Atoll. There, in the absence of predators, they lost their ability to fly, forming a new subspecies known as the Aldabran rail (Dryolimnas cuvieri aldabranus).
Then, 136,000 years ago, the great inundation event wiped them off the atoll; without functional wings, they had no way to escape. But 100,000 years ago, an ice age caused sea levels to drop, making Aldabra habitable once more. So, the rails flew from Madagascar to set up camp on the atoll, where, in the absence of predators, they lost their ability to fly once again.
Essentially, the Madagascan species of rail managed to give rise to two different flightless subspecies in just the space of just a few thousand years. That’s pretty unusual. Scientists from the University of Portsmouth and the Natural History Museum in the UK came to this conclusion after analyzing rail fossils from both before and after the inundation event, finding that both were flightless.
“This scenario may seem surprising, but rails are known to be persistent colonizers of isolated islands and can evolve flightlessness rapidly if suitable conditions exist,” the study authors write in the Zoological Journal of the Linnaean Society. “Therefore, it is likely that the dispersal of nominate Dryolimnas from Madagascar to remote Aldabra occurred on multiple occasions, as did giant tortoises.”
The researchers note that an iguana and numerous lizards also recolonized the atoll, but most of these species were subsequently lost, likely due to the introduction of invasive black rats.
The new research not only marks the first time that iterative evolution has been recorded in rails but provides one of the best examples of the phenomenon in birds in general.
“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 recolonisation events,” explained Professor David Martill of the University of Portsmouth in a statement.
"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,” added lead researcher Dr Julian Hume of the Natural History Museum. “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."