Evolution is an amazing process, helping life adapt to new environments and conditions – and now scientists have uncovered a rare occasion where it got a second chance. About 136,000 years ago, a flightless bird on an island in the Indian Ocean was wiped out, only to re-evolve itself back into existence tens of thousands of years later.
Species are shaped by environmental pressures, which can make evolution a predictable process at times. It's not entirely surprising that sometimes after a species dies off, a new species with some very similar characteristics might arise in the same location later on. After all, evolution is basically following a recipe – if you put similar ingredients through the same process, the end result should be similar the second time around. This reoccurrence is known as iterative evolution.
According to a new study, that's essentially what happened on the island of Aldabra, a ring-shaped coral atoll just north of Madagascar. The fossil record shows that sometime after the island formed 400,000 years ago, the white-throated rail – a bird native to Madagascar – colonized Aldabra. Since there were no natural predators, the birds soon evolved to become flightless.
But about 136,000 years ago, sea levels rose and the island was completely submerged. Since evolution needs time to work its magic, life on Aldabra – including the flightless rails – was wiped out.
Thankfully, the story doesn't end there. Around 100,000 years ago, the island re-emerged when sea levels dropped due to an ice age. White-throated rails migrated there from Madagascar and once again, evolved to become flightless – a textbook example of iterative evolution, and one of the strongest cases ever seen in birds.
The team discovered this process was at work by studying rail bones from before and after the inundation event. The wing bones demonstrated an advanced state of flightlessness, and bones from the ankles showed signs of being clearly on their way towards flightlessness.
"These unique fossils provide irrefutable evidence that a member of the rail family colonized the atoll, most likely from Madagascar, and became flightless independently on each occasion," says Julian Hume, lead researcher on the study. "Fossil evidence presented here is unique for rails, and epitomizes the ability of these birds to successfully colonize isolated islands and evolve flightlessness on multiple occasions."
The research was published in the Zoological Journal of the Linnean Society.
Source: University of Portsmouth
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