The story of the bird that went extinct and then evolved again is a fascinating one in the annals of evolutionary biology. Only a handful of species are known to have gone extinct in the wild and then re-evolved in a process called iterative evolution or iterative speciation. This remarkable phenomenon provides valuable insights into how evolution works and the factors that can drive the emergence of new species over time. In this article, we will explore the unique case of the Aldabra white-throated rail and unravel the central question: what bird went extinct and then re-evolved thousands of years later?
Key Details
– The Aldabra rail is a flightless bird in the rail family that inhabits the Aldabra Atoll in the Indian Ocean.
– It originally evolved on the island of Madagascar and colonized other islands like Aldabra.
– APast sea level rises submerged the Aldabra Atoll over 100,000 years ago, wiping out the local rail population.
– Thousands of years later, after the atoll re-emerged, the rail evolved again independently through a process known as iterative evolution.
– Analysis of fossil evidence and DNA sequencing confirmed that the new rail was nearly identical to and yet genetically distinct from the extinct original Aldabran rail.
This represents one of the few scientifically validated cases of iterative evolution in action. Now let’s dig deeper into the story behind the extinction and re-emergence of this remarkable rail.
The Original Aldabra Rail
The earliest known fossil remains of a rail species on Aldabra date back to around 136,000 years ago. This rail, paleontologically named Dryolimnas cuvieri aldabranus, was a flightless bird perfectly adapted to the Aldabra’s ecosystem. It had evolved from the white-throated rail subspecies originally found on Madagascar that later colonized other islands like Aldabra.
On Aldabra, the birds gradually lost the ability to fly over thousands of years since flight was unnecessary on the remote island which lacked predators. Other adaptations included reduced wings, more robust legs for walking, and claws suited to traversing rough terrain. The rails occupied several ecological niches on Aldabra from scavenging on the beaches to hunting small prey in the shrublands. They became part of the island’s unique biodiversity.
Extinction via Submersion
This first lineage of Aldabra rails was unfortunately wiped out completely when the sea levels rose substantially around 125,000 years ago during the Pleistocene interglacial period. The entire Aldabra Atoll was completely submerged, causing the extinction of all the terrestrial species present there, including the flightless rail population.
With the local ecosystem destroyed, there were no more rails on Aldabra for several thousand years as the atoll remained fully submerged by the high sea levels of the interglacial period. This marked the end of the original Aldabran rail.
Re-emergence of Aldabra and Arrival of Rails
Approximately 118,000 years ago, as sea levels steadily fell once more, the peaks of the Aldabra Atoll slowly started to re-emerge. Gradually the islands became habitable again and species began recolonizing the nascent islands. The Aldabra rail likely returned via a few surviving individuals from nearby Madagascar that arrived on natural rafts of vegetation and then adapted again to the Aldabran habitat.
On the re-emerged atoll, free from predators and the need to fly, these rails began evolving independently into new forms adapted to the island ecosystems. The rails once again lost their ability to fly and over tens of thousands of years diverged into a new iteration of the Aldabra rail reclaiming their niche as an endemic flightless species of the atoll.
Proof of Iterative Evolution
For a long time, scientists were unaware that the modern Aldabra rail was actually a new evolutionary iteration and not the original species. The first clues came from fossil evidence that showed differences between the ancient and contemporary rail fossils.
However, definitive proof was established in a groundbreaking 2016 study by Dr. Julian Hume of the Natural History Museum in London titled “Iterative evolution of flight loss in rail on isolated islands” published in the Zoological Journal of the Linnean Society. Analyzing DNA evidence from multiple rail species, Dr. Hume demonstrated that the modern Aldabra rails had re-evolved into a separate lineage from the ancestral white-throated rails that recolonized Aldabra after its re-emergence.
This legitimate case of iterative evolution was further supported by evidence that showed the two rail species were morphologically similar but with distinct adaptations like differences in hindlimb and pelvis structure. Overall, the study provided undisputed evidence that the extinction and subsequent re-evolution of the rails represented a scientifically valid instance of iterative evolution.
Implications for Evolution
The rare case of iterative evolution demonstrated by the Aldabra rail has noteworthy implications for our understanding of evolutionary processes:
Repeating Patterns in Evolution
The re-emergence of very similar flightless rail species highlights that evolution can repeat itself when environmental pressures act on organisms in predictable ways. The survival advantage offered by flightlessness in the predator-free Aldabran habitat predictably led to parallel evolution of reduced wings and other associated traits in both rail lineages.
Role of Isolation
The fact that this phenomenon occurred on such a remote island emphasizes the pivotal role isolation can play in iterative evolution. The colonizing rails were free to diversify without gene flow from other populations, allowing rapid evolution towards flightlessness to establish independently.
Power of Natural Selection
The rails demonstrate the immense power of natural selection to drive adaptations that allow species to thrive in their ecological contexts, whether past or present. This caused the iterative emergence of this specialist flightless rail perfectly suited to Aldabra’s ecosystem.
Timescale of Evolution
The relatively short timescale of a few tens of thousands of years for the rails to re-evolve demonstrates the potential for evolutionary changes to arise rapidly in isolated populations. This adds to the evidence that evolution does not always require extremely long timeframes.
Iteration | Period Present | Key Traits |
---|---|---|
Original Aldabra Rail | 136,000 – 125,000 years ago | Flightless, reduced wings, robust legs |
Modern Aldabra Rail | From 118,000 years ago | Flightless, reduced wings, robust legs |
Conclusion
In conclusion, the extinction and subsequent iterative evolution of the Aldabra rail provides a compelling example of evolution repeating itself under similar environmental circumstances. Isolated island settings present ideal conditions for this phenomenon to occur when extinctions wipe out local species that then have to recolonize and re-adapt to the same island habitats. Overall, this fascinating case study provides tangible proof of how powerful natural selection can shape the evolutionary trajectory of organisms to optimize their fitness to their ecological niches over timeframes as short as several thousand years. The story of this little flightless rail highlights how evolution does not always move in linear fashion but can in fact cycle back in intriguing ways when enabled by the right environmental context.