What is Extinction?
A species of plants or organisms is considered ‘globally extinct’ when there are no more living specimens of it left and their genetic heritage has been lost forever. Scientifically, extinction is considered simply as an expression of biological evolution. National Geographic states that more than 99 per cent of all organisms that have ever lived on Earth are extinct, which makes it a natural phenomenon. Animals go extinct due to either environmental forces like global change, natural disaster and overexploitation of species for human use or genetic changes like genetic inbreeding or poor reproduction.
About Aldabra Rail
The Aldabra rail is a flightless subspecies of the white-throated rail (Dryolimnas cuvieri), which is indigenous to islands in the Southwestern Indian Ocean—Comros, Madagascar, Mayote, and Seychelles. The birds are chicken-sized and their flightlessness is characterized by weak arm muscles and asymmetrical flight. The Aldabra rail inhabits the Aldabra Islands Atoll, which is a part of the Outer Islands of Seychelles.
As mentioned before, these birds evolved from the high-soaring white-throated rail. A study in the University of Portsmouth calls these birds ‘persistent colonizers’ as they build up on huge landmasses and eventually depart in huge numbers due to overcrowding or lack of food. White-throated rails inhabit Madagascar and neighbouring islands. It is assumed that at some point in history, a number of these birds flew out to the Aldabra Atoll.
Due to lack of predators on the island, the birds ultimately lost their ability to fly. On the Aldabra Atoll, flight became unnecessary for short-term survival. So, in about 20,000 years, the isolated rail population gave rise to the fully flightless birds we know today.
Extinction & Resurrection?
Aldabra atoll comprises of a group of islands which are surrounded by a coral reef, making it completely vulnerable to inundation events. It appears that the atoll was totally submerged underwater multiple times in the past 400,000 years. Unfortunately, 136,000 years ago, the Aldabra atoll in the Indian Ocean was inundated by a major flood that wiped out all the flora and fauna on the island, and the Aldabra rail being flightless couldn’t flee. Tens of thousands of years later, sea levels fell back, once again making life possible on the atoll. Astonishingly, the once-extinct Aldabra rail came back.
Palaeontologists and researchers were able to decode this mystery by studying rail humeri bones dating to at least 1,36,000 years ago, and comparing them with a rail leg bone found in a deposit from 1,00,000 years old. The older specimens were found to be very similar to the bones of the flightless rails that exist on Aldabra today indicating that even back then the rails were completely flightless. The relatively recent specimen, however, suggests that the bird was in the process of losing its flight. This means that after the atoll sunk and reappeared, the Aldabra rails once again showed up, and once again lost their ability to fly due to no danger of predators.
When you hear that an animal goes extinct, it is usually considered to be permanent. The Aldabra rail coming back from extinction was not a miracle, but a process called ‘iterative evolution.’ This is a very rare phenomenon and this was the first time it was noticed in rails. Iterative evolution is a process that involves the evolution of ‘similar or parallel structures’ from the same ancestral lineage but at different times.’ This means that in spite of the subspecies going extinct in the past, it can re-emerge a number of times. What really happened is that the islands resurfaced, and the white-throated rails inhabited the atoll, once again, evolving into the flightless rails.
Today, the flightless rails that exist on various islands are vulnerable to predation by introduced predators like cats and rats. The Aldabra rail is, in fact, the last living member of the genus Dryolimnas and the only flightless rail that still survives in the Indian Ocean. It has a ‘least concerned’ status in IUCN Red List.
Image Source: Google Images
Written by Aakanksha Mantri for MTTN
Infographics by Ambuj Chauhan
Edited by Chintan Gandhi