Evolution Brings Extinct Bird Back to Life! Except it Didn’t

A news story has been circulating a fair bit in the past couple of weeks. This story has been picked up by numerous news and science outlets. How it is being reported and explained is just plain misleading and inaccurate.

The Aldabra Rail is a subspecies of the White-throated Rail.

Here are a few titles that show how the subject is being covered.

Science Magazine – Evolution Brings Extinct Island Bird Back into Existence

Smithsonian Magazine – How Evolution Brought a Flightless Bird Back from Extinction

CBS News – An Extinct Bird Species Has Evolved Back into Existence, Study Says

From these titles, and from the bodies of the articles themselves, readers would think that the same species of bird existed at some point in the past, went extinct (as in died out completely), and then re-evolved!

That does not happen.

Here is what actually did occur.

The small atoll of Aldabra is a pretty spectacular spot. It is very remote. It is quite beautiful. It is home to a bunch of unique animals found no where else on earth. It has one of the longest fossil records on any island in the Indian Ocean.

That fossil record includes a lot of the animals that have called the atoll home over the past few million years. One of those animals was the Aldabra Rail. This rail was a small flightless bird that was probably found hunting through reed beds along the edges of water. The Aldabra Rail went extinct about 136,000 years ago at about the same time that global sea level was rising and submerging oceanic islands like Aldabra. After a few thousand years, sea level dropped and Aldabra became an exposed island once more. Not long after that fossils of a rail on Aldabra start showing up again.

There are a couple of possible explanations. One is that some remnant population of the Aldabra Rail hung on, some how, and did not die. These were flightless birds, so it is not clear how this might have happened, but perhaps a small population managed to survive on a floating raft of vegetation long enough to reach an exposed bit of land. This seems like a very long shot. It is much more likely that the Aldabra Rail simply died out completely. It went extinct.

The other possible explanation is much more likely and widely understood and accepted, and it is this: the Aldabra Rail went extinct when the atoll went under water. Then after it re-emerged, a group of birds likely from the same parent stock of the original Aldabra Rail re-colonized the atoll (quite probably from Madagascar). This new group of colonizers eventually became flightless and filled the same, or very similar, ecological niche as the original Aldabra Rail.

This is a process called iterative evolution and it is pretty rare. The definition of iterative evolution is: the evolution of similar or parallel structures in the development of the same main line.

But iterative evolution does not produce the same species twice. It may produce similar species, but to produce the same species twice would require starting with the same gene pool twice. The group of birds that first colonized Aldabra, and became the Aldabra Rail 1.0, had a unique combination of genes to work with. The group of birds that later colonized Aldabra, and became the Aldabra Rail 2.0, had a unique combination of genes to work with. Those two combinations of genes may have been similar, but they were not the same. Therefore the decedents of those two groups would not be the same.

I really think that the implications of how this story is being reported is really misleading and possible even damaging.

Misleading because they imply that a species can evolve twice. To go back to the definition of  iterative evolution, it the evolution of “similar or parallel structures…” Similar or parallel structures are not the same as identical species. Two rails that evolved at different times in the same place and that are both flightless, are not the same species.

Damaging because there is weight to the idea of extinction. Extinction is forever. It means that an entire evolutionary lineage has ended, and any potential future that that lineage may have had is gone. If the idea of extinction becomes an impermanent one, it looses its urgency and tragedy. People may well not worry about extinction as that species can just re-evolve. No harm, no foul.

Again, no species can ever occur twice. Once a species goes extinct, that is it for that evolutionary lineage. Even if some other lineage emerges that is close, it will not be the same and will not have the same evolutionary trajectory or potential.

When reporting on science, I feel strongly that the ideas behind the science should be accurately represented. I think it is especially distressing when the sources of the misrepresentations are otherwise reputable sources for science.

I hope the current Aldabra Rail has a long future filled with descendants, and I mourn the loss of the previous rail of Aldabra and the lineage it might have left behind, but never will.

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Powered Flight in Vertebrates

Powered flight, the ability to propel oneself through the air against the force of gravity, requires a great deal. From specialized bones to specialized skin, and everything in between, the demands of flight penetrate all aspects of an animals’ life. And yet, despite how mechanically difficult powered flight is to achieve, it has evolved three different times in vertebrate evolution, once in birds, once in bats, and once in the now extinct pterosaurs. One of the things that make the three different evolutions of flight especially interesting is that the three different groups accomplished the feat is such different ways.

A pterosaur wing has a large upper arm bone, two smaller forearm bones, a few wrist bones, and then four fingers. The first three are small fingers that have small claws at their tips and are free to move and grasp objects. The fourth finger is extremely long, extending all the way to the tip of the wing. The bones of this fourth finger are longer and thicker than the bones of the other fingers, and this is because this fourth finger supports the entire wing membrane. The wing membrane attaches all along the rear surface of the fourth finger, the rear surface of the arm, and then to the side of the trunk of the animal and even to the leading edge of the hind legs. That means that, when a pterosaur is in flight, the whole weight of the animal is being supported on once finger of each hand! Some pterosaurs got to an estimated 550 lbs (although most were about 25 lbs) with a 10 or 11 meter wingspan, so those are some strong fourth fingers!

A bat wing is the same upper arm bones of the pterosaur, but the hand is very different. Instead of supporting al the body weight of the animal on just one finger, all five of the fingers of a bat are elongated (the thumb is the only finger that is small and clawed and free to grasp). In between each of these fingers, and extending from the pinky finger to the body are membranes that the bat can stretch or fold as needed by moving the bones in its fingers, hands, and arm. Since all the fingers are sharing the load, each one is proportionately much finer and thinner than the fourth finger of a pterosaur. Additionally, bats have thin muscles that cover the surfaces of the membranes just beneath the skin. By tensing or relaxing these muscles, the bat has very fine control over the sharp and tension of the wing membranes, and this turns out to be very important in bat flight. It is not known if pterosaurs had similar muscles on the membranes of their wings, but it seems likely that they did.

A bird wing also has the same basic upper arm bones seen in the other groups, but instead of elongating bones and making them delicate and distinct, birds go the opposite direction. Most of the bones in the hand and fingers of a bird are fused together. This makes a structure that is short, thick, and strong. Out of this support structure extends feathers. Not a membrane made of skin like a pterosaur or a bat, but a completely different evolutionary innovation. Feathers, and flight feathers in particular, are strong and thin and light. Flexible enough to bend a bit to change shape, but rigid enough to support the weight of the bird and the forces of air-speed, drag, lift and gravity that all flying organisms have to contend with. In modern birds, muscles in the skin at the base of the feathers allow for independent control of each feather. This allows birds to have amazing control over the shape of their wing.

Flight is a fascinating example of how a complex structure or function can arise by natural selection. In these three cases, natural selection favored three very different, and ultimately successful, experiments in how to get an animal airborne. All three were very different solutions, which is worth remembering. Most of the problems out there, even really hard ones, probably have many different solutions, you just have to tinker and figure them out.

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