Iterative Evolution: Did the Aldabra Rail Evolve Twice?


The Aldabra rail that lives on Aldabra Atoll today is flightless, but they descended from an ancestral stock of high-soaring rails. Charles J Sharp/Wikimedia Commons/CC BY-SA 4.0

Out in the Indian Ocean, 248 miles (400 kilometers) to the northwest of Madagascar, there's a shallow lagoon encircled by a ring of islands. Those outcrops make up the Aldabra Atoll, a place where mangroves flourish and 100,000 giant tortoises roam free.

Recently, a different resident caught the world's attention. The Aldabra rail (Dryolimnas cuvieri aldabranus) is a chicken-sized bird found exclusively on the atoll. It's also the only remaining island bird in the Indian Ocean that happens to be flightless. Weak arm muscles and asymmetrical flight feathers keep the bird grounded.

Yet its ancestors could fly. The Aldabra rail evolved from the white-throated rail (Dryolimnas cuvieri), a still-living bird that often takes to the skies. White-throated rails inhabit Madagascar and neighboring islands. Thousands of years ago, a number of these birds flew out to the Aldabra Atoll.

Then, as now, large predators were rare on the atoll. With the threat of predation mostly gone, the birds' descendants gradually lost the ability to fly. That same thing happened to the dodo, another island-dwelling bird whose ancestors surrendered flight.

Flying is a high-energy activity. When there's no need to fly away from predators — and you can get food simply by walking around — why waste the energy? On the Aldabra Atoll, flight became unnecessary for short-term survival. So over many generations, the isolated rail population gave rise to the fully flightless birds we know today.

But wait! It turns out there's a startling plot twist. Apparently, the sequence of events we just described happened more than once. A 2019 study suggests that flighted, colonizing rails came to Aldabra and begat a nonflying subspecies on two different occasions. It's as if natural selection hit the "reset" button.

Scientists call the phenomenon iterative evolution. Today, we're going to explain what this process entails — and what it doesn't.

The Talk of an Atoll

University of Portsmouth biologists Julian P. Hume and David Martill co-authored the groundbreaking new study, which appeared in the Zoological Journal of the Linnaean Society on May 8, 2019.

Since their paper was published, Hume and Martill's work has garnered a lot of press coverage. Unfortunately, their findings have been widely misinterpreted. To hear some media outlets tell it, the modern Aldabra rail somehow went extinct and then resurrected itself from the dead. That's not what happened. And it's not how iterative evolution works.

Photographers love the Aldabra Atoll for its sunny beaches and blue lagoon. If you're a paleontologist, the islands have another draw: a bountiful fossil record going back hundreds of thousands of years.

On Ile Picard, the westernmost island, a dig site has yielded a pair of fossilized arm bones from prehistoric rails. Geologic clues tell us the bones are more than 136,000 years old.

It looks like the dead birds could've used a good flood insurance policy. Judging by the distribution of marine fossils (e.g., oceanic mollusk remains), it appears the atoll was totally submerged underwater multiple times in the past 400,000 years. Most recently, the islands disappeared beneath the waves from about 136,000 to 118,000 years ago due to a rise in sea levels. Afterward, the waters retreated and the atoll re-emerged.

The bones here show the wing bones fossils of the flighted (far right) and flightless Dryolimnas rails.
Dr Julian Hume

Repetition, But Not Resurrection

Now here's where the story takes an unexpected turn. The Ile Picard arm bones look almost identical to the ones we see in living Aldabra rails today — which, as you'll recall, are flightless. Therefore, the birds those fossils belonged to probably couldn't fly either.

So theoretically, when the atoll flooded, the prehistoric rails in question were unable to escape and got wiped out. Poor things.

However, the saga didn't end there. As Hume and Martill explain in their paper, the fossilized foot bone of a much younger rail was once extracted from Grand Terre, another island in the atoll. That specimen is only about 100,000 years of age. Ergo, its owner lived after the sea levels went back down and the Aldabra Atoll resurfaced.

In an intriguing case of déjà vu, this fossil closely resembles the analogous bones in today's nonflying Aldabra rail and the Assumption rail — a bird that went extinct in 1937. (Primary sources indicate that it was flightless, too.)

Chances are, the Grand Terre fossil came from a bird who either couldn't fly or was in the process of losing its ability to do so. Either way, it was the probable ancestor of modern Aldabra rails.

According to Hume and Martill, we're looking at an evolutionary do-over. The flightless islanders who died out when the atoll went under had descended from an ancestral stock of high-soaring rails. Once the islands vanished and then re-emerged, those aerial wanderers repopulated the atoll and evolved into an all-new, flightless subspecies — one that's still at large today.

History repeated itself, loud and clear. That's iterative evolution in a nutshell.

"Stop Me If You've Heard This One"

Iterative evolution can be defined as "the repeated evolution of a specific trait or body plan from the same ancestral lineage at different points in time."

Let's say there's an organism (or a closely related group of organisms) with a fairly conservative build that manages to survive over a long period of geologic time. If multiple groups of similar-looking descendants independently evolved — one after another — from this common ancestor, it'd be a clear-cut case of iterative evolution.

Consider the ammonites. Spiral-shelled relatives of squids and nautiluses, ammonites roamed the oceans throughout the age of dinosaurs. Some experts think that individuals with thinner shells that were compressed from side to side were better-suited for shallow environments with very fast currents. On the other hand, thicker, heavier shells nicely leant themselves to deep areas far offshore.

So there's evidence that — in certain parts of the world — an ancestral stock of thick-shelled ammonites would periodically give rise to thin-shelled descendants who invaded beachside habitats. When the sea levels fell, many of those habitats disappeared and the offshoot ammonites died out. But their thick-shelled ancestors persisted — and when the oceans rose again, they'd sire a new generation of shallow water denizens with thin shells.

And that's just one example. Iterative evolution might also explain the repeated rise and fall of similar-looking sea cows over the past 26 million years. Likewise, sea turtles — specifically the ones with seagrass-centered diets — may have undergone this same process during their evolutionary history.

While natural selection is a powerful force, it cannot revive an extinct species. But when the environmental conditions are right, at least it can produce a good imitation.