Iterative Evolution: Has the Aldabra Rail Experienced Dual Evolutionary Events?

Located in the Indian Ocean, 248 miles (400 kilometers) northwest of Madagascar, a shallow lagoon is surrounded by a cluster of islands. These formations constitute the Aldabra Atoll, a haven for mangroves and home to 100,000 giant tortoises.

Recently, another inhabitant has drawn global interest. The Aldabra rail (Dryolimnas cuvieri aldabranus), a bird the size of a chicken, is unique to the atoll. It stands as the sole surviving flightless bird in the Indian Ocean, with underdeveloped wing muscles and uneven flight feathers preventing it from taking flight.

However, its predecessors were capable of flight. The Aldabra rail originated from the white-throated rail (Dryolimnas cuvieri), a species still in existence that frequently takes flight. These birds are found in Madagascar and nearby islands. Millennia ago, a group of these birds flew to the Aldabra Atoll.

Back then, as is the case today, large predators were scarce on the atoll. With minimal predation risks, the birds' descendants slowly lost their ability to fly. A similar fate befell the dodo, another island bird whose ancestors abandoned flight.

Flight demands significant energy. When escaping predators is unnecessary and food can be found by simply walking, why expend the energy? On Aldabra Atoll, flying became irrelevant for survival. Over countless generations, the isolated rail population evolved into the flightless birds we recognize today.

But here's a surprising twist. The sequence of events we described occurred not once, but twice. A 2019 study indicates that flying rails colonized Aldabra and produced a flightless subspecies on two separate occasions. It’s as though natural selection pressed the "reset" button.

Scientists refer to this phenomenon as iterative evolution. Today, we’ll explore what this process involves — and what it doesn’t.

The Talk of an Atoll

Biologists Julian P. Hume and David Martill from the University of Portsmouth co-wrote the pioneering study, published in the Zoological Journal of the Linnaean Society on May 8, 2019.

Since its publication, Hume and Martill's research has attracted significant media attention. However, their conclusions have often been misunderstood. Some reports suggest the modern Aldabra rail went extinct and then miraculously reappeared. This is not the case, nor does it align with the principles of iterative evolution.

The Aldabra Atoll is a favorite among photographers for its stunning beaches and turquoise waters. For paleontologists, the islands offer another allure: a rich fossil record spanning hundreds of thousands of years.

On Ile Picard, the westernmost island, excavations have uncovered fossilized arm bones from ancient rails. Geological evidence indicates these bones are over 136,000 years old.

It seems the deceased birds could have benefited from flood insurance. Based on the distribution of marine fossils, such as oceanic mollusk remains, the atoll was completely submerged multiple times over the past 400,000 years. Most recently, the islands vanished underwater between 136,000 and 118,000 years ago due to rising sea levels. Later, the waters receded, and the atoll resurfaced.

Repetition, But Not Resurrection

Here’s where the tale takes a surprising twist. The arm bones from Ile Picard closely resemble those of the modern Aldabra rails, which, as noted, are flightless. This suggests the ancient birds these fossils belonged to were also likely incapable of flight.

In theory, when the atoll was flooded, the ancient rails couldn’t escape and were wiped out. A tragic fate indeed.

But the story didn’t end there. As Hume and Martill detail in their study, a fossilized foot bone from a much younger rail was discovered on Grand Terre, another island in the atoll. This specimen is approximately 100,000 years old, meaning its owner lived after the sea levels receded and the Aldabra Atoll reappeared.

In a fascinating case of repetition, this fossil bears a striking resemblance to the corresponding bones in today’s flightless Aldabra rail and the Assumption rail — a bird that became extinct in 1937. (Historical records confirm it was also flightless.)

It’s likely that the Grand Terre fossil belonged to a bird that either couldn’t fly or was gradually losing its flight capability. In any case, it was likely the ancestor of the modern Aldabra rails.

Hume and Martill suggest this represents an evolutionary reset. The flightless birds that perished when the atoll submerged were descendants of flying rails. After the islands re-emerged, these flying ancestors recolonized the atoll and evolved into a new, flightless subspecies — one that still exists today.

History repeated itself unmistakably. This is the essence of iterative evolution.

"Stop Me If You've Heard This One"

Iterative evolution refers to "the repeated emergence of a specific trait or body plan from the same ancestral lineage at different times."

Imagine an organism (or a closely related group) with a relatively stable structure that persists over a long geological period. If multiple groups of similar descendants independently evolve from this common ancestor at different times, it’s a textbook example of iterative evolution.

Take ammonites, for instance. These spiral-shelled relatives of squids and nautiluses thrived in the oceans during the age of dinosaurs. Some researchers believe that those with thinner shells, flattened side to side, were better adapted to shallow, fast-moving waters. In contrast, thicker, heavier shells were more suited to deep, offshore environments.

Evidence suggests that, in specific regions, ancestral thick-shelled ammonites periodically gave rise to thin-shelled descendants that colonized coastal habitats. When sea levels dropped, many of these habitats vanished, leading to the extinction of the thin-shelled ammonites. However, their thick-shelled ancestors endured, and when sea levels rose again, they produced new generations of thin-shelled ammonites adapted to shallow waters.

This is just one instance. Iterative evolution may also account for the repeated emergence and disappearance of similar-looking sea cows over the past 26 million years. Similarly, sea turtles, particularly those with diets centered on seagrass, might have experienced this process during their evolutionary journey.

While natural selection is a formidable force, it cannot bring extinct species back to life. However, under the right environmental conditions, it can create a remarkably similar counterpart.