Speaker 1: Welcome to Brainstuff, a production of iHeartRadio.
Speaker 2: Hey brain Stuff. I'm Lauren vogelbamb In. Today's episode is
Speaker 2: a classic from the podcast's archives. In this one, we
Speaker 2: get into the science of what's called iterative evolution, the
Speaker 2: phenomenon where some combinations of traits turn out to be
Speaker 2: so useful that evolution repeats itself.
Speaker 1: Hey brain Stuff, Lauren vogelbomb. Here out in the Indian Ocean,
Speaker 1: about two hundred and fifty miles or four hundred kilometers
Speaker 1: to the northwest of Madagascar, there's a shallow lagoon encircled
Speaker 1: by a ring of islands. Those outcrops make up the
Speaker 1: Aldabra Atoll, a place where mangroves flourish and one hundred
Speaker 1: thousand giant tortoises roam free. Recently, a different resident caught
Speaker 1: the world's attention. The Aldabra rail is a chicken sized
Speaker 1: bird found exclusively on the atoll. It's also the only
Speaker 1: remaining island bird in the Indian Ocean that happens to
Speaker 1: be flightless. Weak our muscles and asymmetrical flight pats keep
Speaker 1: the bird grounded, yet its ancestors could fly. The Aldabra
Speaker 1: rail evolved from the white throated rail, a still living
Speaker 1: bird that flies very well. Thank you. White throated rails
Speaker 1: inhabit Madagascar and neighboring islands. Thousands of years ago, a
Speaker 1: number of these birds flew out to the Aldabra Atoll. Then,
Speaker 1: as now, large predators were rare on the atoll. With
Speaker 1: the threat of predation mostly gone, the bird's descendants gradually
Speaker 1: lost the ability to fly. That same thing happened to
Speaker 1: the dodo, another island dwelling bird whose ancestors surrendered flight.
Speaker 1: Flying is a high energy activity. When there's no need
Speaker 1: to fly away from predators and you can get food
Speaker 1: simply by walking around, why waste the energy on the
Speaker 1: Aldabra atoll, flight became unnecessary for short term survival, So
Speaker 1: over many generations, the isolated rail population gave rise to
Speaker 1: the fully flightless birds we know today. But it turns
Speaker 1: out there's a startling plot twist. Apparently the sequence of
Speaker 1: events we just described happened more than once. A twenty
Speaker 1: nineteen study suggests that flighted colonizing rails came to Aldabra
Speaker 1: and begot a non flying subspecies on two different occasions.
Speaker 1: It's as if natural selection hit the reset button. Scientists
Speaker 1: call the phenomenon iterative evolution. Today, we're going to explain
Speaker 1: what this process entails and what it doesn't. University of
Speaker 1: Portsmouth biologists Julian P. Hume and David Martil co authored
Speaker 1: the groundbreaking new study, which appeared in the Zoological Journal
Speaker 1: of the Linean Society. Since their paper was published, Hume
Speaker 1: in Martill's work has garnered a lot of press coverage. Unfortunately,
Speaker 1: their findings have been widely misinterpreted. To hear, some media
Speaker 1: outlets tell it, the modern Aldabra rail went extinct and
Speaker 1: then resurrected itself from the dead. But that's not what happened,
Speaker 1: and it's not how iterative evolution works. Photographers love the
Speaker 1: Aldabra Toll for its sunny beaches and blue lagoon. If
Speaker 1: you're a palaeontologist, the islands have another draw, a bountiful
Speaker 1: fossil record going back hundreds of thousands of years. On
Speaker 1: ele Picard, the westernmost island, a dig site has yielded
Speaker 1: a pair of fossilized armbones from prehistoric rails. Geologic clues
Speaker 1: tell us the bones are more than one hundred and
Speaker 1: thirty six thousand years old. It looks like the dead
Speaker 1: birds could have used a good flood insurance policy. Judging
Speaker 1: by the distribution of marine fossils like oceanic mollusk remains,
Speaker 1: it appears the atoll was totally submerged under water multiple
Speaker 1: times in the past four hundred thousand years. More recently,
Speaker 1: the islands disappeared beneath the waves from about one hundred
Speaker 1: and thirty six thousand to one hundred and eighteen thousand
Speaker 1: years ago due to a rise in sea levels. Afterward,
Speaker 1: the waters retreated and the atoll re emerged. And now
Speaker 1: here's where the story takes an unexpected turn. The elip
Speaker 1: card armbones look almost identical to the ones we see
Speaker 1: in living al dabra rails today, which, as you'll recall,
Speaker 1: are flightless. Therefore, the birds those fossils belonged to probably
Speaker 1: couldn't fly either, So theoretically, when the atoll flooded, the
Speaker 1: prehistoric rails in question were unable to escape and got
Speaker 1: wiped out. However, the saga didn't end there, as human
Speaker 1: martill It explained in their paper, the fossilized footbone of
Speaker 1: a much more recent rail was once extracted from Grand Terre,
Speaker 1: another island in the atoll. That specimen is only about
Speaker 1: one hundred thousand years of age orgo. Its owner lived
Speaker 1: after the sea levels went back down and the Aldebra
Speaker 1: atoll resurfaced. In an intriguing case of deja vus, this
Speaker 1: fossil closely resembles the bones in today's non flying Aldebra
Speaker 1: rail and the assumption rail, a bird that went extinct
Speaker 1: in nineteen thirty seven. A primary sources indicate that it
Speaker 1: was flightless too. The chances are the Grand Terra fossil
Speaker 1: came from a bird that either couldn't fly or was
Speaker 1: in the process of losing its ability to do so.
Speaker 1: Either way, it was the probable ancestor of modern aldebra rails.
Speaker 1: According to human Martill, we're looking at an evolutionary do over.
Speaker 1: The flightless birds that died out when the atoll went
Speaker 1: under had descended from an ancestral stock of high soaring rails.
Speaker 1: Once the islands vanished and then re emerged, those aerial
Speaker 1: wanderers repopulated the atoll and evolved into an all new
Speaker 1: flightless subspecies, one that's still at large today. History repeated
Speaker 1: itself loud and clear, and that's iterative evolution and a nutshell.
Speaker 1: Iterative evolution can be defined as the repeated evolution of
Speaker 1: a specific trait or body plan from the same ancestral
Speaker 1: lineage at different points in time. Let's say there's an
Speaker 1: organism or a closely related group of organisms with a
Speaker 1: fairly conservative build that manages to survive over a long
Speaker 1: period of geologic time. If multiple groups of similar looking
Speaker 1: descendants independently evolved one after another from this common ancestor,
Speaker 1: it'd be a clear cutcase of iterative evolution. Consider the ammonites,
Speaker 1: spiral shelled relatives of squids and nautiluses. Ammonites roamed the
Speaker 1: oceans throughout the age of dinosaurs. Some experts think that
Speaker 1: individuals with thinner shells that were compressed from side to
Speaker 1: side were better suited for shallow environments with very fast currents.
Speaker 1: On the other hand, thicker, heavier shells nicely lent themselves
Speaker 1: two deep areas far offshore. So there's evidence that in
Speaker 1: certain parts of the world an ancestral stock of thick
Speaker 1: shelled amonites would periodically give rise to thin shelled descendants
Speaker 1: who invaded beachside habitats. When the sea levels fell, many
Speaker 1: of those habitats disappeared, and the offshoot ammonites died out.
Speaker 1: But their thick shelled ancestors persisted, and when the oceans
Speaker 1: rose again, they'd sire a new generation of shallow water
Speaker 1: denizens with thin shells. And that's just one example. Iterative
Speaker 1: evolution might also explain the repeated rise and fall of
Speaker 1: similar looking sea cows over the past twenty six million years. Likewise,
Speaker 1: sea turtles, specifically the ones with seagrass centered diets, may
Speaker 1: have undergone the same process during their evolutionary history. While
Speaker 1: natural selection is a powerful force, it cannot revive an
Speaker 1: extinct species, but when the environmental conditions are right, it
Speaker 1: can at least produce a good imitation. Today's episode is
Speaker 1: based on article iterative evolution, Did the Aldebra rail Evolve Twice?
Speaker 1: On how Stuffworks dot Com? Written by Mark Mancini.
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