WEBVTT - BrainStuff Classics: Can the Same Animal Evolve Twice?

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<v Speaker 1>Welcome to Brainstuff, a production of iHeartRadio.

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<v Speaker 2>Hey brain Stuff. I'm Lauren vogelbamb In. Today's episode is

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<v Speaker 2>a classic from the podcast's archives. In this one, we

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<v Speaker 2>get into the science of what's called iterative evolution, the

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<v Speaker 2>phenomenon where some combinations of traits turn out to be

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<v Speaker 2>so useful that evolution repeats itself.

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<v Speaker 1>Hey brain Stuff, Lauren vogelbomb. Here out in the Indian Ocean,

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<v Speaker 1>about two hundred and fifty miles or four hundred kilometers

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<v Speaker 1>to the northwest of Madagascar, there's a shallow lagoon encircled

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<v Speaker 1>by a ring of islands. Those outcrops make up the

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<v Speaker 1>Aldabra Atoll, a place where mangroves flourish and one hundred

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<v Speaker 1>thousand giant tortoises roam free. Recently, a different resident caught

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<v Speaker 1>the world's attention. The Aldabra rail is a chicken sized

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<v Speaker 1>bird found exclusively on the atoll. It's also the only

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<v Speaker 1>remaining island bird in the Indian Ocean that happens to

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<v Speaker 1>be flightless. Weak our muscles and asymmetrical flight pats keep

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<v Speaker 1>the bird grounded, yet its ancestors could fly. The Aldabra

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<v Speaker 1>rail evolved from the white throated rail, a still living

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<v Speaker 1>bird that flies very well. Thank you. White throated rails

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<v Speaker 1>inhabit Madagascar and neighboring islands. Thousands of years ago, a

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<v Speaker 1>number of these birds flew out to the Aldabra Atoll. Then,

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<v Speaker 1>as now, large predators were rare on the atoll. With

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<v Speaker 1>the threat of predation mostly gone, the bird's descendants gradually

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<v Speaker 1>lost the ability to fly. That same thing happened to

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<v Speaker 1>the dodo, another island dwelling bird whose ancestors surrendered flight.

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<v Speaker 1>Flying is a high energy activity. When there's no need

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<v Speaker 1>to fly away from predators and you can get food

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<v Speaker 1>simply by walking around, why waste the energy on the

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<v Speaker 1>Aldabra atoll, flight became unnecessary for short term survival, So

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<v Speaker 1>over many generations, the isolated rail population gave rise to

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<v Speaker 1>the fully flightless birds we know today. But it turns

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<v Speaker 1>out there's a startling plot twist. Apparently the sequence of

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<v Speaker 1>events we just described happened more than once. A twenty

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<v Speaker 1>nineteen study suggests that flighted colonizing rails came to Aldabra

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<v Speaker 1>and begot a non flying subspecies on two different occasions.

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<v Speaker 1>It's as if natural selection hit the reset button. Scientists

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<v Speaker 1>call the phenomenon iterative evolution. Today, we're going to explain

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<v Speaker 1>what this process entails and what it doesn't. University of

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<v Speaker 1>Portsmouth biologists Julian P. Hume and David Martil co authored

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<v Speaker 1>the groundbreaking new study, which appeared in the Zoological Journal

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<v Speaker 1>of the Linean Society. Since their paper was published, Hume

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<v Speaker 1>in Martill's work has garnered a lot of press coverage. Unfortunately,

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<v Speaker 1>their findings have been widely misinterpreted. To hear, some media

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<v Speaker 1>outlets tell it, the modern Aldabra rail went extinct and

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<v Speaker 1>then resurrected itself from the dead. But that's not what happened,

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<v Speaker 1>and it's not how iterative evolution works. Photographers love the

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<v Speaker 1>Aldabra Toll for its sunny beaches and blue lagoon. If

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<v Speaker 1>you're a palaeontologist, the islands have another draw, a bountiful

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<v Speaker 1>fossil record going back hundreds of thousands of years. On

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<v Speaker 1>ele Picard, the westernmost island, a dig site has yielded

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<v Speaker 1>a pair of fossilized armbones from prehistoric rails. Geologic clues

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<v Speaker 1>tell us the bones are more than one hundred and

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<v Speaker 1>thirty six thousand years old. It looks like the dead

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<v Speaker 1>birds could have used a good flood insurance policy. Judging

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<v Speaker 1>by the distribution of marine fossils like oceanic mollusk remains,

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<v Speaker 1>it appears the atoll was totally submerged under water multiple

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<v Speaker 1>times in the past four hundred thousand years. More recently,

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<v Speaker 1>the islands disappeared beneath the waves from about one hundred

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<v Speaker 1>and thirty six thousand to one hundred and eighteen thousand

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<v Speaker 1>years ago due to a rise in sea levels. Afterward,

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<v Speaker 1>the waters retreated and the atoll re emerged. And now

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<v Speaker 1>here's where the story takes an unexpected turn. The elip

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<v Speaker 1>card armbones look almost identical to the ones we see

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<v Speaker 1>in living al dabra rails today, which, as you'll recall,

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<v Speaker 1>are flightless. Therefore, the birds those fossils belonged to probably

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<v Speaker 1>couldn't fly either, So theoretically, when the atoll flooded, the

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<v Speaker 1>prehistoric rails in question were unable to escape and got

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<v Speaker 1>wiped out. However, the saga didn't end there, as human

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<v Speaker 1>martill It explained in their paper, the fossilized footbone of

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<v Speaker 1>a much more recent rail was once extracted from Grand Terre,

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<v Speaker 1>another island in the atoll. That specimen is only about

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<v Speaker 1>one hundred thousand years of age orgo. Its owner lived

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<v Speaker 1>after the sea levels went back down and the Aldebra

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<v Speaker 1>atoll resurfaced. In an intriguing case of deja vus, this

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<v Speaker 1>fossil closely resembles the bones in today's non flying Aldebra

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<v Speaker 1>rail and the assumption rail, a bird that went extinct

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<v Speaker 1>in nineteen thirty seven. A primary sources indicate that it

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<v Speaker 1>was flightless too. The chances are the Grand Terra fossil

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<v Speaker 1>came from a bird that either couldn't fly or was

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<v Speaker 1>in the process of losing its ability to do so.

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<v Speaker 1>Either way, it was the probable ancestor of modern aldebra rails.

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<v Speaker 1>According to human Martill, we're looking at an evolutionary do over.

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<v Speaker 1>The flightless birds that died out when the atoll went

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<v Speaker 1>under had descended from an ancestral stock of high soaring rails.

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<v Speaker 1>Once the islands vanished and then re emerged, those aerial

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<v Speaker 1>wanderers repopulated the atoll and evolved into an all new

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<v Speaker 1>flightless subspecies, one that's still at large today. History repeated

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<v Speaker 1>itself loud and clear, and that's iterative evolution and a nutshell.

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<v Speaker 1>Iterative evolution can be defined as the repeated evolution of

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<v Speaker 1>a specific trait or body plan from the same ancestral

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<v Speaker 1>lineage at different points in time. Let's say there's an

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<v Speaker 1>organism or a closely related group of organisms with a

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<v Speaker 1>fairly conservative build that manages to survive over a long

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<v Speaker 1>period of geologic time. If multiple groups of similar looking

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<v Speaker 1>descendants independently evolved one after another from this common ancestor,

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<v Speaker 1>it'd be a clear cutcase of iterative evolution. Consider the ammonites,

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<v Speaker 1>spiral shelled relatives of squids and nautiluses. Ammonites roamed the

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<v Speaker 1>oceans throughout the age of dinosaurs. Some experts think that

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<v Speaker 1>individuals with thinner shells that were compressed from side to

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<v Speaker 1>side were better suited for shallow environments with very fast currents.

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<v Speaker 1>On the other hand, thicker, heavier shells nicely lent themselves

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<v Speaker 1>two deep areas far offshore. So there's evidence that in

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<v Speaker 1>certain parts of the world an ancestral stock of thick

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<v Speaker 1>shelled amonites would periodically give rise to thin shelled descendants

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<v Speaker 1>who invaded beachside habitats. When the sea levels fell, many

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<v Speaker 1>of those habitats disappeared, and the offshoot ammonites died out.

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<v Speaker 1>But their thick shelled ancestors persisted, and when the oceans

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<v Speaker 1>rose again, they'd sire a new generation of shallow water

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<v Speaker 1>denizens with thin shells. And that's just one example. Iterative

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<v Speaker 1>evolution might also explain the repeated rise and fall of

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<v Speaker 1>similar looking sea cows over the past twenty six million years. Likewise,

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<v Speaker 1>sea turtles, specifically the ones with seagrass centered diets, may

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<v Speaker 1>have undergone the same process during their evolutionary history. While

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<v Speaker 1>natural selection is a powerful force, it cannot revive an

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<v Speaker 1>extinct species, but when the environmental conditions are right, it

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<v Speaker 1>can at least produce a good imitation. Today's episode is

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<v Speaker 1>based on article iterative evolution, Did the Aldebra rail Evolve Twice?

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<v Speaker 1>On how Stuffworks dot Com? Written by Mark Mancini.

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<v Speaker 2>Brain Stuff is production of iHeart Radio and partnership book

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<v Speaker 2>HowStuffWorks dot Com, and it's produced by Tyler Klang. Four

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