BrainStuff Classics: Can the Same Animal Evolve Twice?

Remote islands are excellent laboratories for observing evolution at work -- and in the case of a flightless bird called the Aldabra rail, we can observe how its evolution happened twice. Learn about iterative evolution in today's classic episode of BrainStuff, based on this article: https://science.howstuffworks.com/life/evolution/iterative-evolution.htm

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Episode Transcript

Speaker 1 (00:01):
Welcome to Brainstuff, a production of iHeartRadio.

Speaker 2 (00:06):
Hey brain Stuff. I'm Lauren vogelbamb In. Today's episode is
a classic from the podcast's archives. In this one, we
get into the science of what's called iterative evolution, the
phenomenon where some combinations of traits turn out to be
so useful that evolution repeats itself.

Speaker 1 (00:27):
Hey brain Stuff, Lauren vogelbomb. Here out in the Indian Ocean,
about two hundred and fifty miles or four hundred 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 one hundred
thousand giant tortoises roam free. Recently, a different resident caught

(00:47):
the world's attention. The Aldabra rail 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 our muscles and asymmetrical flight pats keep
the bird grounded, yet its ancestors could fly. The Aldabra
rail evolved from the white throated rail, a still living

(01:07):
bird that flies very well. Thank you. 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 bird's descendants gradually
lost the ability to fly. That same thing happened to
the dodo, another island dwelling bird whose ancestors surrendered flight.

(01:31):
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 it turns
out there's a startling plot twist. Apparently the sequence of

(01:54):
events we just described happened more than once. A twenty
nineteen study suggests that flighted colonizing rails came to Aldabra
and begot a non flying 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. University of

(02:17):
Portsmouth biologists Julian P. Hume and David Martil co authored
the groundbreaking new study, which appeared in the Zoological Journal
of the Linean Society. Since their paper was published, Hume
in 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 went extinct and

(02:38):
then resurrected itself from the dead. But that's not what happened,
and it's not how iterative evolution works. Photographers love the
Aldabra Toll for its sunny beaches and blue lagoon. If
you're a palaeontologist, the islands have another draw, a bountiful
fossil record going back hundreds of thousands of years. On
ele Picard, the westernmost island, a dig site has yielded

(02:58):
a pair of fossilized armbones from prehistoric rails. Geologic clues
tell us the bones are more than one hundred and
thirty six thousand years old. It looks like the dead
birds could have used a good flood insurance policy. Judging
by the distribution of marine fossils like oceanic mollusk remains,
it appears the atoll was totally submerged under water multiple
times in the past four hundred thousand years. More recently,

(03:21):
the islands disappeared beneath the waves from about one hundred
and thirty six thousand to one hundred and eighteen thousand
years ago due to a rise in sea levels. Afterward,
the waters retreated and the atoll re emerged. And now
here's where the story takes an unexpected turn. The elip
card armbones look almost identical to the ones we see
in living al dabra rails today, which, as you'll recall,

(03:43):
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. However, the saga didn't end there, as human
martill It explained in their paper, the fossilized footbone of
a much more recent rail was once extracted from Grand Terre,

(04:05):
another island in the atoll. That specimen is only about
one hundred thousand years of age orgo. Its owner lived
after the sea levels went back down and the Aldebra
atoll resurfaced. In an intriguing case of deja vus, this
fossil closely resembles the bones in today's non flying Aldebra
rail and the assumption rail, a bird that went extinct
in nineteen thirty seven. A primary sources indicate that it

(04:28):
was flightless too. The chances are the Grand Terra fossil
came from a bird that 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 aldebra rails.
According to human Martill, we're looking at an evolutionary do over.
The flightless birds that died out when the atoll went
under had descended from an ancestral stock of high soaring rails.

(04:50):
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, and that's iterative evolution and a nutshell.
Iterative evolution can be defined as the repeated evolution of
a specific trait or body plan from the same ancestral

(05:12):
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 cutcase of iterative evolution. Consider the ammonites,

(05:35):
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 lent themselves
two deep areas far offshore. So there's evidence that in

(05:56):
certain parts of the world an ancestral stock of thick
shelled amonites 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

(06:20):
evolution might also explain the repeated rise and fall of
similar looking sea cows over the past twenty six million years. Likewise,
sea turtles, specifically the ones with seagrass centered diets, may
have undergone the 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, it

(06:41):
can at least produce a good imitation. Today's episode is
based on article iterative evolution, Did the Aldebra rail Evolve Twice?
On how Stuffworks dot Com? Written by Mark Mancini.

Speaker 2 (06:57):
Brain Stuff is production of iHeart Radio and partnership book
HowStuffWorks dot Com, and it's produced by Tyler Klang. Four
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