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[NARRATOR:] Our planet has millions of species.
Over 300,000 beetles alone.
17,000 butterflies.
Thousands of mammals, fish and birds,
all astonishingly different.
How did so many species come to be?
To seek insights into that question,
researchers are focusing on places
where species recently arose,
such as the remote Galápagos Islands.
[CARROLL:] Scientists are making observations
and conducting experiments
that would have surprised Charles Darwin.
And they're discovering new insights
into what the great naturalist called the "mystery of mysteries":
How new species form.
[NARRATOR:] The Galápagos Islands are one
of the most spectacular landscapes in the world,
home to a variety of species that live nowhere else.
Biologists Peter and Rosemary Grant have been seeking answers
to how species arise by focusing on one
of the smaller islands, called Daphne Major.
[PETER GRANT:] When we started out, we had
no plan for the long term.
In fact, we thought it was just going
to be just a few years, maybe two years.
[NARRATOR:] Two years have turned into a 40-year odyssey.
The Grants have returned every summer since 1973.
[ROSEMARY GRANT:] Oh, there's a bird.
[PETER GRANT:] Is that 306?
[ROSEMARY GRANT:] Three oh metal six.
[NARRATOR:] Here, they've made some
of the most remarkable observations in the history
of field research as they studied the famed
Galápagos finches.
The finches were first brought to scientists' attention
by Charles Darwin, when his voyage
around South America brought him to this cluster
of islands 600 miles from mainland Ecuador.
These volcanic islands are geologically young.
They began rising from the ocean floor less
than five million years ago.
At first devoid of life,
they now support a modest number of species.
Among them, 13 species of finches found
in various combinations on the different islands.
The birds live in diverse habitats.
[ROSEMARY GRANT:] The islands are very different
from each other.
They differ in size.
They differ in topography and in height.
[NARRATOR:] Larger trees grow
at higher elevations while low islands have mostly cactus,
grasses and shrubs.
In these diverse habitats,
the finches have evolved many ways to survive.
[CARROLL:] So Rosemary,
what's the important difference between these birds?
[ROSEMARY GRANT:] This little warbler finch
with its very fine needlelike beak is perfect
for picking off insects.
This one is the woodpecker finch with a rather more robust beak.
It concentrates on beetle larvae and termite larvae.
Then we have the cactus finch
with a much longer sharp pointed beak
which probes into cactus flowers.
And then these three species are the large,
medium and small ground finches.
So, Sean, a basic idea is, the beaks are tools
and you need the right tool for the right job.
[NARRATOR:] The finches look so different
that Darwin first mistook them
for entirely unrelated kinds of birds.
How did the Galápagos end up with so many species of finches?
[CARROLL:] In terms of the actual history of the finches
of the Galápagos, there were many different possibilities.
Different kinds of finches could have all come
from the mainland separately
or the finches could have all evolved
out there on the islands.
And what do we know about that?
[PETER GRANT:] Well, now we know from DNA evidence that all
of the finches are more related to each other than any one is
to a species on the mainland.
And that tells us only one species arrived
on the archipelago, and diversified into the 13 species
that we see nowadays in the Galápagos.
So they've all come from a single common ancestor.
[NARRATOR:] The question then becomes how did one ancestral
population give rise to many different species, each adapted
to a different lifestyle.
A crucial insight into how adaptation occurs came
when the Grants focused on one species
on the island of Daphne Major.
[PETER GRANT:] Factor of great convenience
for us was the small size of the island.
That meant that we could walk all over the place.
[ROSEMARY GRANT:] The idea was that if we worked really hard,
we could follow every individual or almost every individual.
[NARRATOR:] They rose at 5:30 each morning
to net the island's medium ground finches.
[NARRATOR:] They measured the size and shape
of each bird's beak, the bird's weight
and they tagged them for identification.
[NARRATOR:] Year after year they returned,
at times tracking over 1,000 finches.
[PETER GRANT:] So here's an example
of a bird we know intimately over the whole of its lifespan.
The number is 5960.
We know how many times it bred, which years it bred in,
how many mates it had, how many offspring it produced.
And then how many of those offspring themselves survived
long enough to breed.
[NARRATOR:] Over the first four years, little seemed to change.
Then in 1977 a terrible drought began.
[PETER GRANT:] Virtually no rain fell for the next 18 months.
[ROSEMARY GRANT:] The vegetation practically disappeared apart
from a few trees without any leaves.
And, of course, the cactus bushes were still there.
[NARRATOR:] Now the medium ground finches had
to compete for scarce food.
[PETER GRANT:] They started off with a big food supply
of small seeds, medium seeds, large seeds.
As these small seeds became very scarce,
they had to turn increasingly to the large and hard seeds.
Well, only birds with large beaks can crack open these
woody, spiny fruits.
[NARRATOR:] The birds with the smallest beaks had the
most trouble.
[ROSEMARY GRANT:] They were scraping
about amongst the rocks, and their plumage got so worn
that they could barely fly.
[NARRATOR:] That year, over 80 percent
of the medium ground finches died.
[PETER GRANT:] We would go around looking
for birds that had died.
And it's very sad to pick up a bird and say, "3972.
"Oh no, not that bird.
[NARRATOR:] When they inventoried the surviving medium
ground finches, they discovered
that one trait had made the greatest difference
between life and death.
[PETER GRANT:] What I'm showing here, a distribution
of beak depths of the population in 1976.
The survivors of this group are shown in black.
So the larger the beak, the better your chances?
[PETER GRANT:] The larger the beak, the higher the likelihood
of surviving through the drought of 1977.
[PETER GRANT:] 18.6 grams.
[NARRATOR:] When they looked at the offspring,
they found an even greater surprise.
The average beak depth was more than four percent larger
than the previous generation.
Natural selection had changed the average beak size.
[CARROLL:] Could you have ever imagined measuring
and observing something like this on such a short time scale
until you actually did it?
[PETER GRANT:] When we started, the answer is no.
We could not imagine we would be able to do it.
[NARRATOR:] But was this a fluke?
Or are changes like this happening all the time?
Five years later in 1983,
an unusually strong El Nino brought ten times more rain
than normal.
And the island was overrun by vines
that covered even the cactus.
The rains changed the vegetation on the island,
such that two years later, when drought struck,
larger seeds became scarce.
The birds with larger beaks now had difficulty picking
up the more abundant food:
the small seeds produced by the vines.
That year many more finches with small beaks survived,
and their offspring inherited smaller beaks.
[PETER GRANT:] So the selection had swung
in the opposite direction,
and evolution had occurred as a result.
[CARROLL:] In an amazingly short period of time,
the Grants had measured evolution of beak size,
not once, but twice, demonstrating
that when birds encounter different environments they will
change over a very short amount of time.
[NARRATOR:] Over millions of years,
changes like these occurring throughout the Galápagos
generated all sorts of beak sizes and shapes.
But that's only part of the story.
How did finches with different beaks become distinct species?
Species are defined
as populations whose members don't interbreed.
[CARROLL:] So how does one species split into two?
A typical scenario is that two populations become separated
geographically, and undergo enough change
in their respective habitats, that if or when they come
into contact again, they do not mate.
[NARRATOR:] So in the Galápagos,
the Grants asked what keeps different species
of finches from mating?
[ROSEMARY GRANT:] We were very conscious
that on any given island,
the different species sing very different songs.
This is what a cactus finch sounds like.
Whereas the medium ground finch sounds very much
like this.
[NARRATOR:] So to see if songs keep the species apart,
the Grants, and their student Laurene Ratcliffe,
played each species' songs through a loudspeaker.
[ROSEMARY GRANT:] When we played back the cactus finch song,
cactus finch came to the loudspeaker
and the medium ground finch completely ignored it.
[NARRATOR:] The males only responded
to songs of their own species.
The Grants looked at whether finches might also choose mates
based on appearance.
So they put out stuffed female specimens to see
if males would respond.
[ROSEMARY GRANT:] Clearly they could discriminate.
The male vigorously courted a female of his own species;
completely ignored the other one.
[NARRATOR:] The males only courted females
that had a similar size and similar beak.
Song and appearance both play a role
in keeping different species from mating.
So when populations of the same species are separated,
changes in these traits set the stage
for the formation of new species.
The Grants have shown that both geography and ecology are keys
to the evolution of the Galápagos finches.
The most likely scenario is that, two million years ago,
a single finch population arrived from the mainland.
When their descendants reached another island,
they faced new conditions.
As those isolated populations adapted
to their surroundings, their traits changed.
If the changes included traits involved in mating,
and the populations came
into contact again, they no longer mated.
They had become distinct species.
While unique to these remote islands,
the history of the Galápagos finches offers a general insight
into why the world is populated with so many species.
[PETER GRANT:] The more diverse the environment,
the more opportunities for evolutionary change
to produce those new species.
[NARRATOR:] Over 150 years
after Darwin first recognized their significance,
these unassuming birds still illuminate how the great
diversity of life arose and continues to evolve.


Galapagos Finch Evolution — HHMI BioInteractive Video

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kevin 2015 年 3 月 2 日 に公開
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