[CRICKETS]
[FEET PATTERING]
[SWISH]
[BING]
[SPLASH]
[MUSIC PLAYING]
[CROAKING]
SEAN B. CARROLL: 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 Galapagos
Islands.
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.
[MUSIC PLAYING]
[BIRDS CAWING]
The Galapagos 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.
SEAN B. CARROLL: 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: OK, that's--
PETER GRANT: 306.
ROSEMARY GRANT: 3-0, Metal 6.
SEAN B. CARROLL: Here they've made
some of the most remarkable observations
in the history of field research as they studied
the famed Galapagos finches.
[BIRDS CAWING]
[MUSIC PLAYING]
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 5 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.
SEAN B. CARROLL: 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.
So, Rosemary, what's the important difference
between these birds?
ROSEMARY GRANT: This little warbler finch,
with its very fine, needle-like 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 a 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, the basic idea is the beaks are tools,
and you need the right tool for the right job.
[BIRDS CHIRPING]
[MUSIC PLAYING]
SEAN B. CARROLL: The finches look
so different that Darwin first mistook them
for entirely unrelated kinds of birds.
How did the Galapagos end up with so many species
of finches?
In terms of the actual history of the finches
of the Galapagos, there were many different possibilities.
Different times, those 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 Galapagos.
So they've all come from a single common ancestor.
SEAN B. CARROLL: 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: A factor a great convenience for us
was the small size of the island.
[BIRDS CLUCKING]
That meant that we could walk all over the place.
ROSEMARY GRANT: Oh, there's a bird.
PETER GRANT: I'll leave that one to you.
ROSEMARY GRANT: The idea was that if we worked really hard,
we could follow every individual or almost every individual.
SEAN B. CARROLL: They rose at 5:30 each morning
to net the island's medium ground finches.
They measured the size and shape of each bird's beak, the birds
weight, and they tagged them for identification.
PETER GRANT: The male is in 17418.
SEAN B. CARROLL: 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 survive long enough to breed.
[MUSIC PLAYING]
SEAN B. CARROLL: 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.
SEAN B. CARROLL: 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.
But only birds with large beaks can crack open
these woody, spiny fruits.
SEAN B. CARROLL: 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.
SEAN B. CARROLL: That year, over 80%
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.
Oh!"
SEAN B. CARROLL: 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.
SEAN B. CARROLL: 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.
[MUSIC PLAYING]
18.6 grams.
SEAN B. CARROLL: When they looked at the offspring,
they found an even greater surprise.
The average beak depth was more than 4%
larger than the previous generation.
Natural selection had changed the average beak size.
Could you have ever imagined measuring and observing
something like this on such a short timescale,
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.
[MUSIC PLAYING]
SEAN B. CARROLL: But was this a fluke or are changes like this
happening all the time?
[THUNDER RUMBLING]
Five years later, in 1983, an unusually strong El Nino
brought 10 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.
SEAN B. CARROLL: In an amazingly short period of time,
the Grants had measured evolution of beak size
not once, but twice, demonstrating
that when birds encountered different environments
they will change over a very short amount of time.
[MUSIC PLAYING]
Over millions of years, changes like these occurring
throughout the Galapagos 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.
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.
[MUSIC PLAYING]
So in the Galapagos, the Grants asked,
what keeps different species of finches from mating?
[BIRDS CHIRPING]
ROSEMARY GRANT: We were very conscious
that, on any given island, the different species
sang very different songs.
This is what a cactus finch sounds like--
[CACTUS FINCH CHIRPING]
--whereas the medium ground finch sounds very much like
this--
[MEDIUM GROUND FINCH CHIRPING]
[MUSIC PLAYING]
SEAN B. CARROLL: So to see if songs keep the species apart,
the Grants and their student, Laureen Ratcliffe,
played each species' songs through a loudspeaker.
[BIRDS CHIRPING]
ROSEMARY GRANT: When we played back the cactus finch song,
cactus finch came to the loudspeaker
and the medium ground finch completely ignored it.
SEAN B. CARROLL: 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.
SEAN B. CARROLL: 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.
[MUSIC PLAYING]
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 Galapagos 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 Galapagos 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.
[BIRDS CAWING]
SEAN B. CARROLL: 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.
[MUSIC PLAYING]