Placeholder Image

字幕列表 影片播放

  • [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.

  • "Oh."

  • [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.

  • [CARROLL:] Oh.

  • 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.

  • [chirping]

  • Whereas the medium ground finch sounds very much

  • like this.

  • [chirping]

  • [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.

[NARRATOR:] Our planet has millions of species.

字幕與單字

單字即點即查 點擊單字可以查詢單字解釋

B2 中高級 美國腔

加拉帕戈斯雀的演化--HHMI生物交互式視頻演示 (Galapagos Finch Evolution — HHMI BioInteractive Video)

  • 69 3
    kevin 發佈於 2021 年 01 月 14 日
影片單字