Holly老師的英語新天地

Observing Natural Selection

Witnessing natural selection would not have seemed possible to Darwin because he assumed natural selection was too slow and gradual for our short-term minds to perceive. Yet later biologists have been able to witness flashes of evolutionary change. In the late 1980s, for example, biologist David Reznick began to use the guppies that swim in the streams of Trinidad forests in natural experiment. At lower elevations these guppies face the assault of predatory fishes, but the ones in higher waters live in peace because few of the predators can move upstream past the waterfalls and craggy rocks.

Like all animals, guppies have a timetable for their lives - how long they take to reach sexual maturity how fast they grow during that time, how long they live as adults. Theoretical biologists have predicted that the life history of animals can evolve if mutations that alter it bring the animals more reproductive success Reznick put their predictions to the test.

In ponds with a lot of predators, guppies that grow fast should be more successful than slow-growing ones. With the threat of death hanging over a guppy, it will grow as quickly as possible so that it can start mating as soon as possible and have as many offspring as possible. Of course, the strategy comes with a heavy price. By growing so quickly, a guppy may shorten its own natural life span, and by quickly giving birth to babies, female guppy cannot take time to support her offspring with energy, which put them at risk of dying young. But Reznick reasoned, that the threat ,of an early death offset by other risk.

To see whether this trade-off was real, Reznick relocated guppies that were being terrorized in the downstream by putting them in pools with relatively few predators. Eleven years in these conditions produced guppies that were, on average, in less of a rush. They took 10 percent longer to mature than their ancestors and were over 10 percent heavier by the time they were fully grown They were also laying smaller broods of eggs, but each of the new guppies that hatched from those was bigger.

■Sometimes nature runs evolutionary experiments of its own without any help from humans whatsoever. ■In 1973 Peter and Rosemary Grant, husband-and-wife biologists, arrived on the Galapagos Islands to study the effects of natural selection on the birds. Most years on the Galapagos, the weather follows a standard pattern. ■For the first five months of the years it is hot and rainy, followed by a cool, dry period. ■But in 1977 the wet season never came. A periodic disturbance of the Pacific ocean called La Nina altered weather patterns over the Galapagos, causing a disastrous drought.

On Daphne Island, where the Grants worked, the drought was lethal. Out of the 1,200 medium ground finches Geospiza fortis that lived on the Island, more that 1,000 died. But the Grants discovered that the decimation was not random. G fortis lives mainly on seeds, which it cracks with its strong beak. Small G. fortis can break only small seeds, but larger birds have beaks that are strong enough to break big ones. The drought had lingered for a few months, the small finches ran out of small seeds and began dying off. But the big finches managed to survive, because they could eat seeds that the smaller ones couldn’t get to. (In particular, they depended on a plant called caltrop, which grows spiked shells to protect it seeds).

The survivors on the 1977 drought mated in 1978, and the Grants could see evolution’s mark on their offspring. A new generation of G. fortis was born, and the Grants’ student Peter Boag discovered that, on average, their beaks were 4 percent larger than those of the previous generation. The big-beaked finches, which had fared better during the drought had put their trait to their offspring, altered the profile of the population.

In the years since the drought the finches changed. In 1983, for example, there was a season of heavy rain, abundant seeds favored finches with smaller beaks and Grants found that by 1985 their average size had dropper 5 percent. The finches can change quickly, but it seems that are swinging back and forth like a pendulum.

關於natural selection，Darwin認為是長期且漸進的。後來，科學家卻在短時間內目擊witness（辭彙題）到進化。接者用兩個relatively short evolution’s examples來support natural selection。

1^st，人為空制條件。有一種魚guppy，在predator多的時候，長的快，淡代價是life-span變短。不過研究人於認為這代價被其他危險抵銷offset（辭彙題）了。

2^nd，自然條件變化。我們需要做的事只是observe。講的是finch，有關他們beak 的大小的。又舉了一個bird的例子。一對夫婦學者在一個群島研究一種雀鳥，有依年島上發生致命的lethal（辭彙題）乾旱，他們發現，當乾旱（drought）時，鳥的大量死亡decimation（辭彙題）並不是隨機的。那些有大而硬的beak的bird往往能生存下來，這樣一來這一地區的這種bird很快都有了大而硬的beak。乾旱的時候小種子少，嘴小的鳥死了，嘴長的鳥存活，存活的鳥第二年的鳥交配，後代的鳥嘴整體變長。雨量又充足的時候，有足夠小種子，這個時候，新出生的鳥嘴又變短了。

Q1. According to paragraph 1, compared to the views of modern biologists, Darwin’s view of natural selection was limited by his belief that

A1. natural selection only occurred over a very long period of time IS- natural selection could only be witnessed in some species

Q2. The word “witness” in the passage is closest in meaning to

A2. observe

Q3. The word “offset” in the passage in meaning to

A3. balanced

Q4. Which of the following reason is mentioned in paragraph 3 to explain why growing fast is an advantage for guppies that live in ponds with many predators?

A4. They will product more offspring.

Q5. What is the author’s primary in presenting the information in paragraph 3?

A5. To explain Reznick’s reasoning about why it would be an advantage for guppies with predators to mature quickly

Q6. All of the following are mentioned in paragraph 4 as change in the guppy population eleven years after guppies were transferred to upstream ponds EXCEPT

A6. More offspring were produced.

Q7. The word “lethal” in the passage is closest in meaning to

A7. deadly

Q8. The word “decimation” in the passage is closest in meaning to

A8. destruction

Q9. According to paragraph 6, what disadvantage did small-beaked finches have compared to large-beaked fiches

A9. Small-beaked finches were not able to crack open the large seeds.

Q10. According to paragraph 7, the new generation of G. fortis on average differed from earlier population in

A10. the size of their beaks

Q11. According to paragraph 8, how short-term changes to the environment affect the r natural election of traits in G fortis?

A11. Short-term changes result in quick reversals of the trait selected.

Q12. How were the results of the natural experiment with the finches similar to the results of Reznick’s experiment with the guppies?

A12. In both cases, evolutionary changes occurred in a relatively short period of time.

Q13. Look at the four squares [■] that indicate where the following sentence could be added to the passage.

In such case, if biologists are lucky enough to be present, they simply have to observe.

A13. (B)

Q14. 總結題

Evolutionary changes resulting from changes in the environment can occur in relatively short period on time.

Guppies taken from upstream pools and precede in downstream pools produced offspring that had significantly shorter life spans.

The survival of finches with big beaks during a severe drought in Galapagos resulted in more big beaks finches in the next generation.