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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.

nSometimes nature runs evolutionary experiments of its own without any help from humans whatsoever. nIn 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. nFor the first five months of the years it is hot and rainy, followed by a cool, dry period. nBut 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. fontis 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.

关于natural selection：Darwin 的natural selection 本来需要long time evolution 来验证，本文则是用了两个relatively short evolution's examples 来support natural

selection。

开头，达尔文认为自然选择are too slow for people to witness. 首先说达尔文提出由于物种进化需要很长的时间，因此不可能会被人类观察到。但是最近的一些发现却说明某些物种进化时间很短，可以被科学家所观察。然后，提出了一个关于环境与物种生存周期的假说，还说某个科学家研究一种鱼类证明了这个假说的合理性。

自然选择对生物的影响（与达尔文的不同）两个例子，guppy 在predator 多和少池塘里生活，大小不同（offspring 的大小多少）和一种鸟在小岛上适应干旱( large &small) 讲Darwin 的nature selection，开始讲到生物的自然演变要经历很长的时间，这种限制是Darwin当时不能用experiment 证明nature selection 的原因（第一题考），然后讲了现代生物学家发现在短期可以观察到nature selection 给动物的一些特性带来的变化。

1st 一种鱼，人为控制条件。有一种鱼，在predator 多的时候，life-span, size, mate, reproduction 都有变化，为什么变化。然后，再将一部分放入predator（掠食者）少

的pool 中，offspring(后代)发生了很多changes，比如比它们的ancestor（祖先） size 上要大，下egg 少了，等等。整个实验耗时11y；举例时用guppy 这种鱼，他们生活在下游时由于predator 很多，所以繁殖的速度很快；但当科学家把他们带到上游生活（那里的predator 很少），他们的繁殖的速度明显变慢了。这种鱼，放在下游，还有好多predator，所以这些鱼的繁殖速度加快，baby与鱼比以前大，质量差，畸形多，所以就不会超负荷。

而且长得快。放在上游，并且有很少predator，它们的繁殖速度就慢，而且baby 鱼不大并且生长速度慢。

2ed 一种鸟，自然条件变化。我们需要做的只是observe（观察）。讲的是finch（雀类），有关它们beak（ 喙）的大小的。cause 是lethal drought（致命干旱）。举了一个bird 的例子，有的嘴大有的嘴小。最小的就只能吃小的seed/nut。有一段时间干旱，种子大，死了好多鸟，可调查研究发现，死的鸟是有规律的。死的大多是小嘴的鸟。后来天气恢复正常之后，发现那些后代鸟比以前的嘴大了。雨量充沛的话，鸟的嘴又比以前小了。当干旱（drought）时那些站着长而硬的beak 的bird 往往能生存下来（这里考了个生词decimation＝destruction），这样一来这一地区的这种bird 很快都有了长而硬的beak

 (1) According to paragraph 1, compared to the views of modern biologists, Darwin's view of natural selection was limited by his belief that

答案：(A) natural selection only occurred over a very long period of time IS- natural selection could only be witnessed in some species

(2)The word "witness" in the passage is closest in meaning to

答案：(A) observe

(3)The word "offset" in the passage in meaning to

答案：(D) balanced

(4)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

答案：(A) They will product more offspring.

(5)What is the author's primary in presenting the information in paragraph 3

答案：(C) To explain Reznick's reasoning about why it would be an advantage for guppies with predators to mature quickly

(6)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

答案：(B) More offspring were produced.

(7)The word "lethal" in the passage is closest in meaning to

答案：(C) deadly

(8)The word "decimation" in the passage is closest in meaning to

答案：(A) destruction

(9)According to paragraph 6, what disadvantage did small-beaked finches have compared to large-beaked fiches

答案：(A) Small-beaked finches were not able to crack open the large seeds.

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

答案：(B) the size of their beaks

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

答案：(A) Short-term changes result in quick reversals of the trait selected.

(12)How were the results of the natural experiment with the finches similar to the results of Reznick's experiment with the guppies

答案：(B) In both cases, evolutionary changes occurred in a relatively short period of time.

(13)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.

答案：(B) 第二个方块。

(14)总结题。

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.