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  The Pace of Evolutionary Change

  A heated debate has enlivened recent studiesof evolution.Darwin's original thesis,and theviewpoint supported by evolutionary gradualists,isthat species change continuously but slowly and insmall increments.Such changes are all but invisible over the short time scale of modernobservations,and,it is argued,they are usually obscured by innumerable gaps in theimperfect fossil record.Gradualism,with its stress on the slow pace of change,is a comfortingposition,repeated over and over again in generations of textbooks.By the early twentiethcentury,the question about the rate of evolution had been answered in favor of gradualism tomost biologists'satisfaction.


  Sometimes a closed question must be reopened as new evidence or new arguments basedon old evidence come to light.In 1972 paleontologist Stephen Jay Gould and Niles Eldredgechallenged conventional wisdom with an opposing viewpoint,the punctuated equilibriumhypothesis,which posits that species give rise to new species in relatively sudden bursts,without a lengthy transition period.These episodes of rapid evolution are separated byrelatively long static spans during which a species may hardly change at all.

  有时,已经有了结论的问题必须由在已有证据基础上出现的新的证据和新的论点使其重新展开讨论。在1972年,古生物学者Stephen Jay Gould和Niles Eldredge用相反的论点挑战了世俗的结论,即断点平衡说,它假设了物种演变为新的物种是通过相对突然的爆发,并非通过长时间的过渡时期。迅速的进化期被时间相对更长的静态期分开,而在静态时期,物种是几乎完全不变的。

  The punctuated equilibrium hypothesis attempts to explain a curious feature of thefossil record—one that has been familiar to paleontologist for more than a century but hasusually been ignored.Many species appear to remain unchanged in the fossil record for millionsof years—a situation that seems to be at odds with Darwin's model of continuous change.Intermediated fossil forms,predicted by gradualism,are typically lacking.In most localities agiven species of clam or coral persists essentially unchanged throughout a thick formationof rock,only to be replaced suddenly by a new and different species.


  The evolution of North American horse,which was once presented as a classic textbookexample of gradual evolution,is now providing equally compelling evidence for punctuatedequilibrium.A convincing 50-million-year sequence of modern horse ancestors—eachslightly larger,with more complex teeth,a longer face,and a more prominent central toe—seemed to provide strong support for Darwin's contention that species evolve gradually.But close examination of those fossil deposits now reveals a somewhat different story.Horsesevolved in discrete steps,each of which persisted almost unchanged for millions of years andwas eventually replaced by a distinctive newer model.The four-toed Eohippus preceded thethree-toed Miohippus,for example,but North American fossil evidence suggests a jerky,uneven transition between the two.If evolution had been a continuous,gradual process,one might expect that almost every fossil specimen would be slightly different from every year.


  If it seems difficult to conceive how major changes could occur rapidly,consider this:analteration of a single gene in files is enough to turn a normal fly with a single pair of wings intoone that has two pairs of wings.


  The question about the rate of evolution must now be turned around:does evolutionever proceed gradually,or does it always occur in short bursts?Detailed field studies of thickrock formations containing fossils provide the best potential tests of the competing theories.


  Occasionally,a sequence of fossil-rich layers of rock permits a comprehensive look atone type of organism over a long period of time.For example,Peter Sheldon's studies oftrilobites,a now extinct marine animal with a segmented body,offer a detailed glimpse intothree million years of evolution in one marine environment.In that study,each of eightdifferent trilobite species was observed to undergo a gradual change in the number ofsegments—typically an increase of one or two segments over the whole time interval.Nosignificant discontinuous were observed,leading Sheldon to conclude that environmentalconditions were quite stable during the period he examined.

  偶尔,有一个系列的化石丰富的岩石可以允许人们综合性的观察一种生物在很长一段时间中的变化。比如,Peter Sheldon对于三叶虫,一种已灭绝的身体分节的海洋生物,的研究提供了其对三百万年来在同一海洋环境下进化的一些细节。研究中,八种三叶虫都观察到了其身体节数数量逐渐改变的过程,在整个时间段中,一般身体都增加了一到两节。没有明显的不连贯,这使sheldon得出结论:海洋环境在那段时间是比较稳定的。

  Similar exhaustive studies are required for many different kinds of organisms from manydifferent periods.Most researchers expect to find that both modes of transition from onespecies to another are at work in evolution.Slow,continuous change may be the normduring periods of environmental stability,while rapid evolution of new species occurs duringperiods of environment stress.But a lot more studies like Sheldon's are needed before we cansay for sure.



  1.The word"innumerable"in the passage isclosest in the meaning to






  2.According to paragraph 1,all of the following are true EXCEPT

  A.Darwin saw evolutionary change as happening slowly and gradually.

  B.Gaps in the fossil record were used to explain why it is difficult to see continuous smallchanges in the evolution of species.

  C.Darwin's evolutionary thesis was rejected because small changes could not be observed inthe evolutionary record.

  D.By the early twentieth century,most biologists believed that gradualism explainedevolutionary change.


  3.Which of the sentences below best expresses the essential information in thehighlighted sentence in the passage paragraph 2?Incorrect choices change themeaning in important ways or leave out essential information.

  A.The punctuated equilibrium hypothesis challenged gradualism,which holds that speciesevolve in relatively sudden bursts of brief duration.

  B.The punctuated equilibrium hypothesis developed by Stephen Jay Gould and NilesEldredge was challenged in 1972.C.In 1972 Stephen Jay Gould and Niles Eldredge challengedgradualism by positing that change from one species to another cannot occur without alengthy transition period.

  D.The punctuate equilibrium hypothesis,in opposition to gradualism,holds that transitionsfrom one species to another occur in comparatively sudden burst.

  3.高亮句子的主干部分是断点平衡论挑战了原来的渐进论,然后解释了断点平衡论的内容。A选项which修饰不明,容易产生误解;B选项与原文矛盾,C与原文不符,原文S和N的观点是change的发生是withoutlengthy transition的。D和原文意思相符,并且也包含了所有的主干部分。

  4.According to paragraph 1 and paragraph 2,the punctuated equilibrium hypothesisand the gradualism hypothesis differed about

  A.Whether the fossil record is complete.

  B.Whether all species undergo change.

  C.Whether evolution proceeds an a constant rate.

  D.How many new species occur over long periods of time.


  5.According to paragraph 3,the lack of intermediate fossils in the fossil record ofsome species

  A.has been extensively studied by paleontologist for over a century.

  B.contradicts the idea that most species have remained unchanged for millions of years.

  C.challenges the view that evolutionary change is gradual.

  D.is most common in the fossil records of clam and coral species.

  5.根据lack of intermediate fossils定位到第三段倒数第二句,在往前看一句说,这一情况对于达尔文学说是不和的,而达尔文学说正是渐进论,这一段的最后也说原来的物种突然被替换,而不是渐渐进化改变的。所以C符合原文意思。A与原文第一句破折号后矛盾,原文说这一现象一直被Ignored,B选项原文矛盾,该现象应该是反对了达尔文学说,支持了物种有长期不改变的论点。D虽然正确但是只是本段的细节,不能表达主题,所以不选。

  6.The word"compelling"in the passage paragraph 4 is closest in the meaning to





  6.Compel本身是强迫,此处作为evidence的形容词可以延伸为说服力强的。因此答案选B有说服力的。A是惊奇的,C是有争议的,D是细节的。都不沾边。另外通过句义可以判断,前文说马的进化was once是经典的渐进论的证明,is now(轻微转折)提了"equally"怎么样的证据证明了点断平衡论,前文既然说对gradualevolution很支持,那么后文出现equally,那应该对点断论也是有力证据。因此选B。

  7.Paragraph 4 mentions that North American horses have changed in all the followingways EXCEPT in

  A.the number of toes they have.

  B.the length of their face.

  C.their overall size.

  D.the number of years they live.


  8.The word"alteration"in the passage paragraph 5 is closest in meaning to






  9.According to paragraph 7,Peter Sheldon's studies demonstrated which ofthefollowing about trilobites?

  A.They underwent gradual change over a long time period.

  B.They experienced a number of discontinuous transitions during their history.

  C.They remained unchanged during a long period of environmental stability.

  D.They evolved in ways that cannot be counted for by either of the two competing theories.

  9.根据Peter Sheldon定位到原文第二句。后面对trilobites进行了描述,A选项对应了原文第三句,后文又继续说没有明显的断点。所以A正确。B与原文第四句冲突。C与第三句冲突,D原文没有这种说法。而且上文也表明它符合gradual evolvement。

  10.The word"occasionally"in the passage paragraph 7 is closest in meaning to



  C.once in a while.

  D.to some extent.


  11.The main purpose of paragraph 7 is to

  A.Describe one test of the competing theories.

  B.Provide an example of punctuated equilibrium.

  C.Describe how segmented animals evidence both competing theories.

  D.Explain why trilobites became extinct.

  11.上文说实地勘测会给competing theories提供test,第七段就给了试验例子,那么就证明是A正确。B与原文冲突,这个例子是支持渐进论的。C也不对,理由同上。D与原文目的不符。

  12.Look at the four squares[■]that indicate where the following sentence can beadded to the passage.Where could the sentence best fit?They believe thatenvironmental conditions may play a crucial role in determining which of the twomodes will be in operation over a given period.

  ■【A】Similar exhaustive studies are required for many different kinds of organisms frommany different periods.■【B】Most researchers expect to find that both modes of transitionfrom one species to another are at work in evolution.■【C】Slow,continuous change may bethe norm during periods of environmental stability,while rapid evolution of new speciesoccurs during periods of environment stress.■【D】But a lot more studies like Sheldon's areneeded before we can say for sure.


  13.Directions:selected from the seven phrases below the phrases that correctlycharacterize punctuated equilibrium and the phrases that correctly characterizegradualism.Two of the phrases will not be used.This question is worth 3 points.

  A.States that new species emerge from existing species during relatively brief period of time.

  B.Was first formulated by Charles Darwin.

  C.Explain why North American horses have become smaller over time.

  D.States that new species evolve slowly and continuously from existing species.

  E.Explain the lack of intermediate fossil forms in the fossil record of many species.

  F.Competition is usually strongest when the density of the competing populations is the same.

  G.States that a species will not change unless its environment changes.



  A B C D E F G


  punctuated equilibrium

  A B C D E F G