A heateddebatehas enlivened recent studies ofevolution. Darwin' soriginalthesis, and the viewpoint supported byevolutionarygradualists, is thatspecieschange continuously but slowly and in small increments. Such changes are all butinvisibleover the short timescaleof modern observations, and, it is argued, they are usually obscured byinnumerablegaps in the imperfectfossilrecord. Gradualism, with itsstresson the slow pace of change, is a comfortingposition, repeated over and over again in generations of textbooks. By the early twentiethcentury, the question about therateofevolutionhad been answered infavorof gradualism to most biologists' satisfaction.

Sometimes a closed question must be reopened as newevidenceor new arguments based on oldevidencecome to light. In 1972paleontologistStephen Jay Gould and Niles Eldredge challengedconventionalwisdom with an opposing viewpoint, the punctuatedequilibriumhypothesis, which posits thatspeciesgive rise to newspeciesinrelativelysudden bursts, without alengthytransitionperiod. These episodes of rapidevolutionare separated byrelativelylongstaticspans during which aspeciesmayhardlychange at all.

The punctuatedequilibriumhypothesisattempts to explain acuriousfeature of thefossilrecord--- one that has beenfamiliartopaleontologistfor more than acenturybut has usually been ignored. Manyspeciesappeartoremainunchanged in thefossilrecordfor millions of years --- a situation that seems to be at odds with Darwin' s model ofcontinuouschange. Intermediatedfossilforms, predicted by gradualism, are typically lacking. In most localities agivenspeciesofclamorcoralpersistsessentiallyunchangedthroughouta thickformationof rock, only to be replaced suddenly by a new and differentspecies.

Theevolutionof North American horse, which was once presented as aclassictextbook example ofgradualevolution, is now providing equallycompellingevidencefor punctuatedequilibrium. A convincing 50-million-yearsequenceof modern horse ancestors --- eachslightlylarger, with morecomplexteeth, a longer face, and a moreprominentcentral toe ---seemed to provide strong support for Darwin' s contention thatspeciesevolvegradually. But close examination of thosefossildeposits now reveals asomewhatdifferent story. Horses evolved indiscretesteps, each of which persisted almost unchanged for millions of years and waseventuallyreplaced by adistinctivenewer model. The four-toed Eohippus preceded the three-toed Miohippus, for example, but North Americanfossilevidencesuggests a jerky,uneventransitionbetween the two. Ifevolutionhad been acontinuous,gradualprocess, one mightexpectthat almost everyfossilspecimenwould beslightlydifferent from every year.

If it seems difficult toconceivehowmajorchanges couldoccurrapidly,considerthis: analterationof a singlegenein files is enough to turn a normal fly with a single pair of wings into one that has two pairs of wings.

The question about therateofevolutionmust now be turned around: doesevolutioneverproceedgradually, or does it alwaysoccurin short bursts? Detailedfieldstudies of thick rock formations containing fossils provide the bestpotentialtests of thecompetingtheories.

Occasionally , asequenceof fossil-rich layers of rock permits acomprehensivelook at one type oforganismover a long period of time. For example, Peter Sheldon' s studies of trilobites, a nowextinctmarineanimal with a segmented body,offera detailedglimpseinto three million years ofevolutionin onemarineenvironment. In that study, each of eight different trilobitespecieswas observed toundergoagradualchange in the number of segments --- typically an increase of one or two segments over the whole timeinterval. Nosignificantdiscontinuous were observed,leadingSheldon toconcludethatenvironmentalconditions were quitestableduring the period he examined.

Similarexhaustive studies are required for many different kinds of organisms from many different periods. Most researchersexpectto find that both modes oftransitionfrom onespeciesto another are at work inevolution. Slow,continuouschange may be the norm during periods ofenvironmentalstability, while rapidevolutionof newspeciesoccurs during periods of environmentstress. But a lot more studies like Sheldon' s are needed before we can say for sure.

Questions:
question_stem: The word “innumerable”in the passage is closest in the meaning to
choices: tag: A
text: countless

tag: B
text: occasional

tag: C
text: large

tag: D
text: repeated
answer_key: A
explanation: 正确答案：A。innumerable 是不可计数的，A 是无数的，B 是偶然的，C 是大的，D 是重复的。这个单词是 numerable 加否定前缀，很容易就能推出意思来。

question_stem: According to paragraph 1, all of the following are true EXCEPT
choices: tag: A
text: Darwin saw evolutionary change as happening slowly and gradually

tag: B
text: Gaps in the fossil record were used to explain why it is difficult to see continuous small changes in the evolution of species

tag: C
text: Darwin’s evolutionary thesis was rejected because small changes could not be observed in the evolutionary record

tag: D
text: By the early twentieth century, most biologists believed that gradualism explained evolutionary change
answer_key: C
explanation: 正确答案 C。A 答案对应第一段第二句， B 对应第一段第三句，D 对应一段最后一句。C 与原文冲突，原文一直在说 darwin 理论被人们广泛接受。

question_stem: Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.
choices: tag: A
text: The punctuated equilibrium hypothesis challenged gradualism, which holds that species evolve in relatively sudden bursts of brief duration.

tag: B
text: The punctuated equilibrium hypothesis developed by Stephen Jay Gould and Niles Eldredge was challenged in 1972.

tag: C
text: In 1972 Stephen Jay Gould and Niles Eldredge challenged gradualism by positing that change from one species to another cannot occur without a lengthy transition period.

tag: D
text: The punctuate equilibrium hypothesis, in opposition to gradualism, holds that transitions from one species to another occur in comparatively sudden burst.
answer_key: D
explanation: 正确答案 D。划线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 选项原文没提到 brief duration；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 without lengthy transition 的。D 和原文意思相符，并且也包含了所有的主干部分。正确，选

question_stem: According to paragraph 1 and paragraph 2, the punctuated equilibrium hypothesis and the gradualism hypothesis differed about
choices: tag: A
text: Whether the fossil record is complete

tag: B
text: Whether all species undergo change

tag: C
text: Whether evolution proceeds at a constant rate

tag: D
text: How many new species occur over long periods of time
answer_key: C
explanation: 正确答案 C。根据原文，渐进论是说物种演变是通过长时间的缓慢改变发生的；断点平衡论是说物种演变是短期爆发的。所以选择 C，进化是否是匀速发生的。

question_stem: According to paragraph 3, the lack of intermediate fossils in the fossil record of some species
choices: tag: A
text: has been extensively studied by paleontologist for over a century

tag: B
text: contradicts the idea that most species have remained unchanged for millions of years

tag: C
text: challenges the view that evolutionary change is gradual

tag: D
text: is most common in the fossil records of clam and coral species
answer_key: C
explanation: 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。

question_stem: The word “compelling”in the passage is closest in the meaning to
choices: tag: A
text: surprising

tag: B
text: persuasive

tag: C
text: controversial

tag: D
text: detailed
answer_key: B
explanation: B compelling: 引人注目的，不可抗拒的。和 B 选项的 persuasive（有说服力的）呼应。

question_stem: Paragraph 4 mentions that North American horses have changed in all the following ways EXCEPT in
choices: tag: A
text: the number of toes they have

tag: B
text: the length of their face

tag: C
text: their overall size

tag: D
text: the number of years they live
answer_key: D
explanation: 正确答案：D。对应部分在第四段的第二句和第五句，已用红色字体标出。只有 D 选项没有现。

question_stem: The word “alteration”in the passage is closest in meaning to
choices: tag: A
text: imperfection

tag: B
text: replacement

tag: C
text: change

tag: D
text: duplication
answer_key: C
explanation: 这一句话说一个怎么样的单一基因就足以变一个普通飞禽的一对的正常翅为两对翅膀，所以有推理应该是要改变这个基因。所以选 C

question_stem: According to paragraph 7, Peter Sheldon’s studies demonstrated which of the following about trilobites?
choices: tag: A
text: They underwent gradual change over a long time period

tag: B
text: They experienced a number of discontinuous transitions during their history

tag: C
text: They remained unchanged during a long period of environmental stability

tag: D
text: They evolved in ways that cannot be counted for by either of the two competing theories.
answer_key: A
explanation: A 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。

question_stem: The word “Occasionally”in the passage is closest in meaning to
choices: tag: A
text: undoubtedly

tag: B
text: basically

tag: C
text: once in a while

tag: D
text: to some extent
answer_key: C
explanation: C occasionally：偶然，偶尔。对应 C 选项的 once in a while（偶尔有一次）

question_stem: The main purpose of paragraph 7 is to
choices: tag: A
text: Describe one test of the competing theories

tag: B
text: Provide an example of punctuated equilibrium

tag: C
text: Describe how segmented animals evidence both competing theories

tag: D
text: Explain why trilobites became extinct
answer_key: A
explanation: A 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。