A heated debate has enlivened recent studies of evolution. Darwin' s original thesis, and the viewpoint supported by evolutionary gradualists, is that species change continuously but slowly and in small increments. Such changes are all but invisible over the short time scale of modern observations, and, it is argued, they are usually obscured by innumerable gaps in the imperfect fossil record. Gradualism, with its stress on the slow pace of change, is a comforting position, repeated over and over again in generations of textbooks. By the early twentieth century, the question about the rate of evolution had been answered in favor of gradualism to most biologists' satisfaction.
Sometimes a closed question must be reopened as new evidence or new arguments based on old evidence come to light. In 1972 paleontologist Stephen Jay Gould and Niles Eldredge challenged conventional wisdom with an opposing viewpoint, the punctuated equilibrium hypothesis, 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 by relatively long static spans during which a species may hardly change at all.
The punctuated equilibrium hypothesis attempts to explain a curious feature of the fossil record --- one that has been familiar to paleontologist for more than a century but has usually been ignored. Many species appear to remain unchanged in the fossil record for millions of 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 a given species of clam or coral persists essentially unchanged throughout a thick formation of 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 textbook example of gradual evolution, is now providing equally compelling evidence for punctuated equilibrium. A convincing 50-million-year sequence of modern horse ancestors --- each slightly 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. Horses evolved in discrete steps, each of which persisted almost unchanged for millions of years and was eventually replaced by a distinctive newer model. The four-toed Eohippus preceded the three-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: an alteration of a single gene in 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 the rate of evolution must now be turned around: does evolution ever proceed gradually, or does it always occur in short bursts? Detailed field studies of thick rock 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 at one type of organism over a long period of time. For example, Peter Sheldon' s studies of trilobites, a now extinct marine animal with a segmented body, offer a detailed glimpse into three million years of evolution in one marine environment. In that study, each of eight different trilobite species was observed to undergo a gradual change in the number of segments --- typically an increase of one or two segments over the whole time interval. No significant discontinuous were observed, leading Sheldon to conclude that environmental conditions were quite stable during the period he examined.
Similar exhaustive studies are required for many different kinds of organisms from many different periods. Most researchers expect to find that both modes of transition from one species to another are at work in evolution. Slow, continuous change may be the norm during periods of environmental stability, while rapid evolution of new species occurs during periods of environment stress. But a lot more studies like Sheldon' s are needed before we can say for sure.
Paragraph 1: A heated debate has enlivened recent studies of evolution. Darwin' s original thesis, and the viewpoint supported by evolutionary gradualists, is that species change continuously but slowly and in small increments. Such changes are all but invisible over the short time scale of modern observations, and, it is argued, they are usually obscured by innumerable gaps in the imperfect fossil record. Gradualism, with its stress on the slow pace of change, is a comforting position, repeated over and over again in generations of textbooks. By the early twentieth century, the question about the rate of evolution had been answered in favor of gradualism to most biologists' satisfaction.
1. The word “innumerable” in the passage is closest in the meaning to
O countless
O occasional
O large
O repeated
2. According to paragraph 1, all of the following are true EXCEPT
O Darwin saw evolutionary change as happening slowly and gradually
O Gaps in the fossil record were used to explain why it is difficult to see continuous small changes in the evolution of species
O Darwin’s evolutionary thesis was rejected because small changes could not be observed in the evolutionary record
O By the early twentieth century, most biologists believed that gradualism explained evolutionary change
Paragraph 2: Sometimes a closed question must be reopened as new evidence or new arguments based on old evidence come to light. In 1972 paleontologist Stephen Jay Gould and Niles Eldredge challenged conventional wisdom with an opposing viewpoint, the punctuated equilibrium hypothesis, 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 by relatively long static spans during which a species may hardly change at all.
3. 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.
O The punctuated equilibrium hypothesis challenged gradualism, which holds that species evolve in relatively sudden bursts of brief duration.
O The punctuated equilibrium hypothesis developed by Stephen Jay Gould and Niles Eldredge was challenged in 1972.
O 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.
O The punctuate equilibrium hypothesis, in opposition to gradualism, holds that transitions from one species to another occur in comparatively sudden burst.
4. According to paragraph 1 and paragraph 2, the punctuated equilibrium hypothesis and the gradualism hypothesis differed about
O Whether the fossil record is complete
O Whether all species undergo change
O Whether evolution proceeds an a constant rate
O How many new species occur over long periods of time
Paragraph 3: The punctuated equilibrium hypothesis attempts to explain a curious feature of the fossil record --- one that has been familiar to paleontologist for more than a century but has usually been ignored. Many species appear to remain unchanged in the fossil record for millions of 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 a given species of clam or coral persists essentially unchanged throughout a thick formation of rock, only to be replaced suddenly by a new and different species.
5. According to paragraph 3, the lack of intermediate fossils in the fossil record of some species
O has been extensively studied by paleontologist for over a century
O contradicts the idea that most species have remained unchanged for millions of years
O challenges the view that evolutionary change is gradual
O is most common in the fossil records of clam and coral species
Paragraph 4: The evolution of North American horse, which was once presented as a classic textbook example of gradual evolution, is now providing equally compelling evidence for punctuated equilibrium. A convincing 50-million-year sequence of modern horse ancestors --- each slightly 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. Horses evolved in discrete steps, each of which persisted almost unchanged for millions of years and was eventually replaced by a distinctive newer model. The four-toed Eohippus preceded the three-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.
6. The word “compelling” in the passage is closest in the meaning to
O surprising
O persuasive
O controversial
O detailed
7. Paragraph 4 mentions that North American horses have changed in all the following ways EXCEPT in
O the number of toes they have
O the length of their face
O their overall size
O the number of years they live
Paragraph 5: If it seems difficult to conceive how major changes could occur rapidly, consider this: an alteration of a single gene in files is enough to turn a normal fly with a single pair of wings into one that has two pairs of wings.
8. The word “alteration” in the passage is closest in meaning to
O imperfection
O replacement
O change
O duplication
Paragraph 7: Occasionally , a sequence of fossil-rich layers of rock permits a comprehensive look at one type of organism over a long period of time. For example, Peter Sheldon' s studies of trilobites, a now extinct marine animal with a segmented body, offer a detailed glimpse into three million years of evolution in one marine environment. In that study, each of eight different trilobite species was observed to undergo a gradual change in the number of segments --- typically an increase of one or two segments over the whole time interval. No significant discontinuous were observed, leading Sheldon to conclude that environmental conditions were quite stable during the period he examined.
9. According to paragraph 7, Peter Sheldon’s studies demonstrated which of the following about trilobites?
O They underwent gradual change over a long time period
O They experienced a number of discontinuous transitions during their history
O They remained unchanged during a long period of environmental stability
O They evolved in ways that cannot be counted for by either of the two competing theories.
10. The word “occasionally” in the passage is closest in meaning to
O undoubtedly
O basically
O once in a while
O to some extent
11. The main purpose of paragraph 7 is to
O Describe one test of the competing theories
O Provide an example of punctuated equilibrium
O Describe how segmented animals evidence both competing theories
O Explain why trilobites became extinct
Paragraph 8: ■Similar exhaustive studies are required for many different kinds of organisms from many different periods. ■Most researchers expect to find that both modes of transition from one species to another are at work in evolution. ■Slow, continuous change may be the norm during periods of environmental stability, while rapid evolution of new species occurs during periods of environment stress. ■But a lot more studies like Sheldon' s are needed before we can say for sure.
12. Look at the four squares [■] that indicate where the following sentence can be added to the passage.
They believe that environmental conditions may play a crucial role in determining which of the two modes will be in operation over a given period.
Where could the sentence best fit?
13. Directions: selected from the seven phrases below the phrases that correctly characterize punctuated equilibrium and the phrases that correctly characterize gradualism. Two of the phrases will not be used. This question is worth 3 points.
Gradualism punctuated equilibrium
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Answer Choices
O States that new species emerge from existing species during relatively brief period of time
O Was first formulated by Charles Darwin
O Explain why North American horses have become smaller over time
O States that new species evolve slowly and continuously from existing species
O Explain the lack of intermediate fossil forms in the fossil record of many species
O Competition is usually strongest when the density of the competing populations is the same
O States that a species will not change unless its environment changes
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最近的一個關(guān)于進(jìn)化的研究引發(fā)了激烈的爭論。達(dá)爾文的原始論點(diǎn)和進(jìn)化漸進(jìn)主義者支持的觀點(diǎn)是物種會持續(xù)地改變，但非常緩慢，增量也很小。這種改變是普遍的，但是現(xiàn)在短時間的觀察是不能察覺的，并且，這個觀點(diǎn)聲稱，它們通常被掩蓋于不完美的化石記錄的不可計(jì)數(shù)的缺失中。漸進(jìn)主義及其對物種緩慢變化的引力讓人欣然接受，并在世代的教科書中重復(fù)出現(xiàn)。在20世紀(jì)早前之前，令大部分的生物學(xué)家滿足于利用漸進(jìn)主義來回答關(guān)于進(jìn)化速率的問題。
有時，已經(jīng)有了結(jié)論的問題必須由在已有證據(jù)基礎(chǔ)上出現(xiàn)的新的證據(jù)和新的論點(diǎn)使其重新展開討論。在1972年，古生物學(xué)者Stephen Jay Gould和 Niles Eldredge用相反的論點(diǎn)挑戰(zhàn)了世俗的結(jié)論，即斷點(diǎn)平衡說，它假設(shè)了物種演變?yōu)樾碌奈锓N是通過相對突然的爆發(fā)，并非通過長時間的過渡時期。迅速的進(jìn)化期被時間相對更長的靜態(tài)期分開，而在靜態(tài)時期，物種是幾乎完全不變的。
斷點(diǎn)平衡論試著去解釋化石記錄的一個古怪的特點(diǎn)----在超過一個世紀(jì)的時間里它已經(jīng)為古生物學(xué)者所熟悉，但一直被忽視。許多物種似乎在上百萬年的化石記錄中一直沒有改變，這個情況與達(dá)爾文的模型所支持的物種的持續(xù)變化相悖。進(jìn)化漸進(jìn)論的支持者所預(yù)測的中間狀態(tài)的化石一直沒有出現(xiàn)。在大部分蛤和珊瑚的聚集地，其化石在很厚的巖石中都實(shí)際上沒有變化，只是突然被另一新的并且不同的物種而取代。
北美馬的進(jìn)化曾經(jīng)被用作經(jīng)典的教科書案例來證明漸變進(jìn)化論，現(xiàn)在卻為斷點(diǎn)平衡學(xué)提供了同樣有說服力的證據(jù)。一個有說服力的5千萬年的馬祖先的進(jìn)化模型----每一代都稍稍大一點(diǎn)，有更復(fù)雜的牙齒，更長的臉，和中間更突出的腳趾----這一切都看似強(qiáng)有力的支持了達(dá)爾文的論點(diǎn)，物種是逐步地進(jìn)化的。但是，對這些化石更嚴(yán)謹(jǐn)?shù)尿?yàn)證現(xiàn)在揭示了一個不太一樣的故事。馬是在不連續(xù)的步驟中進(jìn)化的，其中每個進(jìn)化步驟中間都有上百萬年時間保持不變，在最后被一個不同的更新的模型取代。比如四只腳趾的Eohippus 在三只腳趾的moihippus之前，但北美化石證據(jù)表明在這之間有一個不平穩(wěn)的，不均衡的轉(zhuǎn)換過程。如果進(jìn)化一直都是連續(xù)，漸進(jìn)的過程，人們應(yīng)該預(yù)期到的是每年的化石樣本都會存在細(xì)微的差別。
如果很難設(shè)想大的改變會迅速發(fā)生，想想這些：一個單一基因的改變就足以將有一對翅膀的蒼蠅變成兩對翅膀。
關(guān)于進(jìn)化速度的問題現(xiàn)在發(fā)生了轉(zhuǎn)變：進(jìn)化過程是逐漸發(fā)生的么，還是總是突然短時間的爆發(fā)？對含有化石的厚巖層的細(xì)致的現(xiàn)場調(diào)查可以檢驗(yàn)這兩個備受爭論的理論。
偶爾，有一個系列的化石豐富的巖石可以允許人們綜合性的觀察一種生物在很長一段時間中的變化。比如，Peter Sheldon對于三葉蟲，一種已滅絕的身體分節(jié)的海洋生物，的研究提供了其對三百萬年來在同一海洋環(huán)境下進(jìn)化的一些細(xì)節(jié)。研究中，八種三葉蟲都觀察到了其身體節(jié)數(shù)數(shù)量逐漸改變的過程，在整個時間段中，一般身體都增加了一到兩節(jié)。沒有明顯的不連貫，這使sheldon得出結(jié)論：海洋環(huán)境在那段時間是比較穩(wěn)定的。
很多來自不同時期的不同的生物都需要開展相似的相近研究。大多數(shù)研究者希望發(fā)現(xiàn)物種進(jìn)化的這兩種模式都存在。緩慢的，連續(xù)的變化可能是在環(huán)境穩(wěn)定的時間段下的規(guī)律，而快速進(jìn)化的新物種則發(fā)生在環(huán)境變化時期的壓力下。但是，我們需要更多的想Sheldon所做的研究以證明這個觀點(diǎn)。
