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一场激烈的辩论使最近的进化论研究活跃起来。

A heated debate has enlivened recent studies of evolution.

达尔文最初的论点，以及进化渐进派所支持的观点是，物种的变化是持续的，但缓慢且有微小渐进。

Darwin’s original thesis, and the viewpoint supported by evolutionary gradualists, is that species change continuously but slowly and in small increments.

这些变化在现代观测的短时间范围内几乎是看不见的，而且，有人认为，这些变化常常被不完整的化石记录中的无数空白所掩盖。

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.

渐进主义强调变化的缓慢速度，这是一个令人欣慰的立场，在几代教科书中反复出现。

Gradualism, with its stress on the slow pace of change, is a comforting position, repeated over and over again in generations of textbooks.

到二十世纪初，关于进化速度的问题已经得到了答案，大多数生物学家对渐进主义感到满意。

By the early twentieth century, the question about the rate of evolution had been answered in favor of gradualism to most biologists’ satisfaction.

有时，当新的证据或基于旧证据的新论点出现时，一个封闭的问题必须重新讨论。

Sometimes a closed question must be reopened as new evidence or new arguments based on old evidence come to light.

1972年，古生物学家斯蒂芬·杰伊·古尔德（Stephen Jay Gould）和奈尔斯·埃尔德雷吉（Niles Eldredge）用一种相反的观点挑战了传统的观点，即间断平衡假说，该假说假定物种在相对突然的爆发中产生新物种，而没有一个漫长的过渡时期。

In 1972 paleontologists 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.

这些快速进化的阶段被相对较长的静态跨度分隔开来，在此期间，一个物种几乎不会发生任何变化。

These episodes of rapid evolution are separated by relatively long static spans during which a species may hardly change at all.

间断平衡假说试图解释化石记录的一个奇怪特征——一个多世纪以来为古生物学家所熟悉但通常被忽视的一个特征。

The punctuated equilibrium hypothesis attempts to explain a curious feature of the fossil record—one that has been familiar to paleontologists for more than a century but has usually been ignored.

许多物种似乎在数百万年的化石记录中保持不变——这种情况似乎与达尔文的持续变化模型不一致。

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.

由渐进主义预测的中间化石形态通常是不足的。

Intermediate fossil forms, predicted by gradualism, are typically lacking.

在大多数地方，一种特定的蛤蜊或珊瑚在厚厚的岩层中基本保持不变，直到突然被另一种不同的新物种所取代。

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.

北美马的进化，曾经被作为一个经典的渐进进化教科书的例子，现在为间断平衡提供了同样令人信服的证据。

The evolution of North American horses, which was once presented as a classic textbook example of gradual evolution, is now providing equally compelling evidence for punctuated equilibrium.

有一个有说服力的5000万年的现代马祖先的序列——每次都体型稍微大一点，牙齿更复杂，脸更长，中间脚趾更突出——似乎为达尔文的物种逐渐进化的论点提供了强有力的支持。

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.

但是对这些化石沉积物的仔细研究现在揭示了一个有点不同的故事。

But close examination of those fossil deposits now reveals a somewhat different story.

马的进化是分步进行的，每一步都几乎保持不变地延续了数百万年，最终被一种独特的新模式所取代。

Horses evolved in discrete steps, each of which persisted almost unchanged for millions of years and was eventually replaced by a distinctive newer model.

例如，四趾的始祖马先于三趾的中新马，但北美的化石证据表明，这两种动物之间的过渡是不稳定、不均匀的。

The four-toed Eohippus preceded the three-toed Miohippus, for example, but North American fossil evidence suggests a jerky, uneven transition between the two.

如果进化是一个连续渐进的过程，人们可能会认为几乎每一个化石标本都与其他标本略有不同。

If evolution had been a continuous, gradual process, one might expect that almost every fossil specimen would be slightly different from every other.

如果很难想象重大的变化是如何迅速发生的，考虑一下这个：单个基因在苍蝇中的改变就足以使一只正常的有一对翅膀的苍蝇变成有两对翅膀的苍蝇。

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

关于进化速度的问题现在必须被扭转过来：进化是逐步进行的，还是总是在短时间内发生的？

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?

对含有化石的厚岩层的详细现场研究为相互竞争的两种理论提供了最佳的潜在检验。

Detailed field studies of thick rock formations containing fossils provide the best potential tests of the competing theories.

偶尔，一个富含化石的岩层序列可以让我们在很长一段时间内全面地观察一种生物。

Occasionally, a sequence of fossil-rich layers of rock permits a comprehensive look at one type of organism over a long period of time.

例如，彼得·谢尔登（Peter Sheldon）对三叶虫的研究，为我们提供了在一个海洋环境中300万年进化过程的详细观察。三叶虫是一种现已灭绝的海洋动物，身体呈分段状。

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.

在这项研究中，观察到八种不同的三叶虫，每一种都经历了身体段数的逐渐变化————通常是在整个时间间隔内增加一段或两段。

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.

没有观察到明显的不连续，这使得谢尔登得出结论，在他研究期间，环境条件是相当稳定的。

No significant discontinuities were observed, leading Sheldon to conclude that environmental conditions were quite stable during the period he examined.

对于许多不同时期的不同种类的生物体，也需要进行类似的详尽的研究。

Similar exhaustive studies are required for many different kinds of organisms from many different periods.

大多数研究人员希望发现从一个物种到另一个物种的两种转变模式都在进化中起作用。

Most researchers expect to find that both modes of transition from one species to another are at work in evolution.

缓慢的、持续的变化可能是环境稳定时期的常态，而新物种的快速进化发生在有环境压力的时期。

Slow, continuous change may be the norm during periods of environmental stability, while rapid evolution of new species occurs during periods of environmental stress.

但在我们确定之前，还需要更多像谢尔登这样的研究。

But a lot more studies like Sheldon’s are needed before we can say for sure.

The word “innumerable” in the passage is closest in meaning to

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

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.

According to paragraphs 1 and 2, the punctuated equilibrium hypothesis and the gradualism hypothesis differed about

According to paragraph 3, the lack of intermediate fossils in the fossil record of some species

The word “compelling” in the passage is closest in meaning to

Paragraph 4 mentions that North American horses have changed in all of the following ways EXCEPT in

The word “alteration” in the passage is closest in meaning to

According to paragraph 7, Peter Sheldon's studies demonstrated which of the following about trilobites?

The word “Occasionally” in the passage is closest in meaning to

Look at the four squares  that indicate where the following sentence could 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 would the sentence best fit? Click on a square  to add the sentence to the passage.

Directions: Select 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.

The Pace of Evolutionary Change 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 paleontologists 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 paleontologists 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. Intermediate 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 horses, 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 other. If it seems difficult to conceive how major changes could occur rapidly, consider this: an alteration of a single gene in flies 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 discontinuities 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 environmental stress.  But a lot more studies like Sheldon’s are needed before we can say for sure.