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.
enlivened : 使生动；使活跃gradualists：渐进主义者increments：增长数，增量
一场激烈的辩论使最近的进化论研究活跃起来。达尔文最初的论点，以及进化论渐进论者所支持的观点是，物种的变化是持续的，但缓慢且渐进。
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.
obscured：无名的，鲜为人知的；被遮蔽
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善的化石记录中的无数空白所掩盖。
Gradualism, with its stress on the slow pace of change, is a comforting position, repeated over and over again in generations of textbooks.
渐进主义强调变化的缓慢步伐，这是一个令人欣慰的立场，在几代教科书中反复出现。
By the early twentieth century, the question about the rate of evolution had been answered in favor of gradualism to most biologists' satisfaction.
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。
Sometimes a closed question must be reopened as new evidence or new arguments based on old evidence come to light.
come to light. ：真相大白，显露出现
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。
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.
paleontologists：n 古生物学家
punctuated：v.间断；强调；标点(文章)；加重(语气等)
posit：v.假设；认定；认为…为实
punctuated equilibrium hypothesis：间断平衡假说
static ：a 静止的；不变的
1972年，古生物学家史蒂芬·古尔德和奈尔斯 · 埃尔德雷奇用一个相反的观点来挑战传统智慧---- 间断平衡假说，该假说认为物种在相对突然的爆发中产生新物种，而不需要很长的过渡期。 这些快速进化的阶段被相对较长的静态跨度所分隔，在此期间，一个物种几乎不会发生任何改变。
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.
clam：n.蛤；蛤蜊；蚌
间断平衡假说试图解释化石记录中一个奇怪的特征，这个特征对古生物学家来说已经熟悉了一个多世纪，但通常被忽略了。 许多物种在数百万年的化石记录中似乎没有改变——这种情况似乎与达尔文的持续变化模型不一致。 渐进主义所预测的中间化石形式通常是缺乏的。 在大多数地区，一种特定的蛤蜊或珊瑚在整个厚厚的岩层中基本保持不变，只是突然被一个新的和不同的物种所取代。
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.
compelling ：引人注目的；强制的；令人信服的
prominent ：adj.重要的；著名的；杰出的；显眼的
contention ：n.争论；争吵；争执；（尤指争论时的）看法
discrete ：adj.离散的；分离的；各别的
precede ：v.居先于；在…之上；优于；比…重要
jerk ：突然的；急促的；颠簸的；不平稳的
北美马的进化，曾经作为教科书中渐进进化的经典例子，现在也为间断平衡提供了同样令人信服的证据。 一个令人信服的5000万年的现代马祖先序列——每一个都稍微大一点，有更复杂的牙齿，更长的脸，和更突出的中央脚趾——似乎为达尔文的论点提供了强有力的支持，即物种是逐渐进化的。 但是对这些化石沉积物的仔细研究现在揭示了一个有些不同的故事。 马是以分散的步伐进化而来的，每一步都几乎不变地持续了数百万年，最终被一种独特的新模式所取代。 例如，四趾的 Eohippus 先于三趾的 Miohippus，但是北美的化石证据表明，在两者之间存在着一种断断续续、不均衡的过渡。 如果进化是一个连续的、渐进的过程，人们可能会认为几乎每一个化石标本都会略有不同。
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.
conceive：v.构想；设想；想象；怀孕
如果看起来很难想象主要的变化是如何迅速发生的，那么考虑一下这个: 一个单一基因在果蝇中的改变就足以把一只有一对翅膀的正常果蝇变成一只有两对翅膀的果蝇。
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.
关于进化速度的问题现在必须回过头来: 进化是逐渐进行的，还是总是在短时间内爆发？ 对含有化石的厚岩层进行详细的野外研究，为这些相互竞争的理论提供了最好的可能性测试。
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.
segmented ：分割的;切断的;划分的
interval ：间隔；区间；时间间隔
有时，一系列富含化石的岩石层允许我们在很长一段时间内全面地观察一种生物。 例如，彼得 · 谢尔顿对三叶虫---- 一种身体分节的现已灭绝的海洋动物---- 的研究，提供了对一个海洋环境中300万年进化的详细一瞥。 在这项研究中，观察到八个不同的三叶虫物种的每一个部分的数量都在逐渐变化——通常在整个时间间隔内增加一个或两个部分。 没有重大的不连续性观察，导致谢尔登得出结论，环境条件是相当稳定的在被他研究的那个时期。
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.
norm ：n.规范；标准；定额；常态
需要对许多不同时期的许多不同种类的生物进行类似的详尽研究。 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过渡模式都在进化中起作用。 缓慢、连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则发生在环境压力时期。 但是在我们确定之前还需要更多像谢尔顿这样的研究。
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