爱华网The Pace of EvolutionaryChange
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A heated debate has enlivened recent studies ofevolution. Darwin’s original thesis, and the viewpoint supported byevolutionary gradualists, is that species change continuously butslowly and in small increments. Such changes are all but invisibleover the short time scale of modern observations, and, it isargued, they are usually obscured by innumerable gaps in theimperfect fossil record. Gradualism, with its stress on the slowpace of change, is a comforting position, repeated over and overagain in generations of textbooks. By the early twentieth century,the question about the rate of evolution had been answered in favorof gradualism to most biologists’ satisfaction.
激昂的辩论使得最近对进化的研究变的活跃起来。达尔文的起源理论以及得到进化渐进主义者支持的观点都表明：物种的改变是持续但缓慢的，是以小幅的增量在变化。这种改变以现在的观察来看短时间内几乎都是不可见的，而且，从不完美的化石记录上无数的间隙上来看，这种改变也通常是模糊不清的。渐进主义（偏重慢节奏的改变），是一种令人欣慰的位置，在一代又一代的教科书上不断重复着。在20世纪早期，人们对于进化率这个问题是倾向于渐进主义的观点也符合当时生物学家的要求。
Sometimes a closed question must be reopened asnew evidence or new arguments based on old evidence come to light.In 1972 paleontologist Stephen Jay Gould and Niles Eldredgechallenged conventional wisdom with an opposing viewpoint, thepunctuated equilibrium hypothesis, which posits that species giverise to new species in relatively sudden bursts, without a lengthytransition period. These episodes of rapid evolution are separatedby relatively long static spans during which a species may hardlychange at all.
有时候，一个封存已久的问题随着那些新的证据或者基于旧的的证据的新论点的出现也会被重新打开。1972年古生物学家StephenJay Gould 和NilesEldredge用相反的观点质疑传统观点，也就是强调均衡理论，这个理论假设一些物种不经历较长的转变时期一瞬间进化成新物种。这些快速进化的一连串事件被长期的静止的时间间隔开，而在这些静止的时间段里物种是根本不进化的。
The punctuated equilibrium hypothesis attemptsto explain a curious feature of the fossil record --- one that hasbeen familiar to paleontologist for more than a century but hasusually been ignored. Many species appear to remain unchanged inthe fossil record for millions of years --- a situation that seemsto be at odds with Darwin’s model of continuous change.Intermediated fossil forms, predicted by gradualism, are typicallylacking. In most localities a given species of clam or coralpersists essentially unchanged throughout a thick formation ofrock, only to be replaced suddenly by a new and differentspecies.
这种强调均衡的理论尝试解释化石记录的奇怪之处——这点古生物学家已经了解超过一个世纪了却常常把他忽略。就化石记录来看，很多物种看似几百万年都没有发生变化——这点似乎与达尔文的渐变模型冲突。被渐变主义预测的中间化石形态显然是缺乏的。在很多的聚居所，一些特定的蛤或者珊瑚的物种在整个厚厚的岩石形成过程都没有发生改变，而突然间被一个新的不同的物种取代了。
The evolution of North American horse, which wasonce presented as a classic textbook example of gradual evolution,is now providing equally compelling evidence for punctuatedequilibrium. A convincing 50-million-year sequence of modern horseancestors --- each slightly larger, with more complex teeth, alonger face, and a more prominent central toe ---seemed to providestrong support for Darwin’s contention that species evolvegradually. But close examination of those fossil deposits nowreveals a somewhat different story. Horses evolved in discretesteps, each of which persisted almost unchanged for millions ofyears and was eventually replaced by a distinctive newer model. Thefour-toed Eohippus preceded the three-toed Miohippus, for example,but North American fossil evidence suggests a jerky, uneventransition between the two. If evolution had been a continuous,gradual process, one might expect that almost every fossil specimenwould be slightly different from every year.
北美马的进化，曾经一度作为物种渐变进化的教科书的典型范例，现在正为强调均衡理论提供具有相同的说服力。现代马祖先的一个令人信服的5000万年化石序列——每一个体型都略微有一些大，拥有更复杂的牙齿，更长的脸以及更突出的脚趾——看起来可以为达尔文强调的物种渐变进化论提供充足的支持。但是对这些化石密切的检查向人们揭示了一个不同的结果。马的进化呈现出不连续的节奏，而进化之间的每一个阶段都是保持几百万年不变的，然后最终被一个不同的新的形式取代。举个例子，四趾始祖鸟存在于三趾中新马之前，而北美的化石证据表明这两者之间有一个不平稳的改变。如果进化是一个持续的渐变的过程，那么人们应该能够看到几乎每一个化石样本每一年都会不一样。
If it seems difficult to conceive how majorchanges could occur rapidly, consider this: an alteration of asingle gene in files is enough to turn a normal fly with a singlepair of wings into one that has two pairs ofwings.
如果去设想变化为何这么剧烈有难度的话，那么不妨这样考虑，基因组中一个基因的改变足以将一个单对翅膀的苍蝇改变成双对翅膀的。
Occasionally, a sequence of fossil-rich layersof rock permits a comprehensive look at one type of organism over along period of time. For example, Peter Sheldon’s studies oftrilobites, a now extinct marine animal with a segmented body,offer a detailed glimpse into three million years of evolution inone marine environment. In that study, each of eight differenttrilobite species was observed to undergo a gradual change in thenumber of segments --- typically an increase of one or two segmentsover the whole time interval. No significant discontinuous wereobserved, leading Sheldon to conclude that environmental conditionswere quite stable during the period he examined.
偶尔，富含化石岩层的序列给了我们看待长时期的生物类型的一个综合视角。比如说,PeterSheldon对于三叶虫（一种现在已经灭绝了的拥有分段身体的海洋动物）的研究，提供了对3百万年前海洋环境进化的一个见解。在他的研究中，八种不同的的三叶虫每一种在多个阶段经历渐渐的改变——通常是在整个时间间隙的一到两个阶段出现增长，这使Sheldon做出结论：他检验的这个时期环境条件比较稳定。
Similar exhaustive studies are required for manydifferent kinds of organisms from many different periods. Mostresearchers expect to find that both modes of transition from onespecies to another are at work in evolution. Slow, continuouschange may be the norm during periods of environmental stability,while rapid evolution of new species occurs during periods ofenvironment stress. But a lot more studies like Sheldon’s areneeded before we can say for sure.
类似的详细的研究需要来自不同时期的不同生物种类。大多数的研究者希望找到这种从一种生物到另外一种生物转变的模型在进化方面也适用。在环境稳定时期的渐渐变化是一个基准，而在环境出现压力的时期就会出现快速进化。而我们需要更多的像Sheldon这样的研究才能信服。
以上是ThePace of Evolutionary Change的译文，希望大家提出异议。
