剑桥雅思10Test4Passage3阅读原文翻译 When evolution runs backwards

剑桥雅思10Test4Passage3阅读原文翻译 When evolution runs backwards

剑桥雅思10 Test4 Passage3阅读原文翻译

第1段

The description of any animal as an ‘evolutionary throwback’ is controversial. For the better part of a century, most biologists have been reluctant to use those words, mindful of a principle of evolution that says ‘evolution cannot run backwards’. But as more and more examples come to light and modern genetics enters the scene, that principle is having to be rewritten. Not only are evolutionary throwbacks possible, they sometimes play an important role in the forward march of evolution.

将任何动物描述为“进化倒退”是有争议的。该世纪的大部分时间里，大多数生物学家一直不愿意使用这些词汇。他们谨记着进化的一条基础原则，“进化不能后退”。但随着越来越多的例子出现，以及现代基因科学的诞生，这条规则不得不被改写。进化的倒退不仅是可能的，它们还在向前的进化中起着重要作用。

第2段

The technical term for an evolutionary throwback is an ‘atavism’, from the Latin atavus, meaning forefather. The word has ugly connotations thanks largely to Cesare Lombroso, a 19th-century Italian medic who argued that criminals were born not made and could be identified by certain physical features that were throwbacks to a primitive, sub-human state.

用于描述进化后退的技术词汇是“返祖现象”，它来自于拉丁语atavus，意思是祖先。该词具有一定的丑陋含义。这在很大程度上要归功于19世纪意大利医务人员Cesare Lombroso。他认为犯罪分子是天生的，而不是后期造成的。并且可以根据特定的身体特征来进行识别。而这些特征可以追溯到原始的亚人类状态。

第3段

While Lombroso was measuring criminals, a Belgian palaeontologist called Louis Dollo was studying fossil records and coming to the opposite conclusion. In 1890 he proposed that evolution was irreversible: that ‘an organism is unable to return, even partially, to a previous stage already realised in the ranks of its ancestors’. Early 20th-century biologists came to a similar conclusion, though they qualified it in terms of probability, stating that there is no reason why evolution cannot run backwards—it is just very unlikely. And so the idea of irreversibility in evolution stuck and came to be known as ‘Dollo’s law’.

Lombroso在描述罪犯时，一名叫做Louis Dollo的比利时古生物学家正在研究化石记录，并得出相反的结论。1890年，他提出进化是不可逆的，“一种生物无法回到其祖先在进化序列中已经完成的任何先前阶段，即使是部分回归也不可能”。二十世纪早期的生物学家得出了相似的结论，尽管他们在概率上对其进行了限定，认为并没有任何理由支持进化无法后退，它仅仅是不太可能。因此，这种进化不可逆的观念扎根下来，并被称为“Dollo定律”。

第4段

If Dollo’s law is right, atavisms should occur only very rarely, if at all. Yet almost since the idea took root, exceptions have been cropping up. In 1919, for example, a humpback whale with a pair of leg-like appendages over a metre long, complete with a full set of limb bones, was caught off Vancouver Island in Canada. Explorer Roy Chapman Andrews argued at the time that the whale must be a throwback to a land-living ancestor. ‘I can see no other explanation,’ he wrote in 1921.

如果Dollo定律是正确的，那么返祖现象哪怕确实存在，也应该十分少见。然而，几乎从这一概念扎根以来，例外就一直出现。例如，1919年，加拿大温哥华岛上捕获了一头座头鲸。它有一双类似于腿的、一米多长的附属物，并拥有完整的四肢骨骼。探险家Roy Chapman Andrews那时认为这头鲸鱼一定退化成了其生活在陆地上的祖先状态。他于1921年写到，“我找不到任何其他解释”。

第5段

Since then, so many other examples have been discovered that it no longer makes sense to say that evolution is as good as irreversible. And this poses a puzzle: how can characteristics that disappeared millions of years ago suddenly reappear? In 1994, Rudolf Raff and colleagues at Indiana University in the USA decided to use genetics to put a number on the probability of evolution going into reverse. They reasoned that while some evolutionary changes involve the loss of genes and are therefore irreversible, others may be the result of genes being switched off. If these silent genes are somehow switched back on, they argued, long-lost traits could reappear.

从那时起，发现了如此之多的其他例子，以至于坚持进化不可逆这一理论不再有什么意义。这就提出了一个难题：那些几百万年前消失的特征怎么会再次突然出现？1994年，美国印第安纳大学Rudolf Raff和他的同事决定利用基因学量化进化倒退的可能性。他们考虑到，虽然一些进化变化涉及到基因的缺失，并因此是不可逆的。但其他的则可能是基因被关闭的结果。他们认为，如果这些沉默的基因以某种方式再次被打开，消失了很长时间的特征就会再次出现。

第6段

Raff’s team went on to calculate the likelihood of it happening. Silent genes accumulate random mutations, they reasoned, eventually rendering them useless. So how long can a gene survive in a species if it is no longer used? The team calculated that there is a good chance of silent genes surviving for up to 6 million years in at least a few individuals in a population, and that some might survive as long as 10 million years. In other words, throwbacks are possible, but only to the relatively recent evolutionary past.

Raff的团队继续计算这一现象发生的可能性。他们认为沉默的基因会积累随机突变，并最终使得它们变得毫无用处。所以，如果一个基因不再被使用，那么它在一个物种中可以存在多长时间呢？该团队计算，有很大概率，沉默的基因在种群中的一些个体里可以存在600万年，一些甚至可能延续1000万年那么久。换句话说，退化完全可能，但仅仅是退回相对较近的进化阶段。

第7段

As a possible example, the team pointed to the mole salamanders of Mexico and California. Like most amphibians these begin life in a juvenile ‘tadpole’ state, then metamorphose into the adult form—except for one species, the axolotl, which famously lives its entire life as a juvenile. The simplest explanation for this is that the axolotl lineage alone lost the ability to metamorphose, while others retained it. From a detailed analysis of the salamanders’ family tree, however, it is clear that the other lineages evolved from an ancestor that itself had lost the ability to metamorphose. In other words, metamorphosis in mole salamanders is an atavism. The salamander example fits with Raff’s 10-million-year time frame.

该团队列举了墨西哥和加利福尼亚的钝口螈为证。与大多数两栖动物类似，它们以一种年幼的“蝌蚪”形态开始自己的一生，然后转变为成年形态。只有一个品种例外：蝾螈。它因一生都保持着年幼状态而闻名。这一现象最简单的解释是，蝾螈这一单独分支失去了变形的能力，而其他分支则保留了这一能力。然而，通过对蝾螈家族谱系的细致分析发现，其他分支显然是从某一个已经失去变形能力的祖先进化而来的。话句话说，钝口螈的变形能力是一种返祖现象。钝口螈的例子符合Raff一千万年的时间框架。

第8段

More recently, however, examples have been reported that break the time limit, suggesting that silent genes may not be the whole story. In a paper published last year, biologist Gunter Wagner of Yale University reported some work on the evolutionary history of a group of South American lizards called Bachia. Many of these have minuscule limbs; some look more like snakes than lizards and a few have completely lost the toes on their hind limbs. Other species, however, sport up to four toes on their hind legs. The simplest explanation is that the toed lineages never lost their toes, but Wagner begs to differ. According to his analysis of the Bachia family tree, the toed species re-evolved toes from toeless ancestors and, what is more, digit loss and gain has occurred on more than one occasion over tens of millions of years.

然而，更近一些被报道的例子打破了时间限制，表明沉默的基因可能还不是故事的全部。在去年发表的一篇论文中，耶鲁大学生物学家Gunter Wagner公布了一些自己对一种名为Bachia的南美洲蜥蜴的进化历史所做的研究。它们许多都有微型四肢。一些看起来更像是蛇而不是蜥蜴。一些已经完全失去了后肢的指头。然而，其他物种的后肢上则出现了最多四个指头。最简单的解释是，有指头的分支从未丧失过这一特征。但Wagner持不同意见。根据他对Bachia家族谱系的研究，有指头的物种从没有指头的祖先那里再次进化出了指头。此外，这种脚趾的消失和再现在过去上千万年里曾不只出现过一次。

第9段

So what’s going on? One possibility is that these traits are lost and then simply reappear, in much the same way that similar structures can independently arise in unrelated species, such as the dorsal fins of sharks and killer whales. Another more intriguing possibility is that the genetic information needed to make toes somehow survived for tens or perhaps hundreds of millions of years in the lizards and was reactivated. These atavistic traits provided an advantage and spread through the population, effectively reversing evolution.

那么，到底是怎么回事呢？一种可能是，这些特征先是消失，然后只是简单的再次出现而已。就跟一些相似的结构可能独立出现在不相关的物种身上一样，比如鲨鱼和虎鲸的背鳍。另外一种更为有趣的可能是，需要用来制造指头的基因信息在蜥蜴中存在了数千万甚至上亿年，然后被再次激活了。这些返祖体征提供了一项优势，并在族群中传播开来，有效地逆转了进化进程。

第10段

But if silent genes degrade within 6 to 10 million years, how can long-lost traits be reactivated over longer timescales? The answer may lie in the womb. Early embryos of many species develop ancestral features. Snake embryos, for example, sprout hind limb buds. Later in development, these features disappear thanks to developmental programs that say ‘lose the leg’. If for any reason this does not happen, the ancestral feature may not disappear, leading to an atavism.

但如果沉默的基因在600万到1000万年的时间里退化，那么已经消失了很久的特征又是如何在更长的时间跨度里重现的呢？答案可能存在于子宫内部。许多物种的早期胚胎都发展出祖先的特征。例如，蛇的胚胎会长出后腿突起物。在随后的发育过程中，这些特征会由于发育程序的命令（丢掉这些腿）而消失。如果因为某种原因，这一机制没有起作用，那么祖先的特征就不会消失，导致返祖现象。

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