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剑桥雅思10Test4 Passage3原文READING PASSAGE 3
You should spend about 20 minutes on Questions 27-40, which are based on Reading Passage 3 below.
When evolution runs backwards
Evolution isn’t supposed to run backwards — yet an increasing number of examples show that it does and that it can sometimes represent the future of a species
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.
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.
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’.
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.
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.
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.
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.
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.
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.
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.
Questions 27-31
Choose the correct letter, A, B, C or D.
Write the correct letter in boxes 27-31 on your answer sheet.
27 When discussing the theory developed by Louis Dollo, the writer says that
A it was immediately referred to as Dollo’s law.
B it supported the possibility of evolutionary throwbacks.
C it was modified by biologists in the early twentieth century.
D it was based on many years of research.
28 The humpback whale caught off Vancouver Island is mentioned because of
A the exceptional size of its body.
B the way it exemplifies Dollo’s law.
C the amount of local controversy it caused.
D the reason given for its unusual features.
29 What is said about ‘silent genes’?
A Their numbers vary according to species.
B Raff disagreed with the use of the term.
C They could lead to the re-emergence of certain characteristics.
D They can have an unlimited life span.
30 The writer mentions the mole salamander because
A it exemplifies what happens in the development of most amphibians.
B it suggests that Raff’s theory is correct.
C it has lost and regained more than one ability.
D its ancestors have become the subject of extensive research.
31 Which of the following does Wagner claim?
A Members of the Bachia lizard family have lost and regained certain features several times.
B Evidence shows that the evolution of the Bachia lizard is due to the environment.
C His research into South American lizards supports Raff’s assertions.
D His findings will apply to other species of South American lizards.
Questions 32-36
Complete each sentence with the correct ending, A-G, below.
Write the correct letter, A-G, in boxes 32-36 on your answer sheet.
32 For a long time biologists rejected
33 Opposing views on evolutionary throwbacks are represented by
34 Examples of evolutionary throwbacks have led to
35 The shark and killer whale are mentioned to exemplify
36 One explanation for the findings of Wagner’s research is
A the question of how certain long-lost traits could reappear.
B the occurrence of a particular feature in different species.
C parallels drawn between behaviour and appearance.
D the continued existence of certain genetic information.
E the doubts felt about evolutionary throwbacks.
F the possibility of evolution being reversible.
G Dollo’s findings and the convictions held by Lombroso.
Questions 37-40
Do the following statements agree with the claims of the writer in Reading Passage 3?
In boxes 37-40 on your answer sheet, write
YES if the statement agrees with the claims of the writer
NO if the statement contradicts the claims of the writer
NOT GIVEN if it is impossible to say what the writer thinks about this
37 Wagner was the first person to do research on South American lizards.
38 Wagner believes that Bachia lizards with toes had toeless ancestors.
39 The temporary occurrence of long-lost traits in embryos is rare.
40 Evolutionary throwbacks might be caused by developmental problems in the womb.
以上是小编为大家带来的剑桥雅思10Test4 Passage3原文，下面我们一起来了解一下这部分阅读的参考译文：
Test 4 Passage 3参考译文：
进化反向进行
进化不应该反向进行，但是越来越多的例子表示确实可以如此而且进化反向有时候展示着一个物种的未来。
把任何动物当作一种返祖现象的描述是带有争议性的。在一个世纪的大部分时间里，大多数生物学家不愿意用反向进化等这些词，他们铭记着一个进化原则即“进化是不可以反向进行的。”但越来越多的相关例子为人知晓，同时出现了现代遗传学，这些都表明原则正不得不被改写。反向进化不单单变得有可能，它们还有时候在进化的未来发展上扮演着重要的角色。
一个反向进化的术语为“返祖现象”，该词来自拉丁语atavus，意思是“祖先” 。该词有一个不好的含义，这绝大部分得归功于Cesare Lombroso， 他是一位19世纪的意大利军医，他主张犯人是天生的而不是后天养成的，而且犯人可以通过一些身体特征被识别，而这些特征是亚人类特征的再现。
当Lombroso在观测犯人时，一位比利时的古生物学家Louis Dollo正在研究化石记录并得出了相反的结论。在1890年，他提出进化是不可以逆转的：“一个有机体不能够回转到之前它的祖先已经实现了的阶段，哪怕只是一部分。”在20世纪早期，生物学家得出一个相似的结论，即尽管他们认同返祖现象的可能性，并表示没有理由证明为什么进化不能被反向运行，但他们就是认为发生的可能性极小。所以进化的不可逆性这一观点的研究停住了，并被称为“多洛氏法则”。
如果多洛氏法则是正确的，返祖现象就算真的有，应该也很少发生。然而，几乎自这种想法产生起，就已经出现特例了。比如，在1919年，一头座头鲸在加拿大温哥华岛被捕获，它带有一双长达1 米、像脚的附肢，且有着一套完整的肢骨。探险家Roy Chapman Andrews在那时表明这头鲸一定是某种陆地生活的祖先动物的反向进化结果。“我想不到任何其他的解释。”他在1921年写道。
自从那时起，很多其他的例子已被发现，所以进化是不可逆转的这种观点再也无法成立了。这同时产生了一个困惑：消失了几千万年的特征是如何能重新出现的?在1994年，美国印第安纳大学的Rudolf Raff和他的同事决定采用遗传学研究使得进化逆转增加一定的可能性。他们论证到一些进化过程中因包含了基因丢失的情况而无法逆转，而另一些进化过程或是因为基因的闭合。如果这些休眠基因以某种方式再次激活，他们表示，生物长时间丢失的特征可以重现。
Raff的团队继续计算进化逆转发生的可能性。休眠基因随机突变次数增加，他们推理到，这最终会导致休眼基因失效。那么，如果一个基因长期不被使用，它能在一个物种中存活多长时间呢?该团队计算出休眠基因很可能存在于一个物种的某些个体中，可存活高达六百万年，甚至有的可以存活一千万年。换句话说，进化逆转是可能的，但这仅相对于较近期的进化史而言。
作为一个可能成为例证的事件，团队列举了墨西哥和加利福尼亚的鼹钝口螈。像大多数的两栖类动物一样，这种生物以幼小的蝌蚪状开始他们的生命，然后变形成成年的状态——除了其中一个种类，蝾螈，它们通常会以它的幼年形态一直生活下去。最简单的解释就是蝾螈血统一直丧失了变形的能力，但是其他种类还保持着这样的能力。然而，从对鼹钝口螈的血缘谱的详细分析来说，这是一个明显的事实，其他血统的鼹钝口螈都是从一个本身已经失去变形能力的祖先那里进化而来的。换句话说，变形在鼹钝口螈之中就是一种返祖现象。鼹钝口螈的例子与Raff的100万年的学说框架相符合。
然而，已知的更近的报道说明这个时间界限被打破，它指出了休眠基因不完全是全部的解释。在去年发表的一篇文幸中，耶鲁大学的生物学家Gunter Wagner汇报了一些关于南美蜥蜴Bachia进化史研究的工作。 它们中的一些拥有非常小的肢节;有一些看起来更像蛇而非蜥蜴;有一些完全失去了后肢的趾头。然而，其他的种类则彰显出了后肢的四个趾头。最简单的解释就是这些有趾的蜥蜴品种从没有失去过趾头，但Wagner并不认同。根据他对Bachia的族谱的区别，有趾的物种从它们无趾的祖先进化而成，更有甚者，脚趾的消失和产生在过去的数百万年间发生过不止一次。
因此，到底发生了什么?其中一个可能性就是这三种特性只是失去了，之后又简单地重现。这就像相似的结构可以独立地产生在没有血缘关系的物种中，就像鲨鱼和杀人鲸的背部的鳍一样。另一种更加有吸引力的可能性是那些用来生长趾头的基因信息在蜥蜴上存活了几百或者是几千万年，并且这种基因信息被重新激活了。这些返祖性的特征提供了一种优势，这种优势适用于所有物种，能有效地进行进化逆转。
但是，如果休眠基因在60万到100万年内退化，这种长时间消失的特性是怎样在这么长的时间范围内被重新激活的?这个答案也许在子宫里面可以被找到。 许多物种早期的胚胎形成了祖先的特性。例如蛇的胚胎萌发出后肢的肢芽。这些特性在后期发展中由于某些进化模式而消失了，该程式可能导致“腿部消失”。如果因为任何原因这些事情没有发生，祖先的种种特性也许就不会消失，从而导致返祖现象
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