When Evolution Runs Backwards

  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, meaningforefather. The word has ugly connotations thanks largely to Cesare Lombroso, a 19th-century Italianmedic 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 studyingfossil 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 thatevolution 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 e 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 molesalamanders 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, theancestral feature may not disappear, leading to an atavism.

  Complete each sentence with the correct ending, A-G, below.

  Drag the correct letter, A-G, into boxes 32-36 on your answer sheet.

  32   For a long time biologists rejected 32

  33   Opposing views on evolutionary throwbacks are represented by 33

  34   Examples of evolutionary throwbacks have led to 34

  35   The shark and killer whale are mentioned to exemplify 35

  36   One explanation for the findings of Wagner's research is 36

  A. the question of how certain long-lost trails 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.

  正确答案:32. F33. G34. A35. B36. D

  阅读解析:32.本题起始位置并没有从文章中部开始,而是从开头就进行考查,不太符合常规,关键词定位就更加重要了。在首段中,可以将 long time 与 a century 对应,再加上 biologists 原词重现确认出题位置。题干的意思是很长一段时间,生物学家拒绝什么。原文的意思是近一个世纪以来,大部分生物学家不情愿去使用“进化返祖”这个词,因为他们信守“进化是不可逆转的”这一进化原理。以这句话对应选项,也就是说生物学家拒绝进化是可以逆转的,答案也就能确定了。

  33.本题的意思是关于进化逆转现象的两种对立观点由什么来代表,所以要回到原文中找关于进化逆转现象的不同观点。在第二段中,Lombroso 提到返祖现象是犯罪者天生的,而第三段中 Dollo 认为物种是根本不可能出现进化逆转现象的。他们代表着进化逆转理论的两种观点,能明确对应到 G 选项上。

  34.本题的意思是各种进化返祖的例子导致了什么。关键是回到原文中找到 examples 的对应。根据顺序性原则,在第五段中明确提到 so many other examples 的原词对应,往后分析作者提出的一个疑惑:这些消失了数百万年的特征怎么会突然重现?以这句话对应选项,可以轻松认定答案为 A。long-lost traits 对应原文中的 characteristics that disappeared millions of years ago, 而 reappear 是原词出现。

  35.本题与上一道题所隔的距离较远,但是关键词 shark and killerwhale 在第九段为原词重现。原文的意思是一个可能的解释就是某些特征消失后又重现出来,就像相似的结构在不相关的物种中独立发展出来一样,比如鲨鱼和虎鲸的背鳍。举 shark and killer whale 为例是为了说明前面提及的观点,可以认定 B 为答案。B 选项中的 feature 对应原文中的 structure,而 different species 对应原文中的 unrelated species。

  36.本题有点难度,但是考虑到本题是顺序题型,只能在原文中继续往下找。再加上本题是最后一题,可以考虑排除法的方式对应,只剩下 C, D, E 三个选项。在其中可以寻找与原文有对应的信息。紧跟着上一道题之后,原文中的 another possibility 能够与题干中的 one explanation 对应,原文的意思是能够长出脚趾的基因信息可能在蜥蜴中幸存数千万甚至上亿年然后被重新活化。对应剩余三个选项,只有 D 选项最能表达这个意思,该选项中 genetic information 原词对应,continued existence 也能与 survived for tens or perhaps hundreds of millions of years 一致,所以答案也就确定了。