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  3. The description of any animal as an "evolutionary throwback" is controversial. A Belgian paleontologist called Louis Dollo propo

The description of any animal as an "evolutionary throwback" is controversial. A Belgian paleontologist called Louis Dollo propo

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问题     The description of any animal as an "evolutionary throwback" is controversial. A Belgian paleontologist called Louis Dollo proposed that evolution was irreversible: that "an organism is unable to return, even partially, to a previous stage already realized 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.
    So many 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 that are able to live both on land and in water, these begin life in a juvenile "tadpole" state, then change 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 change, 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 change. In other words, changing in mole salamanders is an atavism. The salamander example fits with Raffs 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 tiny limbs; some look more like snakes than lizards and a few have completely lost the toes on their back limbs. Other species, however, sport up to four toes on their back 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, toes loss and gain has occurred on more than one occasion over tens of millions of years.
The humpback whale caught off Vancouver Island is mentioned because________.
选项 A、its body has an exceptional size
B、it exemplifies Dollo’s law
C、it has caused a lot of local controversy
D、it has unusual traits
答案D
解析 本题的问题是：为什么作者提到在温哥华岛附近海域捕获的座头鲸?定位到第二段。根据第二段第一、二句，如果多洛法则是正确的，返祖现象应该很少发生，或根本不会发生。然而，这种想法自产生起，就已经有特例(exceptions)不断出现。因此在加拿大温哥华捕捉到的座头鲸这个例子用来支持第二句的观点，选项D的unusual和原文exceptions是同义替换，为正确选项。同理，选项B属于正反混淆。选项C在文中没有提及，属于无中生有。选项A并不是作者提及座头鲸的原因，属于曲解文意。第二段：很多特例涌现，证明返祖现象的确存在。
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