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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.
Which one is true about "silent genes"?
选项 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.
答案C
解析 本题关键词是silent genes，问题是：关于“休眠基因”哪个说法正确?定位到第三和第四段。根据第三段段尾句，如果用某种方式再次激活这些休眠基因(these silent genes)，一个物种长时间丢失的特征可能会重现，选项C中的certain characteristics与原文long-lost-traits是相同含义，为正确选项。选项A、B均属于无中生有，原文没有这两项相关信息。根据第四段第三、四句话，一个基因如果长期不被使用，它很可能在一个物种的某些个体中存活长达六百万年，有的甚至可以存活一千万年(as long as 10 million years)，所以不是无限期的(unlimited life span)。因此选项D属于扩大范围。第三段：拉夫认为进化可逆转的原因是一些休眠基因得以再次激活。第四段：拉夫团队认为近期进化逆转是可能发生的。
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