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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 of the following does Wagner claim?
选项 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 Raffs assertions.
D、His findings will apply to other species of South American lizards.
答案A
解析 本题关键词是Wagner’s research，问题是：关于瓦格纳的研究正确的是哪项?定位到第六段。根据第六段末句，有趾的物种从无趾的祖先进化而成，在过去的数千万年间趾头不止一次(more than one occasion)地消失和产生(toes loss and gain)，选项A中的several times和原文more than one occasion是同义替换，为正确选项。选项B属于无中生有，原文没有提及环境影响了巴克蜥的进化。根据第六段首句，最近报道的几个实例(包括瓦格纳的研究)打破了这个时间界限，这与拉夫团队提出的时间界限到10(30万年)矛盾，因此选项C属于正反混淆。选项D属于无中生有，原文仅提到瓦格纳对巴克蜥的研究，并未提及是否可以推广适用于其他南美蜥蜴。第六段：瓦格纳认为休眠基因的说法不可靠，返祖现象可能是常态。
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