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  3. It is not often that biologists have a chance to watch natural selection in action. The best-known cases—the evolution of resist

It is not often that biologists have a chance to watch natural selection in action. The best-known cases—the evolution of resist

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问题     It is not often that biologists have a chance to watch natural selection in action. The best-known cases—the evolution of resistance to antibiotics in bacteria and to pesticides in insects—are responses to deliberate changes people have made in the environment of the creatures concerned. But mankind has caused lots of accidental changes as well, and these also offer opportunities to study evolution.
    Recently, two groups of researchers, one at New York University (NYU) and the other at the Woods Hole Oceanographic Institute in Massachusetts, have taken advantage of one of these changes to look at how fish evolve in response to environmental stress. The stress in question is pollution by polychlorinated biphenyls (PCBs). These chemicals—widely used in the middle decades of the 20th century to manufacture electrical insulation, coolants, sealants and plasticisers—often ended up dumped in lakes, rivers and coastal waters. Eventually, such dumping was banned (in America, this happened in 1977). But PCBs are persistent chemicals, and their effects are felt even today. In particular, they disrupt the immune systems of animals such as fish, cause hormonal imbalances and promote tumours.
    As is the way of evolution, however, some fish species have developed resistance to PCB poisoning. Isaac Wirgin, at NYU, and Mark Hahn, at Woods Hole, have been studying PCB-re-sistant fish, to see how they do it. After that, the two researchers will be able to look at how these populations evolve yet again as the environment is cleaned up.
    The species of interest to Dr Wirgin is the Atlantic tomcod of the Hudson river in upstate New York. Part of the Hudson was polluted with PCBs by two General Electric plants. Dr Hahn is looking at a different animal, the killifish, in New Bedford harbour, Massachusetts, which was polluted by other producers. Both Hudson tomcod and New Bedford killifish are able to tolerate levels of PCB far higher than those that would kill such fish in cleaner waters. The question is, why?
    PCBs do their damage by binding to a protein called the aryl hydrocarbon receptor, or AHR, thus stopping it working properly. AHR is a transcription factor, meaning that it controls the process by which messenger molecules are copied from genes. These messenger molecules go on to act as the blueprints for protein production, so preventing a transcription factor from working can cause all sorts of problems. Both Hudson tomcod and New Bedford killifish, however, have unusual AHR molecules. And it is this that seems to explain their immunity.
    A protein is a chain of chemical units called amino acids. In tomcods, AHR is composed of 1,104 such units. Except that in Hudson tomcod it frequently isn’t. These fish generally have 1,102 amino acids in their AHRs. The two missing links in the chain (a phenylalanine and a leucine, for aficionados) are encoded in the gene for ordinary tomcod AHR by six genetic "letters" that are missing from the DNA found in PCB-resistant Hudson tomcod. The shortened version of AHR does not bind nearly so easily to PCBs. It still, however, seems to work as a transcription factor. The result is fish that are more or less immune to PCB poisoning.
    In the case of the New Bedford killifish the situation is similar, but more complicated. There are no missing amino acids. Dr Hahn has, however, found nine places along the amino-acid chain of killifish AHR where the link in the chain varies between individuals. Altogether, he has identified 26 such variations. Two of them seem particularly resistant to the effects of PCBs. It is not that the pollutants do not bind to the protein—they do. But the protein does not seem to mind. It appears to work equally well, whether or not it has PCB passengers on board.
    These fishy cases are reminiscent of the peppered moth in Britain. This, too, evolved in response to industrial pollution. It developed black wings, so that it was invisible when it settled on soot-covered tree trunks. Now, with the clean air brought by anti-pollution legislation, British peppered moths are once again peppered.
    Both the Hudson river and New Bedford harbour are being cleaned up, too. This year, for example, General Electric will dredge 1.8m cubic metres (2.4m cubic yards) of PCB-contaminated sediment out of 60km (35 miles) of the Hudson. Dredging will continue over the next few years, after which the river should be PCB-free. It is possible that the tomcod and the killifish will then evolve again, just as the peppered moth did, if their PCB-resistant proteins are not absolutely as good as the original versions—which they might not be, given that evolution did not find them before. If that happens, Dr Wirgin and Dr Hahn will be watching.
                                              From The Economist, October 29, 2011
What’s the main idea of the passage?
选项 A、Human-being has made both deliberate and accidental changes to environment.
B、How fish evolve in response to environmental stress.
C、The effects of PCBs pollution on the sea environment.
D、How the PCB-resistant fish evolve in natural courses.
答案B
解析 本题为主旨题。本篇文章主要是围绕由环境污染导致的鱼类进化展开，选项A说“人类有意无意地改变着环境”是本文的大背景，正因如此，才会有科学家就此展开调查研究。选项B“鱼类如何在环境压力下进化”在文章第二段提到，是整个调研的目的，也是文章的主题。而选项D“对聚氯联苯产生抗体的鱼类如何随着自然演进而改变”，错误的关键在于，鱼类不是“随着自然演进改变”而是“随着环境变化(在文章中是人为的)而改变”。选项C将重点放到了环境污染上，显然有些背离主题。因此选B。
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