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  3. You do not usually get something for nothing. Now, a new study reveals that the evolution of an improved learning ability could

You do not usually get something for nothing. Now, a new study reveals that the evolution of an improved learning ability could

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问题     You do not usually get something for nothing. Now, a new study reveals that the evolution of an improved learning ability could come at a particularly high price: an earlier death.
    Past experiments have demonstrated that it is relatively easy through selective breeding to make rats, honey bees and—that great favourite of researchers—fruit flies a lot better at learning. Animals that are better learners should be competitive and, thus, over time, come to dominate a population by natural selection. But improved learning ability does not get selected amongst these animals in the wild. No one really understands why.
    Tadeusz Kawecki and his colleagues at the University of Fribourg in Switzerland have measured the effects of improved learning on the lives of fruit flies. The flies were given two different fruits as egg-laying sites. One of these was laced with a bitter additive that could be detected only on contact. The flies were then given the same fruit but without an additive. Flies that avoided the fruit which had been bitter were deemed to have learned from their experience. Their children were reared and the experiment was run again.
    After repeating the experiment for 30 generations, the children of the learned flies were compared with normal flies. The researchers report in a forthcoming edition of Evolution that although learning ability could be bred into a population of fruit flies, it shortened their lives by 1.5% . When the researchers compared their learned flies to colonies selectively bred to live long lives, they found even greater differences. Whereas learned flies had reduced their life spans, the long-lived flies learned less well than even average flies.
    The authors suggest that evolving an improved learning ability may require a greater investment in the nervous system which takes resources away from processes that delay ageing. However, Dr. Kawecki thinks the effect could also be a by-product of greater brain activity increasing the production of Reactive Oxygen Species(ROS), which can increase oxidation in the body and damage health.
    No one knows whether the phenomenon holds true for other animals. So, biologists, at least, still have a lot to learn.
Past experiments prove selective breeding can make animals better______.
选项 A、commanders
B、competitors
C、survivors
D、learners
答案D
解析 细节题。从文章第二段第一句“Past experiments havedemonstrated that it is relatively easy through selective breeding tomake rats，honey bees and—that great favourite of researchers—fruit flies a lot better at learning．”可知，实验证明选择性繁殖可以提高动物的学习能力，故选D。
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