2018年PETS三级阅读理解练习题（1）

　　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 early death.

　　Past experiments have demonstrated that it is relatively easy through selective breeding to make rats, honey bees and —that great favorite 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 understand why.

　　Tadeuzs 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 fruit 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 experiments was run again.

　　After repeating the experiment for 30 generations, the children of the learned flies were com-pared 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 15%. 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.

　　1. Past experiments prove selective breeding can make animals better_______

　　A. CommandersB. Competitors

　　C. survivorsD. learners

　　2. In this experiment, scientists observed that________

　　A. some flies avoided the fruit without an addictive

　　B. some flies preferred the fruit with an addictive

　　C. the eggs of the flies were not damaged

　　D. the impact on the flies did not last long

　　3. The forthcoming report says that_______

　　A. long-lived flies are better at laying eggs

　　B. long-lived flies are poorer in learning

　　C. learned flies have a relatively long life

　　D. learned flies live as long as average ones

　　4. According to Dr.Kawecki, greater brain activity______

　　A. reduces oxygen consumption

　　B. regulates the nervous system

　　C. speeds up the ageing process

　　D. stabilizes the ageing process

　　5. We learn from the text that_______

　　A. the research findings need to be tested further

　　B. biologists are doing similar research on other animals

　　C. the animal world usually follows the same universal laws

　　D. biologists are applying their findings to other areas