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  3. Over the years, biologists have suggested two main pathways by which sexual selection natural selection for characters that conf

Over the years, biologists have suggested two main pathways by which sexual selection natural selection for characters that conf

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问题     Over the years, biologists have suggested two main pathways by which sexual selection natural selection for characters that confer success in competition for a mate as distinguished from competition with other species; also: the choice of a mate based on a preference for certain characteristics may have shaped the evolution of male birdsong. In the first place, male competition and intrasexual selection produce relatively short, simple songs used mainly in territorial behavior. In the second place, female choice and intersexual selection produce longer, more complicated songs used mainly in mate attraction; like such visual ornamentation as the peacock’s tail, elaborate vocal characteristics increase the male’s chances of being chosen as a mate, and he thus enjoys more reproductive success than his less ostentatious rivals. The two pathways are not mutually exclusive, and we can expect to find examples that reflect their interaction. Teasing for microscopic examination them apart has been an important challenge to evolutionary biologists.
    Early research confirmed the role of intrasexual selection. In a variety of experiments in the field, males responded aggressively to recorded songs by exhibiting territorial behavior near the speakers. The breakthrough for research into intersexual selection came in the development of a new technique for investigating female response in the laboratory. When female cowbirds raised in isolation in sound-proof chambers were exposed to recordings of male song, they responded by exhibiting mating behavior. By quantifying the responses, researchers were able to determine what particular features of the song were most important. In further experiments on song sparrows, researchers found that when exposed to a single song type repeated several times or to a repertoire of different song types, females responded more to the latter. The beauty of the experimental design is that it effectively rules out confounding variables; acoustic isolation assures that the female can respond only to the song structure itself.
    If intersexual selection operates as theorized, males with more complicated songs should not only attract females more readily but should also enjoy greater reproductive success. At first, however, researchers doing fieldwork with song sparrows found no correlation between larger repertoires and early mating, which has been shown to be one indicator of reproductive success; further, common measures of male quality used to predict reproductive success, such as weight, size, age, and territory, also failed to correlate with song complexity.
    The confirmation researchers had been seeking was finally achieved in studies involving two varieties of warblers. Unlike the song sparrow, which repeats one of its several song types in bouts before switching to another, the warbler continuously composes much longer and more variable songs without repetition. For the first time, researchers found a significant correlation between repertoire size and early mating, and they discovered further that repertoire size had a more significant effect than any other measure of male quality on the number of young produced. The evidence suggests that warblers use their extremely elaborate songs primarily to attract females, clearly confirming the effect of intersexual selection on the evolution of birdsong.
The passage indicates that researchers raised female cowbirds in acoustic isolation in order to______.
选项 A、eliminate confounding variables
B、approximate field conditions
C、measure reproductive success
D、quantify repertoire complexity
答案A
解析 细节题。文中“The beauty of the experimental design is that it effectively rulesout confounding variables；acoustic isolation assures that the female can respondonly to the song structure itself．”从该句可以得出研究者之所以把femalecowbirds放在语音封闭的环境中饲养，是出于消除不利变量的考虑。
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