GRE阅读做题指导手记
GRE阅读以艰深着称，很多理论讲解使得考生觉得如雾里云中，下面，我带大家来一起做一篇题目
Over the years， biologists have suggested two main pathways by which sexual selection may have shaped the evolution of male birdsong。
这是本文的主题句，告诉我们一下的`信息：
1．本文探讨的是一个现象，是关于鸟的歌曲的性选择问题。
2．本文是一个新老观点对比型的文章，关键词在第一段中间或者第二段开头
3．本文谈到了进化论，那么在文章后面很可能会否定这个说法，并且可能会进行批驳。
4．下文会以性内和性间选择分别展开进行论述。
In the first， male competition and intrasexual selection produce relatively short， simple songs used mainly in territorial behavior。 In the second， 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 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 breekphvoqgl bor research$intk mnpevsexual 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。
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