GRE阅读以艰深著称，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 peacocks tail, elaborate vocal characteristics increase the males 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.