GRE閱讀以艱深著稱，很多理論講解使得考生覺得如霧裡雲中，下面，我帶大家來一起做一篇題目
Over the years, biologists have suggested two main pathways by which sexual selection may have shaped the evolution of male birdsong.
這是本文的主題句，告訴我們一下的.資訊：
1.本文探討的是一個現象，是關於鳥的歌曲的性選擇問題。
2.本文是一個新老觀點對比型的文章，關鍵詞在第一段中間或者第二段開頭
3.本文談到了進化論，那麼在文章後面很可能會否定這個說法，並且可能會進行批駁。
4.下文會以性內和性間選擇分別展開進行論述。
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.
