[考研英语]考研英语阅读模板(十二).doc

[考研英语]考研英语阅读模板（十二） 阅读原文 来自：新东方在线? 2011-10-26 09:12:00?| 分享到微博?|?写给作者?|?更多▼ ??? ??? SAMPLE 12 　　[生物学类] 　　题目序号?题型归类 　　第1题?细节推导题型 　　第2题?写作手法题型 　　第3题?审题定位与中心主旨题型 　　第4题?词汇题型 　　第5题?审题定位题型 　　 　　Flatfish, such as the flounder, are among the few vertebrates that lack approximate bilateral symmetry (symmetry in which structures to the left and right of the body’s midline are mirror images). Most striking among the many asymmetries evident in an adult flatfish is eye placement: before maturity one eye migrates, so that in an adult flatfish both eyes are on the same side of the head. While in most species with asymmetries virtually all adults share the same asymmetry, members of the starry flounder species can be either left-eyed (both eyes on the left side of head) or right-eyed. In the waters between the United States and Japan, the starry flounder populations vary from about 50 percent left-eyed off the United States West Coast, through about 70 percent left-eyed halfway between the United States and Japan, to nearly 100 percent left-eyed off the Japanese coast. 　　Biologists call this kind of gradual variation over a certain geographic range a “cline” and interpret clines as strong indications that the variation is adaptive, a response to environmental differences. For the starry flounder this interpretation implies that a geometric difference (between fish that are mirror images of one another) is adaptive, that left-eyedness in the Japanese starry flounder has been selected for, which provokes a perplexing question: what is the selective advantage in having both eyes on one side rather than on the other? 　　The ease with which a fish can reverse the effect of the sidedness of its eye asymmetry simply by turning around has caused biologists to study internal anatomy, especially the optic nerves, for the answer. In all flatfish the optic nerves cross, so that the right optic nerve is joined to the brain’s left side and vice versa. This crossing introduces an asymmetry, as