文档介绍：2012 年考研英语阅读理解模拟试题及答案(一) Flatfish, such as the flounder, are among the few vertebrates that lack approximate bilateral symmetry (symmetry in which structures t o the left and right of the body ’s midline are mirror images). Most striking among the many asymmetries evident in an adult flatfish i s eye placement: before maturity one eye migrates, so that in ana dult flatfish both eyes are on the same side of the head. While in most species with asymmetries virtually all adults share the same as ymmetry, members of the starry flounder species can be either left- eyed (both eyes on the left side of head) or right-eyed. In the wate rs between the United States and Japan, the starry flounder populati ons vary from about 50 percent left-eyed off the United States Wes t Coast, through about 70 percent left-eyed halfway between the Un ited States and Japan, to nearly 100 percent left-eyed off the Japan ese coast. Biologists call this kind of gradual variation over a certain geo graphic range a“ cline ” and interpret clines as strong indications tha t the variation is adaptive, a response to environmental differences. For the starry flounder this interpretation implies that a geometric d ifference (between fish that are mirror images of one another) is ad aptive, that left-eyedness in the Japanese starry flounder has been s elected for, which provokes a perplexing question: what is the selec tive 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 sided ness of its eye asymmetry simply by turning around has caused bio logists to study internal anatomy, especially the optic nerves, for th e answer. In all flatfish the optic nerves cross, so that the right opt ic nerve is joined to the brain ’s left side and vice versa. This cross ing introduces an asymmetry, as one optic nerve must cross above or below the other. G. H. Parker reasoned that if, for example, a fl atfish ’s left eye migrated when the right o