gre考试阅读真题解析(A)_GRE阅读

gre考试阅读真题解析(A)

　　20. Which of the following best describes the passage?

　　 It discusses an apparent inconsistency and suggests a reason for it.

　　 It outlines a sequence of historical events.

　　 It shows why a commonly held view is inaccurate.

　　 It evaluates an explanation and finally accepts that explanation.

　　 It contrasts two views of an issue.

　　What causes a helix in nature to appear with either a dextral twist or a sinistral twist is one of the most intriguing puzzles in the science of form. Most spiral-shaped snail species are predominantly dextral. But at one time , handedness was equally distributed within some snail species that have become predominantly dextral or, in a few species, predominantly sinistral. What mechanisms, control handedness and keep left-handedness rare?

　　It would seem unlikely that evolution should discriminate against sinistral snails if sinistral and dextral snails are exact mirror images, for any disadvantage that a sinistral twist in itself could confer on its possessor is almost inconceivable. But left- and right-handed snails are not actually true mirror images of one another. Their shapes are noticeably different. Sinistral rarity might, then, be a consequence of possible disadvantages conferred by these other concomitant structural features. In addition, perhaps left- and right-handed snails cannot mate with each other, having incompatible twist directions. Presumably an individual of the rarer form would have relative difficulty in finding a mate of the same hand, thus keeping the rare form rare or creating geographically separated right-and left-handed populations.