新GRE逻辑阅读 每日一练15

　　编辑点评： GRE阅读考试的材料，非常强调逻辑的关系，所以逻辑要点是大家在解GRE阅读题的时候的关键，本文就为大家例举了这样一篇逻辑短文，供大家参考和练习。

　　随着GRE阅读考试的难度日益增加，大家需要有针对性地进行练习，本文为大家例举的就是逻辑短文的练习，文末还附有答案，供大家更好地练习。

　　Flatfish, such as the flounder, are among the few vertebrates that lack approximate bilateral symmetry . Most striking among 5 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 10 starry flounder species can be either left-eyed 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 15 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 rang a cline and interpret clines as strong indications that the variation is adaptive, a 20 response to environmental differences. For the starry flounder this interpretation implies that a geometric difference is adaptive, that left-eyedness in the Japanese starry flounder has been selected for, which provokes a 25 perplexing questions: 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, 30 especially the optic nerves, for the answer. In all flat fish 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 one optic nerve must cross above or below the other. G. H. Parker 35 reasoned that if, for example, a flatfish s left eye migrated when the right optic nerve was on top, there would be a twisting of nerves, which might be mechanically disadvantageous. For starry flounders, then, the left-eyed variety would be selected against, since in a 40 starry flounder the left optic nerve is uppermost. The problem with the above explanation is that the Japanese starry flounder population is almost exclusivel left-eyed, and natural selection never promotes a purely less advantageous variation. As other explanations 45 proved equally untenable, biologists concluded that there is no important adaptive difference between left- eyedness and right-eyedness, and that the two characteristics are genetically associated with some other adaptively significant characteristic. This 50 situation is one commonly encountered by evolutionary biologists, who must often decide whether a characteristic is adaptive or selectively neutral. As for the left-eyed and right-eyed flatfish, their difference, however striking, appears to be an 55 evolutionary red herring.

　　10. According to the passage, starry flounder differ from most other species of flatfish in that starry flounder

　　are not basically bilaterally symmetric

　　do not become asymmetric until adulthood

　　do not all share the same asymmetry

　　have both eyes on the same side of the head

　　tend to cluster in only certain geographic regions For the following question, consider each of the choices separately and select all that apply

　　11. The author would be most likely to agree with which of the following statements about left-eyedness and right-eyedness in the starry flounder?

　　□A They are adaptive variations by the starry flounder to environmental differences.

　　□B They do not seem to give obvious selective advantages to the starry flounder.

　　□C They occur in different proportions in different locations.

　　12. According to the passage, a possible disadvantage associated with eye migration in flatfish is that the optic nerves can

　　adhere to one another

　　detach from the eyes

　　cross

　　stretch

　　twist

　　13. Which of the following best describes the organization of the passage as a whole?

　　A phenomenon is described and an interpretation presented and rejected.

　　A generalization is made and supporting evidence is supplied and weighed.

　　A contradiction is noted and a resolution is suggested and then modified.

　　A series of observations is presented and explained in terms of the dominant theory.

　　A hypothesis is introduced and corroborated in the light of new evidence.

　　答案：C/BC/E/A

　　上文的题目大家答对了几题?大家不仅要多做题，更要善于总结，这样才会有针对性地提高。