新gre逻辑阅读每日一练15_GRE阅读

　　35 reasoned that if, for example, a flatfishs 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 exclusively

　　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

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