2014年考研英语阅读的经典试题及答案(12)_大学英语阅读

2014年考研英语阅读的经典试题及答案(12)

　　The ease with which a fish can reverse the effect of the sidedness of itseye asymmetry simply by turning around has caused biologists to study internalanatomy, especially the optic nerves, for the answer. In all flatfish the opticnerves cross, so that the right optic nerve is joined to the brains left side and vice versa. Thiscrossing introduces an asymmetry, as one optic nerve must cross above or belowthe other. G. H. Parker reasoned that if, for example, a flatfishs left eye migrated when the rightoptic nerve was on top, there would be a twisting of nerves, which might bemechanically disadvantageous. For starry flounders, then, the left-eyed varietywould be selected against, since in a starry flounder the left optic nerve isuppermost.

　　The problem with the above explanation is that the Japanese starryflounder population is almost exclusively left-eyed, and natural selectionnever promotes a purely less advantageous variation. As other explanationsproved equally untenable, biologists concluded that there is no importantadaptive difference between left-eyedness and right-eyedness, and that the twocharacteristics are genetically associated with some other adaptivelysignificant characteristic. This situation is one commonly encountered by evolutionarybiologists, who must often decide whether a characteristic is adaptive orselectively neutral. As for the left-eyed and right-eyed flatfish, theirdifference, however striking, appears to be an evolutionary red herring.