2017英语专八阅读100篇高级篇附答案解析（74）

2017英语专八阅读100篇高级篇附答案解析（74）

　　Flatfish, such as the flounder, are among the few vertebrates that lackapproximate bilateral symmetry . Most striking among the many asymmetriesevident in an adult flatfish is eye placement: before maturity one eyemigrates, so that in an adult flatfish both eyes are on the same side of thehead. While in most species with asymmetries virtually all adults share thesame asymmetry, members of the starry flounder species can be either left-eyed or right-eyed. In the waters between theUnited States and Japan, the starry flounder populations vary from about 50percent left-eyed off the United States West Coast, through about 70 percentleft-eyed halfway between the United States and Japan, to nearly 100 percentleft-eyed off the Japanese coast.

　　Biologists call this kind of gradual variation over a certain geographicrange a “cline” and interpret clines as strong indications that the variation isadaptive, a response to environmental differences. For the starry flounder thisinterpretation implies that a geometric difference is adaptive, that left-eyedness in the Japanesestarry flounder has been selected for, which provokes a perplexing question:what is the selective advantage in having both eyes on one side rather than onthe other?

　　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 brain’s 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 flatfish’s 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.

　　1. According to the text, starry flounder differ form most other speciesof flatfish in that starry flounder

　　[A] are not basically bilaterally symmetric.

　　[B] do not become asymmetric until adulthood.

　　[C] do not all share the same asymmetry.

　　[D] have both eyes on the same side of the head.

　　2. Which of the following best describes the organization of the text asa whole?

　　[A] A phenomenon is described and an interpretation presented andrejected.

　　[B] A generalization is made and supporting evidence is supplied andweighed.

　　[C] A contradiction is noted and a resolution is suggested and thenmodified.

　　[D] A series of observations is presented and explained in terms of thedominant theory.

　　3. The text supplies information for answering which of the followingquestions?

　　[A] Why are Japanese starry flounder mostly left-eyed?

　　[B] Why should the eye-sidedness in starry flounder be consideredselectively neutral?

　　[C] Why have biologists recently become interested in whether acharacteristic is adaptive or selectively neutral?

　　[D] How do the eyes in flatfish migrate?

　　4. Which of the following is most clearly similar to a cline as it isdescribed in the second paragraph of the text?

　　[A] A vegetable market in which the various items are grouped accordingto place of origin.

　　[B] A wheat field in which different varieties of wheat are planted toyield a crop that will bring the maximum profit.

　　[C] A flower stall in which the various species of flowers are arrangedaccording to their price.

　　[D] A housing development in which the length of the front strutssupporting the porch of each house increases as houses are built up the hill.

　　5. Which of the following phrases from the text best expresses the author’s conclusion about the meaning ofthe difference between left-eyed and right-eyed flatfish?

　　[A] “Most striking”

　　[B] “variation is adaptive”

　　[C] “mechanically disadvantageous”

　　[D] “evolutionary red herring”

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