2. 考研
  3. Write a summary to cover the important points of the following passage. Do not put your own ideas, opinions, or interpretations

Write a summary to cover the important points of the following passage. Do not put your own ideas, opinions, or interpretations

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问题     Write a summary to cover the important points of the following passage. Do not put your own ideas, opinions, or interpretations into the summary, and limit your summary at about 100～150 words.
    Flatfish, such as the flounder, is among the few vertebrates that lack approximate bilateral symmetry (symmetry in which structures to the left and right of the body’s midline are mirror images). Most striking among 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 starry flounder species can be either left-eyed (both eyes on the left side of head) 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 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 range a cline and interpret clines as strong indications that the variation is adaptive, a response to environmental differences. For the starry flounder this interpretation implies that a geometric difference (between fish that are mirror images of one another) is adaptive, that left-eyedness in the Japanese starry flounder has been selected for, which provokes a 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, especially the optic nerves, for the answer. In all flatfish 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 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 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 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 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 evolutionary red herring.
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答案 With both eyes on the same side of the head, flatfish is among the few vertebrates that are asymmetric. While in most species with asymmetries virtually all adults share the same asymmetry, starry flounders are different. They are either left-eyed or right-eyed. And the rate of the left-eyed varies over geographic range. This variation was thought to be adaptive at first place. Biologists studies the internal anatomy, especially the optic nerve to find the answer. It turns out that the left-eyed would be selected against. However, the Japanese starry flounder population is almost exclusively left-eyed, and natural selection never promotes a purely less advantageous variation. So, the interpretation above was rejected. As other explanations proved equally untenable, biologists concluded that there is no important adaptive difference between left-eyedness and right-eyedness.
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