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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