Flatfish, such as the flounder, are 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
10 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 rang 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.