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summer science quiz #2

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  • summer science quiz #2

    Some comments on the origins and utility of bi-lateral symmetry
    (i.e., even numbers of limbs) in animals.

    >My solution: Bilateral symmetry in the animal body plan produces the even
    >number of limbs, but those animals that develop with radial symmetry (like
    >the starfish) can have odd numbers of limbs. Why does bilateral symmetry
    >predominate in the animal kingdom would perhaps be a better question. My
    >answer to that question is: pure dumb luck.

    Dumb luck is possible, but before we accept this non-functional
    explanation, we should consider whether bi-lateral symmetry might
    have a functional role that lead to its being selected for (rather
    than merely tolerated) in the evolutionary history of animals.

    Bi-lateral symmetry is locomotor appendages is not universal, cf.
    star fish, and snails (the latter have only one foot, an odd number).
    But it is very common (including fish, which don't have legs, but do
    have bi-laterally symmetric fins, and even have bi-laterally
    symmetric muscles attached to their otherwise unitary tails). One
    thing that distinguishes bi-laterally symmetric animals from
    asymmetric ones is that the former move faster (relative to the
    viscosity of their media). Bi-lateral asymmetry in quick-moving
    animals is useful simply because it makes for more efficient
    locomotion when speed matters. Star fish are in no hurry, but if
    they had evolved in the direction of faster locomotion, their
    asymmetric bodies would have imposed rather severe constraints on how
    the increase in speed could be accomplished (i.e., consider what odd
    interlimb coordination patterns would be required for a starfish to
    "run"). Bi-laterally symmetric animals, such as fish or humans, have
    a great advantage when it comes to organizing and achieving

    I don't think the "choice" was dumb luck.

    Thomas A. Stoffregen, Associate Professor
    Department of Psychology
    University of Cincinnati
    P. O. Box 210376
    Cincinnati, OH 45221-0376

    Office: 513-556-5535; Lab: 513-556-5682; Fax: 513-556-1904


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