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No. 44: Mar-Apr 1986

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Evolving on half a wing (and a prayer?)

Just about everyone agrees that half a wing is of little use to an animal "straining" to develop the capability of flight. So, how did the marvelously crafted wings of birds, insects, and mammals evolve in infinitesimal steps? Biologists, including Darwin himself, have long puzzled over this. Stephen Jay Gould in a recent article explores a currently favored way of circumventing the negligible additional survival value of half a wing, or even 90% of a wing. This solution (?) maintains that protowings were not "intended" for flight at all but were developed initially as aerodynamic stabilizers, thermoregulatory systems, sexual attractors or other functions requiring large areas.

Gould describes the experiments of Kingsolver and Koehl in which protowings were modelled and tested for their thermoregulatory and flight values. Surprisingly, there was a sharp transition, as the size of the protowing increased, from good thermoregulation but poor flight capability to the reverse -- good flight capability and poor thermoregulation. In other words, a structure developed for one purpose, if enlarged, might be useful for something else!

(Gould, Stephen Jay; "Not Necessarily a Wing," Natural History, 94:14, October 1985. See also: Lewin, Roger, "How Does Half a Bird Fly?" Science, 230:530, 1985.)

Comment. The work of Kingsolver and Koehl, though doubtless of high quality, does not come to grips with the fact that a wing for flight is a highly sophisticated combination of skeleton, feathers, membrane, muscles, nervous system, control system, aerodynamic design, etc. -- most of which have nothing to do with thermoregulation. Are the flying crustaceans mentioned on page ** a failed evolutionary attempt at useful flight? And where did their feathery appendages come from? Somewhere we -- all of us -- are missing something!

From Science Frontiers #44, MAR-APR 1986. 1986-2000 William R. Corliss