Researchers Say Organs May Shift Functions : New Evolution Theory Takes Wing

A major sticking point of evolutionary theory has always been the question of how complex structures initially arose.

The Darwinian theory of natural selection suggests that complex structures arise incrementally, a little at a time. A wing, for example, might start out as a small nub extending from the body and grow gradually.

The problem with this approach, according to biologist Stephen Jay Gould of Harvard University, is that “you can’t fly with 2% of a wing.”

In other words, if you can’t fly at all with a little bit of a wing, then there is no evolutionary pressure for it to grow into a full-sized wing.

Creationists frequently use such arguments in attempting to refute evolution. Even some biologists have argued that such adaptations as wings may have arisen from a drastic mutation.

New evidence, however, suggests another theory in which an organ evolved for one purpose can be shifted to an alternative purpose with only minor changes. Mimi Koehl of the University of California, Berkeley, and Joel Kingsolver of Brown University, in a recent issue of the technical journal Evolution, report experimental results that strongly suggest that this process of change of function--called “exaptation”--can explain how insect wings first arose.

The question of how wings evolved has been a philosophical one because theories have been difficult to test. Some scientists, for example, speculate that the first wings, called protowings, played a role in courtship, a potential role that, at this late date, is impossible to study.

The most plausible explanation now seems to be that protowings served to absorb heat from sunlight to help the insect maintain the higher body temperature required for a high level of physical activity.

Koehl and Kingsolver prepared epoxy models of insect bodies of various sizes. They then fitted these artificial bodies with artificial protowings, also of various sizes, and tested their ability to gather and conduct heat and their lifting ability in a wind tunnel.

They found that increasing the size of the wing increased the amount of heat trapped and carried to the body, but only up to a limit. If the protowing was more than about half an inch long, the heat trapped by the additional size was dissipated in transmitting the heat to the body. Evolution would thus clearly favor wing growth up to a size of about half an inch, after which no additional benefit could be gained.

Koehl and Kingsolver’s studies also showed that aerodynamic lift was sufficient for gliding only if the wing was longer than about an inch to an inch and a half. Again, evolution would favor increasing the length of the wing, but only once it was longer than about an inch.

The problem, then, is explaining what force would cause the wings to grow from the size where they are efficient as thermal regulators to a size where they are efficient airfoils.

Koehl and Kingsolver think they also have an answer to that. They have found that the minimum size of the wing required for flight, about an inch, is relatively constant, no matter how big the body is.

“Consider an insect that is, say, an inch long and that has a proto-wing span of half an inch,” Kingsolver said. “If that species should, over the course of time, grow to twice its size (so that the wing would be an inch long) for reasons completely unrelated to flight or to thermo-regulation, it would suddenly find that it could glide. Normal evolution could then take over to increase the efficiency of the wings.”