Study Suggests Organs May Shift Their Functions : A New Theory Takes Flight on the Wings of Insects
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, contending that complex structures such as the wing must have arisen full-grown from the hand of God. 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 evolution of wings presents two sorts of questions,” Kingsolver said in a telephone interview. “The first sort are: What body structures did wings arise from? And, did wings evolve in a terrestrial or an aquatic environment?
“The second sort is: What was the function of wings once they arose?”
The first sort of question is the type that would normally be studied by looking at the fossil record. “Unfortunately, there are no good fossils from this crucial transitional period” 400 million years ago, Kingsolver said.
The second question has been more philosophical in nature 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 popular theory is that protowings might have helped provide body orientation in leaping insects so they could land on their feet. Two decades ago, however, J. W. Flower of the University of Bristol performed aerodynamic calculations indicating that legs are more important than protowings in providing such stability.
The calculations also showed that selection for orientation would lead to decreased size of the insects, since smaller insects fall more slowly. Nonetheless, many scientists still favor this theory.
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. That theory was first proposed in 1978 by M. M. Douglas, who was then a graduate student at the University of Kansas.
The best thing about this explanation is that it is susceptible to test.
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.
Growth of Wings
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.”
This means, Kingsolver and Koehl wrote, “that geometrically identical forms may serve different functions at different body sizes.”
By invoking this change of function with increasing size, the two scientists have apparently solved the riddle of how at least one complex organ could have been produced by evolution. And in the process, they may have removed one of the stumbling blocks to a greater understanding of evolution.
