About 150 years ago Darwin proposed that the different developing body parts of an organism might compete with each other for resources as they grow within an egg or a womb. That competition, Darwin believed, would determine the final size of the body parts. More recently biologists have favored the idea that genetic constraints, not competition, determine the size of developing organs. But now it seems that Darwin may have been right after all.
Biologists Fred Nijhout of Duke University and Douglas Emlen of the University of Montana had been studying metamorphosis in insects. A caterpillar, for instance, has small clusters of cells destined to become the wings, legs, and mouthparts of a butterfly. The cells don't grow much as the caterpillar eats leaf after leaf. But when the caterpillar stops feeding, just before metamorphosis, the cells "suddenly grow like gangbusters," says Nijhout. This peculiarity, he realized, gave him the perfect opportunity to test Darwin's theory. Since the caterpillar wasn't eating, Nijhout could observe the development of body parts when a limited supply of nutrients was available for growth-the perfect condition for competition.
The researchers first anesthetized caterpillars and removed the cells that would ordinarily become hind wings. When the butterflies emerged from their cocoons two weeks later, their front wings were larger than those of normal butterflies. The mass of the front wings had increased by the same amount that would have been in the missing wings. Next Nijhout and Emlen looked at horned beetles. When they treated beetle larvae with a hormone that curbs horn growth, the beetles grew bigger eyes. They also noticed that beetles bred to grow big horns invariably had smaller eyes than beetles bred to have small horns.
Was Darwin right after all? "The simple explanation would be that these things are truly competing for nutrients," Nijhout says. But it's also possible, he adds, that each part interferes with the others' growth, producing substances that stunt other tissues. "We know very little about these higher-level physiological controls."