Keeping Things in Proportion
Evolutionary biologists take some things for granted: for instance, that the length of a creature's arms or wings will be proportional to its body size. But a new study shows this isn't always the case. Researchers have bred butterflies to have disproportionate wing-to-body ratios, suggesting that natural selection plays a larger than expected role in determining animal form.
The proportionality of animal shapes has long fascinated and divided evolutionary scientists. Some argue that these relationships are the result of genetic constraints that limit variation in shape and size. Others prefer the idea that natural selection is the true driver of body form.
Evolutionary biologists Paul Brakefield of Leiden University in the Netherlands and Anthony Frankino of Princeton University in New Jersey, set out to settle the debate by breeding the Bicyclus anynana butterfly. The team culled each generation, selecting only butterflies at the extremes of wing and body size ratio. To their surprise, canceling out the natural wing-to-body size rule of thumb was easy. After only 13 generations of artificial selection, the team had two distinctly different groups of butterflies: ones with small wings and a beefy body; and ones with large wings and petite body.
Frankino and Brakefield then set up a mating contest between the modified males and normal males to see what would happen when natural selection was allowed to govern reproduction. In two breeding trials, normal males were 3 times more successful than the modified males in mating, the team writes in the 4 February issue of Science. "At least in the short term, there seemed to be no limit to how much we could change the body-to-wing ratio through artificial selection, but natural selection immediately selected for the proportional form," said Frankino.
"This research unambiguously resolves the controversy" of whether genetic controls constrain proportionality, said evolutionary biologist Doug Emlen of the University of Montana, Missoula. Developmental biologist Joseph Kunkel of the University of Massachusetts in Amherst says the study does not address the real issue. Researchers should stop searching for genetic limits on variation and instead "focus on discovering the mechanisms that allow species to stabilize a 'normal' body style," he said.