Evolution On The Fast Track

November 20, 1996


MADISON _ By altering as few as a half-dozen genes of a small African butterfly, scientists have shown that some animals can be sent down the road to becoming a new species in the blink of an eye.

New findings, reported today (Nov. 21) by an international team in the scientific journal Nature, paint the first comprehensive picture of how animal patterns, in response to selective pressure in the wild or in the laboratory, can evolve rapidly through simple genetic change.

The new discovery helps explain biological diversity at the genetic and molecular level, seasonal variation in animal appearance, and how geographically separated animals of a single species can sport vastly different patterns of spots, stripes and colors.

The research was conducted by a team that included Sean B. Carroll of the Howard Hughes Medical Institute at the University of Wisconsin-Madison, Paul M. Brakefield at the University of Leiden in the Netherlands and Vernon French of the University of Edinburgh in Scotland.

It has long been known that under the varying circumstances of nature, when different members of a single species are separated geographically for a period of time, they can exhibit dramatic differences in appearance. This is known to occur widely in the animal kingdom among insects, fish, birds and reptiles.

"The question," said Carroll, "is how different are these animals? The answer may be as few as a handful of genes." The set of experiments described in today's Nature capitalizes on the ability of a butterfly known to scientists as Bicyclus anynana to change the pattern of spots on its wings in response to seasonal changes in its African home.

During the rainy season, the butterfly exhibits characteristic wing eyespots that serve as decoys for prowling lizards and birds, luring them away from vital body parts. During the dry season, when there is little food and the butterflies are inactive against a dull brown backdrop, new generations emerge that lack the target-like eyespots.

"In the dry season, the presence of eyespots is a big disadvantage. They are like bulls-eyes. Predators pick them right off," said Carroll.

In butterflies _ as in snakes, lizards, birds and mammals _ color and pattern distinguish an animal and help conceal it from enemies and prey. But when the environment changes or populations migrate, the animal must change, too, or imperil its survival, said Carroll, a molecular biologist.

By selecting for butterflies with slight differences in eyespot size and interbreeding them over many generations, the scientists developed lines of butterflies with dramatically different eyespot patterns. Moreover, the team identified several individual genes that control the number, size or pattern of eyespots, and determined when during development those genes came into play.

"What these experiments illustrate is just how flexible these patterns are," said Carroll. "We can mimic evolution in the lab, virtually sending these animals down the road to becoming different species."

The fact that just a few genes have such a big and rapid influence in how an animal looks goes a long way toward explaining biological diversity, Carroll said. The changes emerged in just a few years in the laboratory, over the course of about 20 generations of the butterfly.

"If we can accomplish this with a just few genetic changes over a short period of time, and if you think of the millions of species of animals out there, we can get a glimpse of how evolution unfolds over fifty or a hundred million years."
_ Terry Devitt, (608) 262-8282, trdevitt@facstaff.wisc.edu

University of Wisconsin-Madison

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