EAST LANSING, Mich. - In studying the evolution of populations of organisms, "use it or lose it" often applies - whether talking about a bacteria's sweet tooth or, possibly, the mechanisms that maintain youth.

Scientists at Michigan State University have found that successful adaptation to a specific environment is all about trade-offs. In a paper published in the Oct. 12 edition of the British science journal Nature, they work to untangle one of the big questions in evolution: Do populations become more specialized and give up functions because it's efficient or because unused functions allow genetic mutations to accumulate?

"What leads to a reduction in function is a big question, and we've never been able to identify which mechanism is more important," said Vaughn Cooper, a postdoctoral researcher in the laboratory of renowned evolutionary biologist Richard Lenski, with whom he co-authors the Nature paper. "The two mechanisms are always coinciding in evolution and are hard to disentangle."

The study examined 20,000 generations of E.coli bacteria to trace the genetic trail of evolution.

The many generations of rapidly multiplying E.coli are the stage to play out two different possibilities for why this happens. The MSU team raised 12 strains of E. coli exclusively on glucose, to see if they would lose their ability to grow on other sugars.

They did.

But the question remained: Why? Were the genes that allowed them to eat other sugars storing up devastating mutations that were no longer significant to the bacteria? Or did shedding a diverse diet make them stronger glucose eaters?

The research indicates primarily the latter. The mechanism is called antagonistic pleiotropy - growing stronger by dumping unnecessary attributes and focusing on what works best.

In the world outside a test tube, the process can be seen at work in the fish that live in the eternal darkness of caves. The fish have evolved blind.

One could suppose the fish lost their eyes because of generations of unchecked mutations in the genes that underlie eye development. But the MSU work points to antagonistic pleiotropy to explain this, saying that natural selection may favor the elimination of eyes - either because eyes are costly or because of selection for another trait like an enhanced sense of smell.

Both theories of selection are thought to be responsible for the aging process in populations, Cooper said.

One thought is that selection is on our side when in childbearing age. Our bodies focus on developing the attributes that are best suited for successful reproduction - including youth.

On the down side, with age comes an accumulation of mutations that aren't beneficial since in the big evolutionary picture the organism is not regarded as necessary.

"The genes that are responsible for survival later in life are not favored by selection," Cooper said. "Our lab hopes to identify particular mutations of how an organism adapts and why it becomes specialized."

The creating of evolution in a test-tube continues work Lenski introduced in January 1995 in a Science publication using some 2,000 generations of E. coli to "replay life's tape" to recreate scenarios of evolution.

The work is funded by the National Science Foundation.