Biologists have observed a theory of species evolution known as the founder effect in action for the first time, U.S. university researchers reported.

The founder effect, first outlined by German evolutionary biologist Ernst Mayr in 1942, says that when a small group of individuals from a genetically diverse population of some species migrates away and "founds" a new colony, the founders' genes play a dominant evolutionary role in the new population for generation after generation.

Biologists, including scientists who did postdoctoral work the University of California, said they wanted to see if the founder effect was real -- it had never been observed in action because evolution takes place so slowly.

They visited heavily forested Iron Cay, a Bahamas island spared the ravages of 2004's Hurricane Frances, and took brown anole lizard couples from the island at random to seven tiny treeless islands nearby where no lizards remained after the Category 4 hurricane.

On each island they released a single lizard pair, they said.

The Iron Cay lizards long ago evolved long hind legs to run swiftly along broad tree branches to avoid predators, the researchers reported.

But on the seven islands slammed by Frances, the anole lizards that drowned had short hind legs, better suited for darting in and out of the short, tangled, scrubby bushes that thrived there, the San Francisco Chronicle reported, citing the researchers.

If the founder effect held up, succeeding generations of the transplanted lizards would maintain their long hind legs, even though the original lizard residents had short legs, the researchers postulated.

The researchers returned to the islands every year to observe and measure the legs on each new generation of lizards, which now populate the islands.

After five or six generations, the founder effect appeared to hold up, with the new generations still sporting the long hind legs of their ancestors, the scientists reported.

But they saw another evolutionary force emerge, they said.

The lizards' long legs began shortening as each generation adapted to the scrub-bush environment, they said.

The scientists realized they were also witnessing Charles Darwin's natural selection, a key mechanism of evolution, they said.

"In this case, we've seen both the founder effect and natural selection operating right before our eyes -- for the first time," Jason Kolbe, a postdoctoral fellow at U.C. Berkeley and U.C. Davis and now at the University of Rhode Island, wrote in the American Association for the Advancement of Science journal Science Express.