Evolutionary “founder effect” detected

Thomas Schoener and David Spiller, both professors in the UC Davis Evolution and Ecology Department, are among the co-authors of a new study appearing in Science magazine that provides evidence for an evolutionary phenomenon called the “founder effect.”

The founder effect is the evolutionary theory that a small number of “founders,” or initial members of a population in a specific location, can have a long-lasting effect on the genetic composition of subsequent generations.

“We actually saw a hurricane [in 2004] wipe out a population of lizards on small islands that we had been monitoring,” Schoener said . Since the cays, or tiny islands, were cleared of lizards, they were able to introduce male and female lizard pairs on a number of island cays as an experiment to test the possible effects of the founder effect.

“Most of the vegetation [after a hurricane] survives,” Schoener said . “The other things come back fairly quickly. Spiders get completely extirpated, but most of the arthropods can come back pretty quickly. Either they fly, or with the case of the spiders they balloon.”

Schoener, as part of a previous study, had introduced larger founding populations to small islands, with a ratio of three females to two males. For the current study, however, the researchers decided to test the smallest possible number of lizards which could potentially form a founding colony: one female and one male.

“That’s as small as you can get,” Schoener said .

They chose seven pairs of lizards from forests containing large trees on a large island to be introduced on seven small cays where there were small bushes.

Scientists monitored the lizards to see if the lengths of the lizards’ hind legs became shorter over subsequent generations. They measured the leg lengths of the male and female founders on a particular cay to see if the founder leg lengths had a lasting effect on subsequent generations or whether processes such as natural selection or genetic drift were enough to explain the lizards’ evolving leg lengths.

Schoener found that both natural selection and founder effects were involved in the leg lengths. They were able to find lasting influences of the initial leg lengths on subsequent generations.

“Adaptive evolution,” said David Reznick, a biology professor at UC Riverside who was not involved in the study, is “genetic change in populations that makes them better able to survive and produce offspring in a given environment.”

For example, lizards living among small bushes would be expected to develop smaller hind legs after successive generations to facilitate walking on the small branches.

It was not known whether “just one male and one female harbor sufficient genetic variation for adaptive evolution to be possible,” Reznick said . “Their experiment suggests that’s true and that was not expected.”

“I think the uniqueness of this study derives from its focus on an evolutionary process that doesn’t get much attention. Natural selection certainly dominates the study of evolution and the public’s perception of it,” said Michael Kinnison, a biology professor at the University of Maine who was not involved in the study. “However, in reality, natural selection is one process among several that are thought to shape genetic diversity.”

“What their study shows is that there was in fact a founder effect,” Reznick said. “There’s a real trace of adaptive evolution there and it happened very fast, in spite of the very small number of founders.”

For the current study, Schoener said, “We kind of took a chance, and it paid off.”

BRIAN RILEY can be reached at science@theaggie.org.