Orr offers new theory on how species split

Orr

In the August issue of Evolution, due out this month, the University's Allen Orr has put forth the first new explanation in nearly 70 years of the genetic foundations of speciation, the process whereby a single species can split into two.

Orr, an evolutionary biologist, suggests that, faced with environmental change, organisms can evolve through a mix of many minute genetic tweaks, a lesser number of moderate changes, and a few major mutations. The new idea runs counter to standard theory on evolutionary genetics, which holds that only the tiniest of genetic changes contribute to speciation.

No new theory on the genetic architecture of adaptation has been proposed since biologist R.A. Fisher's 1930 assertion that speciation is the result of only minor changes in genes--a view that's come under fire over the last 15 years. Orr's new work is the most thorough attention given to the genetic basis of adaptation in decades, said Nick Barton, a noted evolutionary biologist at the University of Edinburgh in Scotland.

"Allen Orr gives an admirably thorough treatment, which is much more sophisticated than Fisher's brief argument," Barton said. "Overall, I think that Orr's paper will stimulate a good deal of theoretical and empirical work on what is a key question in evolution: what kind of genetic changes contribute to adaptation."

Speciation, which can take anywhere from 100,000 to 1 million years to split a single harmonious species into two reproductively incompatible ones, takes forms ranging from the subtle to the mammoth.

Turning Fisher's argument on its head, Orr suggests that more significant genetic changes do, in fact, contribute to speciation. While few major mutations are needed, the number of more minor mutations rises exponentially with their genetic insignificance.

Orr's theory is based on mathematical predictions and computer simulations, and assumes that a population is well-positioned to adapt to environmental pressures. He now plans to examine whether his theory holds up in the lab.

Orr is an associate professor of biology. His research is sponsored by the National Institutes of Health and the David and Lucile Packard Foundation.