UGA GIves Evolutionary Biology a New Approach

In a new approach to evolutionary biology, scientists have estimated how fast mammals can undergo large-scale evolution.

The recently published study traces changes in the size of mammals subsequent to the time dinosaurs went extinct. It surveyed a wide variety of mammals falling into 28 distinct categories, including rodents, monkeys, whales and elephants. The scope in time and space was also broad, covering the last 70 million years, and multiple continents and oceans.

The rate of change in size was measured in terms of generations rather than years to allow comparison between organisms with differing life spans. According to the findings of the study, it takes on the order of 24 million generations for an animal the size of a mouse to evolve into one as big as an elephant. On the other hand, they say, large decreases in size resulting in dwarfism can occur more quickly, in as little as 100,000 generations.

The research team includes dean John Gittleman and research scientist Patrick Stephens. They are working with evolutionary biologists from Monash University in Australia, among them team leader Alistair Evans, a professor in the School of Biological Sciences at Monash.

Evans says the study is unusual since earlier studies in the field of evolutionary biology have generally considered only much shorter periods of time--tens or hundreds of generations, not hundreds of thousands or millions,

"This is the first study to try to quantify evolutionary rates at such a large time scale and across so many groups of species," says Gittleman. "This is important because it can begin to tell us a lot about how species adapt. Some have the capacity to evolve more quickly than others. That might bode well for those species in the future as major environmental changes occur."

"We can now show that it took at least 24 million generations to make the proverbial mouse-to-elephant size change--a massive change, but also a very long time," says Evans. "A less dramatic change, such as rabbit-sized to elephant-sized, takes 10 million generations."

Co-author Erich Fitzgerald, a curator at Museum Victoria in Melbourne, Australia, says the rate of increase in the size of whales was twice that of terrestrial mammals.

"This is probably because it's easier to be big in the water," he says, "It helps support your weight."

Evans was surprised to find body size decreased more than ten times faster than it increased.

"The huge difference in rates for getting smaller and getting bigger is really astounding," he said, "We certainly never expected it could happen so fast."

The paper, entitled "The maximum rate of mammal evolution," appeared in a recent issue of the Proceedings of the National Academy of Sciences.

OTHER UGA RESEARCH:

An alternative account of mammalian evolution