Modeling the varied avalanches of evolution.

When fossil records point to the demise of a large number of species at roughly the same time, paleontologists often invoke a catastrophe - whether a meteorite impact or a volcanic eruption - as the cause. Similarly, they look for environmental factors to explain the sudden emergence of a variety of new species.

Though such explanations may prove correct, it's also possible that some large changes in an ecosystem occur not because of a cataclysm but because a minor event has started a chain reaction that leads rapidly to widespread disruption. This avalanche behavior can occur in any complex system that automatically organizes itself into a so-called critical state, precariously poised on the edge of catastrophe.

Physicists have explored the possibility that self-organized criticality occurs when sand grains are added to sandpiles, creating avalanches ranging in size from just a few grains to entire slopes (SN: 7/15/89, p.40). They have also studied this behavior in collapsing foams (SN: 3/31/90, p.207), coalescing water droplets (SN: 10/23/93, p. 261), and patterns of earthquake activity,

Now, researchers have developed an extremely simple mathematical model that displays self-organized criticality and appears to capture the way in which biological evolution proceeds via intermittent bursts of activity separated by long periods of quiescence. This model exhibits behavior strongly reminiscent of "punctuated equilibrium," a notion suggested more than 2 decades ago by paleontologists Stephen Jay Gould of Harvard University and Niles Eldredge of the American Museum of Natural History in New York City as an alternative to gradual, step-by-step evolution.

In the new model, proposed by Per Bak of the Brookhaven National Laboratory in Upton, N.Y., and Kim Sneppen of the Niels Bohr Institute in Copenhagen, Denmark, each species in an ecosystem is represented by a random number that corresponds to the "barrier" this species must overcome to evolve further. Arranging these numbers in a line, one picks at each step the smallest number (the species most likely to mutate) and replaces it and the numbers immediately next to it with new random numbers. Thus, the model incorporates the idea that the evolution of a single species also affects the evolution of species with which it interacts.

This model leads to evolutionary avalanches, in which a given species may remain unchanged for long periods, only to go through a number of mutations in a brief spurt. Sometimes, just a few species are affected. At other times, changes are widespread.

Applied to real biology, this model demonstrates that when successful mutations are rare, an ecosystem shows intermittent bursts of evolutionary activity, Sneppen says. Large events in evolutionary history may simply reflect the natural fluctuations of a self-organized critical system rather than the consequences of catastrophes.

Sneppen described the model at an American Physical Society meeting held this week in Pittsburgh.