The best-known example of equilibrium thinking in evolution is John Maynard Smith's celebrated concept of an "evolutionarily stable strategy"--the strategy (or mix of strategies) that emerges when each organism maximizes its fitness given the strategies chosen by other organisms. An ESS is virtually indistinguishable from the economist's concept of equilibrium--in fact, the equilibrium of an economic model almost always is an ESS.

But isn't evolution a domain of constant change, where equilibrium concepts are useless? Consider, if you will, the race--literally--between carnivores and herbivores. Other things being equal, a faster carnivore is better able to catch its meal; a faster herbivore is better able to avoid becoming one. So you might expect both predators and prey to become ever faster as they evolve. But speed comes at a cost (fast but skinny cheetahs tend to get killed by slower but more robust lions), so it is more likely that at some point, this race will settle down to an equilibrium, in which carnivores are optimally fast given the speed of herbivores, and vice versa. Sure enough, the evidence indicates that chases across the savanna have taken place at about the same speed for the last 50 million years. "The data," writes Maynard Smith in his authoritative text, Evolutionary Genetics, "suggest that there has been an arms race, but one in which, for long periods, neither side was able to make further improvement."

Obviously, change does take place: I am not an Australopithecus. But evolutionists have found equilibrium thinking extremely useful. Wouldn't it be strange if economists didn't?