During the experiment, the workers gathered samples every 15 days and measured the microbes' average size. Previous work had suggested that larger bacteria can survive more readily than smaller ones in a low-nutrient environment. For the first few hundred generations (each of which may last just a few hours), the organisms showed little or no change, but then their average size suddenly shot up by almost 30 percent. Another period of stasis then gave way to another upward surge in size. Finally, thousands of generations passed without any further bursts in microbial bulk.

The researchers calculated that millions of bacterial mutations must have occurred over the course their experiment. But only very infrequently did a mutation emerge that significantly boosted the E. coli's fitness, or ability to survive. Lenski's group concluded that the sudden surges in size occur when those "rare, beneficial mutations sweep successively through the population." Once the mutation takes hold, the population returns to equilibrium again as it "awaits" yet another mutation.

Critics have objected that the jerky pattern of size increases observed by the Michigan State group results from examining evolution on too fine a scale. According to the opposing view, less frequent sampling of the bacterial population would have shown evolutionary growth more consonant with the traditional, gradualist conception. Eldredge himself cautions that the punctuated equilibrium was intended to explain the evolution of multicellular, sexually reproducing animals. The processes underlying bacterial evolution are quite different, he points out.

The Michigan State team acknowledges that its experimental approach is far from perfect. On the other hand, so is the fossil record. "We are testing important questions in evolutionary theory," Elena says, "using a simple system that has a large population size and short generations." The researchers now intend to analyze the DNA of successive generations of E. coli to determine what genetic mutations corresponded to the sudden plumping up of their subjects. With luck, Cooper adds, these experiments will illuminate "how microevolutionary processes may impinge on macroevolutionary trends."

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