The fossil record is notoriously stingy in doling out clues about the history of life. Biologists agonize over whether they are inferring a distorted view of the past from the bits of bone that they pluck from the vast expanse of the earth's accumulated sediments. But because evolution proceeds so slowly, scientists cannot test their ideas by watching it unfold in real time.
Or can they? In an ingenious experiment conducted at Michigan State University, researchers have observed evolutionary changes as a family of bacteria passed through 3,000 generations. The procedure took a mere four years; conducting an analogous experiment with Homo sapiens would require roughly 60,000 years, a timeline too long for most granting agencies. The Michigan State researchers reported in the June 21 issue of Science that their observations support a provocative evolutionary theory called punctuated equilibrium, often known by its nickname "punk eek."
Punctuated equilibrium was proposed in 1972 by Stephen Jay Gould of Harvard University and Niles Eldredge of the American Museum of Natural History. Gould and Eldredge challenged the traditional view of Darwinian evolution as a gradual, stately unfolding. Charles Darwin himself had fretted over the gaps separating different species. But he hoped that eventually those gaps would be filled in by new fossil discoveries, showing how natural selection gradually transformed one species into another.
Gould and Eldredge noted that most of those gaps have persisted during the century following the publication of Darwin's On the Origin of Species. The fossil record still shows species persisting unaltered for eons before giving rise to new species in the blink of an eye, geologically speaking. Gould and Eldredge argued that those apparent sudden changes should be taken at face value. They proposed that species experience long stretches of general stasis, interrupted by the abrupt emergence of new evolutionary forms over relatively few generations.
Punk eek has had its ups and downs. After an enthusiastic reception by journalists and some scientists, the theory was seized on by creationists as a sign that the theory of evolution was not universally accepted. Gould and Eldredge retorted that their model does not deny evolution but merely offers an alternative description of how it occurs.
Since then, evolutionary biologists have debated the theory's validity without coming to a resolution. In a retrospective article published three years ago in Nature, Gould and Eldredge acknowledged that their model was merely one of many recent ideas in science that emphasize the role of discontinuity and randomness in nature. "We have either been toadies and panderers to fashion, and therefore destined for history's ashheap, or we had a spark of insight about nature's constitution," they wrote
The recent studies by the Michigan State team may help tip the balance toward the latter view. Richard E. Lenski, Santiago F. Elena and Vaughn S. Cooper carried out their evolutionary tests on the favorite guinea pig of microbiologists, the intestinal bacterium Escherichia coli. By restricting the amount of nutrients available to the bacteria, the investigators increased the competition among them and thus the pressure of natural selection. To make matters more interesting, however, the researchers worked only with a variant of E. coli that cannot swap genetic material with other simple cells, as many bacteria can. Unable to borrow useful genes, the studied bacteria could evolve only through random genetic mutation
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."