"Rather than attempt to reconstruct history with fossils, Richard Lenski, an evolutionary biologist at Michigan State University, decided to watch convergence and contingency unfold in real time, in the controlled environment of his laboratory. In 1988, he separated a single population of Escherichia coli bacteria into 12 separate flasks containing liquid nutrients, and let them each evolve separately. Every few months for the past 26 years, he or one of his students has frozen a sample of the bacteria. This archive of frozen microbes gives Lenski the ability to replay E. coli’s tape of life from any point he wishes, simply by thawing out the samples. Along the way, he can examine how the bacteria change both genetically and in ways that are visible under a microscope. Lenski says, “The whole experiment was set up to test how reproducible evolution was.”
In 11 of Lenski’s flasks, the E. coli cells grew physically larger, but bacteria in one flask divided itself into separate lineages—one with large cells and the other with small cells. “We call them the smalls and the larges,” says Lenski. “They have coexisted now for 50,000 generations.” No other population in the experiment did the same; a historically contingent event seemed to have taken place. Even 26 years later, none of the other E. coli lineages evolved it. In this case, contingency seems to have won out over convergence."
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