An experiment in which a lab population of Escherichia coli has been growing with glucose as a limiting nutrient for almost 20 years has provided an opportunity to examine the tempo and mode of genomic evolution. Genomes of E. coli clones were sampled after 2,000, 5,000, 10,000, 15,000, 20,000 and 40,000 generations. Adaptation decelerated sharply, but genomic evolution was nearly constant for 20,000 generations. Such regularity is considered characteristic of neutral evolution, but several lines of evidence indicate that most of these mutations were beneficial. The population later evolved an elevated mutation rate and accumulated hundreds of additional mutations dominated by a neutral signature. Clearly, the coupling between genomic and adaptive evolution is a complex matter.
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