For more than two decades there has been intense debate over the hypothesis that most morphological evolution occurs during relatively brief episodes of rapid change that punctuate much longer periods of stasis, A clear and unambiguous case of punctuated evolution is presented for cell size in a population of Escherichia coli evolving for 3000 generations in a constant environment, The punctuation is caused by natural selection as rare, beneficial mutations sweep successively through the population, This experiment shows that the most elementary processes in population genetics can give rise to punctuated evolutionary dynamics.