Genetic Changes Accompanying Increased Fitness in Evolving Populations of Escherichia coli

Two populations of Escherichia coli, each initiated with a single clone containing a derivative of the plasmid pBR322, were maintained for long periods in glucose-limited continuous culture. In both populations, after an extensive number of generations had elapsed, clones were isolated in which the transposon Tn3 from the plasmid had integrated into the bacterial chromosome. In both cases examined, the transpositions were shown to increase relative fitness approximately 6-7%, in the environment in which the populations were maintained. The loci of integration were mapped to approximately 13.2 min (population 1) and approximately 32.8 min (population 2).