COMPETITION BETWEEN HIGH AND LOW MUTATING STRAINS OF ESCHERICHIA COLI

The dynamics of bacterial populations are often characterized by several distinctive features: under optimal growth conditions they double every few hours; they usually contain in excess of 106 individuals; higher fitness mutants have a good chance of arising in a population since average mutation rates are approximately 10-6 to 10-7 per gene replication; new favorable mutations, in the absence of genetic recombination, always increase to fixation in linkage with the genome of the parent clone in which they originally occurred; and higher fitness mutants often exhibit 10% to 20% higher growth rates than their parental clones. Consequently, when populations of such organisms are exposed to a new environment, a series of replacement cycles rapidly ensues, each cycle corresponding to the fixation of a higher fitness mutation in linkage with the genome of its parent clone. The linkage between the new mutation and the genome of the parent clone, and the rapidity of these clonal replacements, are two features that distinguish such asexual populations from ones that reproduce sexually. The existence of such cycles in asexual populations was first studied systematically by Atwood et al. (1951) in a study with laboratory populations of Escherichia coli in long-term cultures. By comparing the relative fitness of a series of bacterial clones isolated from these cultures, Atwood et al. were able to show that populations underwent a succession of clonal changes, each clonal replacement