On the mode of selection in escherichia coli

Selection experiments between a lac ⁺ wild type and a lac ^– mutant of Escherichia coli were conducted using a serial transfer prcedure at two levels of total sugar concentration, i.e., 0.750 g/l and 1.0 g/l. The relative frequencies of both genotypes were assayed every 48 hours, at the time of transfer and the frequency changes were followed for about four weeks. The frequency of lac ^– decreased steadily and it was eventually eliminated from the mixed populations in all replicate cultures. The regression of logit p_t was shown to be not linear but curvilinear, indicating that fitness differentials are frequency-dependent. The selection is also probably density-dependent.