On the mode of selection inEscherichia coli

Summary Competition experiments between a lac⁺ prototroph and a lac⁻ auxotroph ofEscherichia coli were conducted by serial transfer procedure on four levels of sugar concentration. Changes of the relative frequencies of both genotypes were followed and the relative fitness was estimated. Fitness was proved to be density-dependent. No explicit dependence of fitness upon genotype frequencies could be detected by the simple method ofKosuda (1981).