The Increased Potential for Selection of the Lac Operon of Escherichia Coli

The fitness effects of six lac operons from natural isolates of Escherichia coli were determined in chemostats, in a test of the idea that selective differences among natural alleles are greater in novel conditions than in the prevailing environment, resulting in a greater genetic variance in fitness in novel conditions. Fitnesses were determined in the common milk sugar lactose, the natural substrate of the lac operon, and in three rare β-galactosides, lactulose, galactosyl-arabinose, and methyl-galactopyranoside, that are novel for E. coli. Significantly greater fitness differences were observed among the lac alleles in each of the novel β-galactosides than in lactose. An alternative explanation of the experimental findings is discussed. General evolutionary causes and consequences of selection potentials are discussed, and an outline of the work necessary to further elucidate the physiological basis of the observed potential for selection of the lac operon of E. coli is presented.