THE INCREASED POTENTIAL FOR SELECTION OF THE LAC OPERON OF ESCHERICHIA-COLI

The fitness effects of six lac operons from natural isolates of Escher ichia coli were determined in chemostats, in a test of the idea that s elective differences among natural alleles are greater in novel condit ions than in the prevailing environment, resulting in a greater geneti c variance in fitness in novel conditions. Fitnesses were determined i n the common milk sugar lactose, the natural substrate of the lac oper on, and in three rare beta-galactosides, lactulose, galactosyl-arabino se, and methyl-galactopyranoside, that are novel for E. coli. Signific antly greater fitness differences were observed among the lac alleles in each of the novel beta-galactosides than in lactose. An alternative explanation of the experimental findings is discussed. General evolut ionary causes and consequences of selection potentials are discussed, and an outline of the work necessary to further elucidate the physiolo gical basis of the observed potential for selection of the lac operon of E. coli is presented.