Adaptive mutation: How growth under selection stimulates Lac⁺ reversion by increasing target copy number

The AM model is consistent with a large body of experimental data obtained by others (10, 39) and is supported by additional experiments designed to test it more directly (2, 16, 21). This model explains how growth under selection can increase the probability of selected mutationsby adding copies of the mutational target. The observed general mutagenesis is explained as a nonessential side effect due to occasional coamplification of lac with the nearby dinB⁺ gene. We suggest the Cairns-Foster system has made a major contribution by revealing a genetic mechanism by which growth under strong selection can enhance production of adaptive mutations without changing either the rate or target specificity of mutation. Because AM relies on standard genetic events, it is likely to operate in any organism and contribute to genetic adaptation in many biological situations.