Adaptive Point Mutation and Adaptive Amplification Pathways in the Escherichia coli Lac System: Stress Responses Producing Genetic Change

Evolution by natural selection includes two main steps: the generation of heritable variations (e.g., mutations) and the differential proliferation of the variants in the environment. When the neo-Darwinists synthesized a modern view of natural selection and genetics in the early 20th century, they specified a simplifying assumption that Darwin (12) had not: that the rates of formation of mutations would be independent of exposure to selective environments (e.g., see reference 47). Thus, evolution, and the mutations driving it, should be constant and gradual. That some spontaneous mutations form independently of interaction with the environment is certainly true (42, 46, 52; see also many subsequent papers). These form before an organism encounters a selective environment, with a definable relationship to cell divisions (“growth-dependent mutations”), probably because many result from DNA replication errors. However, work with several microbial assay systems indicates the existence of additional mutation pathways that appear to be induced in response to the environment (reviewed in references 16, 58, and 60). These mutation mechanisms, called stationary-phase or stress-induced mutation, operate specifically under growth-limiting stress and may sometimes produce mutations that confer a growth advantage in the growth-limiting environment, called adaptive mutations. The problem is, are they really something different from growth-dependent mutations?