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The neo-Darwinists suggested that evolution is constant and gradual, and thus that genetic changes that drive evolution should be too. However, more recent understanding of phenomena called adaptive mutation in microbes indicates that mutation rates can be elevated in response to stress, producing beneficial and other mutations. We review evidence that, in Escherichia coli, two separate mechanisms...
''Adaptive'' or ''stationary-phase'' mutation is a collection of apparent stress responses in which cells exposed to a growth-limiting environment generate genetic changes, some of which can allow resumption of rapid growth. In the well-characterized Lac system of Escherichia coli, reversions of a lac frameshift allele give rise to adaptive point mutations. Also in this system, adaptive gene amplification...
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