The separation of sex and reproduction in bacteria and most other microbes makes their evolutionary adap- tation primarily dependent on mutation as the ‘raw material’. At first sight, producing as many mutations as possible may thus seem a profitable strategy for microbes, because it would allow them to respond rapidly to changing environmental conditions. How- ever, mutations come in many forms and only very few are beneficial for adaptation of the organism to its environment, while many more have deleterious effects (Sturtevant, 1937). This begs the question to what level the mutation rate should evolve to balance the necessity of adaptive change with the cost of deleterious mutations (Sniegowski et al., 2000). Whatever the best mutation rate might be theoretically, actual changes of the mutation rate come about by natural selection acting on mutants with different mutation rates. Since these forces can be different for individual mutation-rate mutants within a population and entire populations of such mutants (see below), the observed mutation rate does not always reflect a single theoretical optimum. Actual mutation rates are rather a product of the interplay between short- and long-term evolutionary forces acting on these mutants, where short-term forces affect the frequency of individual mutation-rate mutants within populations, and long-term forces affect the frequency of populations of these mutants relative to other populations. The clonal structure of most microbial populations causes competition between populations to be a relevant force.
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