Evolution of high mutation rates in experimental populations of E. coli

Most mutations are likely to be deleterious, and so the spontaneous mutation rate is generally held at a very low value1. Nonetheless, evolutionary theory predicts that high mutation rates can evolve under certain circumstances2,3,4. Empirical observations have previously been limited to short-term studies of the fates of mutator strains deliberately introduced into laboratory populations of Escherichia coli5,6,7, and to the effects of intense selective events on mutator frequencies in E. coli8. Here we report the rise of spontaneously originated mutators in populations of E. coli undergoing long-term adaptation to a new environment. Our results corroborate computer simulations of mutator evolution in adapting clonal populations4, and may help to explain observations that associate high mutation rates with emerging pathogens9 and with certain cancers10.