Rapid adaptation to near extinction in microbial experimental evolution

Theory indicates that responses to natural selection maximize immediate fitness benefits, leading to adaptations to current environmental conditions and those of the immediate past. Over a century of advances in theory, experiment, and observation have documented innumerable adaptations demonstrating the efficacy of natural selection to finely tune species to their respective environments. However, theory also suggests that natural selection is not a panacea, and that improvements in competitive ability do not necessarily increase long-term survival. Here we show that adaptation in experimental populations of microbes can dramatically reduce population sizes to near extinction levels in a stressful environment. The long-term potential for extinction differed from that identified in short-term ecological observations, but the eventual outcome is consistent with limitations on specific modes of adaptation. These results suggest that additional emphasis on the limitations of adaptation can provide insight on when and how improvements in competitive ability provide longer-term benefits.