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 longerterm
benefits.

• Rapid adaptation to near extinction in microbial experimental evolution