Mutational reversions during adaptive protein evolution.

Adaptation is often regarded as the sequential fixation of individually, intrinsically beneficial mutations. Contrary to this expectation, we find a surprisingly large number of evolutionary trajectories on which natural selection first favors a mutation, then favors its removal, and later still favors its ultimate restoration during the course of antibiotic resistance evolution. The existence of reversion trajectories implies that natural selection may not follow the most parsimonious path separating two alleles, even during adaptation. Altogether, this discovery highlights the unusual and potentially circuitous routes natural selection can follow during adaptation.