Evolutionary paths of least resistance

Abstract

When organisms face a challenging environment, they may respond by adapting along different potential evolutionary paths. Each potential path will likely correspond to a different biological mechanism for fitness increase, and which of these alternative mechanisms will evolve depends on a number of factors, including the relative fitness benefits and costs of a given mechanism, the number and type of mutations necessary to evolve the mechanism, and random chance. If one of these evolutionary paths is consistently chosen over the others, then we can think of it as the path of least resistance. In PNAS, Bratulic et al. (1) study the evolutionary paths of least resistance in Escherichia coli bacteria exposed to the antibiotic ampicillin. The authors study E. coli strains carrying a plasmid expressing TEM-1, a protein that confers resistance to ampicillin. The experimental strain has an elevated rate of translation errors, whereas a control strain has a normal translational error rate. The experiment is set up such that only the coding sequence of TEM-1 can evolve, but not the E. coli strains themselves nor the rest of the plasmid. Bratulic et al. find that under an elevated translational error rate, the path of least resistance is to evolve reduced translation-initiation efficiency for TEM-1 when ampicillin concentration is low, and robustness to translation errors when ampicillin concentration is high (Fig. 1). In contrast, a reduction in translational error rate, which could have evolved through modified synonymous choice, is not observed.