Fig. 3.

Hard and soft sweeps in drug resistance evolution. In the hard sweep scenario, the evolutionary process toward a drug-resistant phenotype can be driven by a few mutations, and they quickly dominate over other alleles in a population, giving a strong selective signature, as shown with a negative Tajima's D value (D). However, the evolutionary process is often less than ideal, and the accumulation of several mutations either within the same gene or in multiple genes contribute to the eventual drug resistance outcome, making the selective signature less obvious and less easy to track.