Figure 8.—

Schematic of a selective sweep. (A) A schematic of a selective sweep showing that, unlike with clonal interference, the sterile mutation initially occurs in a highly fit individual and is never outrun by another lineage. If, at the time the sterile mutation arose, there was variation in fitness within the population, the relative fitness of the sterile lineage will decrease during the sweep as the mean fitness of the nonsterile subpopulation increases. By the end of the sweep, the sterile lineage will be competing against a population similar in fitness to the background in which the sterile mutation arose. (B) An example of a selective sweep, shown as the change in the log ratio of steriles to nonsteriles over time fit to a linear model (corresponding to a constant relative fitness of the sterile lineage) and a power law model (corresponding to a decreasing relative fitness of the sterile lineage). Similar plots for the other seven sweep populations are shown in Figure S2. (C) The change in the relative fitness of the sterile lineage relative to the population mean fitness can be evaluated at any point by calculating the derivative of the best power-law fit to the data. The initially fast increase slows to a value similar to the fitness provided by the sterile mutation alone, s_ste, by the end of the sweep. Populations 1–8 are BYS1-A07, -A08, and -F05 and BYS2-C03, -C06, -D06, -D07, and -E03, respectively.