Figure 2. Mutator fixation probability is not frequency-dependent.

Fixation probability, $P_{fix}\left(x_0\right),$ of a mutator initiated at frequency $x_0$ (circles: orange for $x_0=1/N,$ purple for ${\displaystyle x_0>1/N}$). Data from simulations in Figure 1. $P_{fix}\left(x_0\right)$ scales with but never crosses the fixation probability of a neutral mutation ($x_0$; black dashed line). Thus, mutators are favored at all starting frequencies. The expected fixation probability $P_{fix}\left(x_0\right)$ (solid orange line), calculated from the fixation probability of a single mutator, $P_{fix}\left(x_0=1/N\right)$ = 5.6×10⁻⁴ (orange point) using Equation 1 is indistinguishable from the $P_{fix}\left(x_0\right)$ observed in simulations, demonstrating that the per-capita fixation probability at every frequency is independent of $x_0$ and equal to $P_{fix}\left(x_0=1/N\right)$.