The quantitative effect of sexual selection on fixation of an allele for obligatory sex. Plot of the critical genome mutation rate (U) above which obligatory sex fixes in a facultative population, as a function of the strength of sexual selection (χ). Contour lines with different costs of sex C
= 0.1,0.2, 0.3, 0.4, and 0.5 are shown. The white area above the C
= 0.5 contour is the parameter range in which obligatory sex fixes even with a two-fold cost of sex. The plot was generated using a = 0.99, s = 0.05, and N = 50, with free recombination. Fixation of the allele O was defined as reaching a frequency higher than 99.9% within 2500 generations.