Conditions for fixation of an allele for obligatory sex in populations with different levels of facultative sex. The contours represent the critical mutation rate above which obligatory sex fixes. Contour lines are shown for different frequencies of sexual reproduction among the facultative sexuals: a = 0.1,0.4, 0.7, 0.9, and 0.99. The white area above the a = 0.1 contour is the parameter range in which obligatory sex fixes even in a population where only 0.1 of the offspring are produced asexually. The plot was generated using C
= 0.2, 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.