Conditions for fixation of an allele for obligatory sex under environmental changes. The contours represent the critical mutation rate above which obligatory sex fixes. Contour lines with t = 0,0.2 and 0.4 are shown. The white area above the t = 0 contour is the parameter range in which obligatory sex fixes even in a constant environment. The plot was generated using C
= 0.5, a = 0.99, s = 0.05, and N = 50, with free recombination. Environmental changes occurred every 10 generations (A) or every 25 generations (B). Fixation of the allele O was defined as reaching a frequency higher than 99.9% within 2500 generations and retaining it for four consecutive cycles of environmental change.