Figure 2.

Simplified model of alternating asexual and sexual reproduction driven by genotype-specific environmental lethality due e.g. to parasitism. Assuming genotype-specific lethality is sufficient to completely eliminate the susceptible genotype, species survival requires sex to generate a (one, for simplicity) new genotype. In this case, the Hamilton’s rule benefit b for somatic cells of cooperating with germ cells to enable sex rises with lethality; in the simplest case, they are the same (red) curve. When the cost threshold c sufficient to enable sex (green dashed line) is reached, sexual reproduction occurs (blue bars) until the existing genotype is eliminated by the still-rising environmental lethality. The cycle then repeats with the new, sexually-generated genotype