Figure 2. Parasite evolution in relation to host sexual dimorphism and likelihood of encountering the other host sex.

In red and blue are parameter combinations, which lead to monomorphic or dimorphic parasite populations, respectively. The higher the degree of host sexual dimorphism and the lower the probability of encountering the same host sex, the higher the likelihood is that a parasite will adapt specifically to its common host sex (A). When one host is different from the other, and so rare that a parasite cannot persist in it (e.g., males in a facultative sexual species like many rotifers, cladocerans, and aphids), then the parasite species may specialize entirely on the common sex (B). When one host is very different from the other in a trait important for the parasite (e.g., a primary sexual trait), then, disregarding the rate at which the opposite sex is encountered, the parasite may specialize entirely on the more suitable host (C). When males and females are very different from the parasite's point of view and the parasite encounters both sexes equally often (D), the parasite might evolve phenotypic plasticity (e.g., Wolbachia).