What selective pressures would favor hermaphroditism in some individuals of an animal species which reproduces sexually?

Some species which reproduce sexually will replace one sex with (or add) hermaphrodites. The coexistence of males and hermaphrodites is androdioecious, females and hermaphrodites gynodioecious, and all three trioecious. In the absence of a male or female to mate with, a hermaphrodite is capable of self-fertilization. On Earth these strategies are found in plants, nematodes, and rarely fish (the same terminology is applied to both plants and animals in papers).

In all these cases the species only ever has two sexes. A sex is defined by the types of gametes produced: males produce sperm, females ovum. Hermaphrodites do not constitute a distinct sex because 1) they produce ovum and sperm and 2) they reproduce by fusing ovum and sperm. To constitute a third sex, a species must produce three structurally distinct types of gametes and all three must be required to reproduce.

posted about 7 years ago

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