Other situations in which hermaphroditism is thought likely to evolve are when population densities are low enough that finding mates is difficult (Baker 1955).Under these conditions, being a self-compatible hermaphrodite allows “reproductive assurance” because of the ability to produce offspring without another individual, and because a single hermaphroditic pair has a 100% chance of outcrossing whereas a single dioecious pair has only a 50% chance of outcrossing (that is, there is a 50% chance of a pair being of the same sex in dioecious populations).
Diagram of the evolutionary transition in reproductive systems from hermaphroditism to dioecy (or vice versa) and the two common transitionary reproductive systems: androdioecy (males hermaphrodites) and gynodioecy (females hermaphrodites).
Interestingly, the evolution of androdioecy or gynodioecy as the transitionary phase is not equally likely, in either evolutionary scenario (that is, dioecy to hermaphroditism or vice versa).
The earliest species to be described as androdioecious were primarily plants.
Charlesworth (1984) suggested that all 17 plant species that had been described to the date of publication were actually cryptically dioecious, with the “hermaphrodites” actually being functional females.
Because in most species, sperm (or pollen) is abundant relative to eggs (ovules), an all-male mutant has greater difficulty competing with hermaphrodites than an all-female mutant (Lloyd 1975; Charlesworth 1984; Pannell 2002; Wolf and Takebayashi 2004).
This is especially true when hermaphrodites self-fertilize (Lloyd 1975; Charlesworth 1984).Specifically, they noted that when the fitness set relating the allocation of male and female gametes is convex, hermaphroditism should be advantageous.They suggested that this type of relationship should be found in low-mobility species, in species in which resources needed for male gamete production are offset from those needed for female gametes, and, in plants, where the costs associated with attracting pollinators are greater than the costs of producing gametes.First, transgenderism isn't the same as being intersex. Someone else may have only internal differences (testicals instead of ovaries, etc) and the urethra would align with their external anatomy (if female, it'd be in the “normal” female location; if male, it'd route through the penis).Two, the term hermaphrodite is generally considered outdated or offensive. A third case would be someone with a “deformity” causing the urethra to route through a micropenis situated in the location of a clitoris but still have a vagina opening.Of these mixed mating systems, trioecy is the rarest, followed by androdioecy, with gynodioecy being most common (Charlesworth and Charlesworth 1978; Charlesworth 1984; Jarne and Charlesworth 1993; Pannell 2002).