Page "Polyphenism" Paragraph 6

Population-dependent and reversible sex determination, found in animals such as the blue wrasse fish, have less potential for failure.

In the blue wrasse, only one male is found in a given territory: larvae within the territory develop into females, and adult males will not enter the same territory.

If a male dies, one of the females in his territory becomes male, replacing him.

While this system ensures that there will always be a mating couple when two animals of the same species are present, it could potentially decrease genetic variance in a population, for example if the females remain in a single male's territory.

Furthermore, this system is inherently unstable on a small scale because a single mutation causing a fish to remain permanently male would spread quickly through the population ( due to high female availability ) and might eventually cause loss of females in the species, and therefore extinction.