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I think the question already says it all. I know there exist animals that have both herm and female populations and other's with herm and male populations, I'm not personally aware of any animal that has herms, males and females but I suspect that also exists somewhere. I'm just curious which of these sex combinations is most common, ie which is most likely to evolve.

If you could also speak to why one sex combination may be more prone to evolve over another I'd be very interested in that, but I'd settle just for knowing which combination is more common.

I'm asking specifically about simultaneous hermaphrodites here, I know sequential hermaphrodites exist but that would just complicate the question.

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Androdioecy (the coexistence of males and hermaphrodites) is very rare in animals, and vanishingly rare in vertebrates (just three species of fish). Coexistence of males and parthenogenetic females is considerably more common, at least in insects (often in different generations).

Gynodioecy (the coexistence of females and hermaphrodites) in animals is thought to be rarer by an order of magnitude. In fact, before 1975 no examples were known. Gynodioecy is, however, much more common and widespread than androdioecy in plants (though still <1% of angiosperms).

You can find details of different sexual systems and the number of species they occur in from the Tree of Sex.

Sources:

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    $\begingroup$ The resource you list doesn't include nematodes (or many other major groups of invertebrates) — Caenorhabditis elegans is a famous example of an androdioecious animal. Can you provide one or more references that directly address the OPs question — whether androdioecy or gynodioecy is more common in animals? Both seem to regarded as rare, but a cursory search only turned up one example of the latter in animals (Epiactis prolifera), which would suggest that the former is fact relatively more common ... $\endgroup$
    – tyersome
    Nov 6 '21 at 23:29

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