The title says it all. I know that 'complex' is a pretty ambiguous term to use, but I can't think of a more scientific term/definition for my meaning. I can't think of any large (larger then a small rodent) hermaphrodite species, or any that we tend to think of as having complex structures; hermaphrodite species all seem pretty boring really.

I get why sex is a huge advantage, but by that regard hermaphrodites seem quite powerful. The ability to both produce your own young and impregnate others allow for more young. Plus, it provides the opportunity for a species to choose to self fertilize if no mate is available. I realize sexual reproduction is preferable, by it's nice to have Autogamy as a backup option just in case.

I understand why there are advantages to two sexes, particularly in terms of sexual dimorphism which allows male to be better at potentially reproducing with many females by specializing in it. I completely understand how two genders would evolve and be successful.

What I find odd is that nearly all larger or complex species seem to have genders. I would have thought that, like many other mating strategies, there would be variance across species; with some choosing to be hermaphrodites to increase the maximum children they can produce as a species, by having twice the child bearers, and others choosing to go the sexual dimorphism route by having roles that they are best at. I mean even Obligate parthenogenesis has evolved separately many larger specie like lizards, despite it arguably being an evolutionary dead end in the long term due to the lose of the advantages of sexual reproduction. If adaptations that completely toss away the power of sex evolve all over the place why do new hermaphrodite species never seem to evolve, and existing hermaphrodite species never seem to grow bigger or more diverse?

In particular sequential hermaphrodite, like many aquatic fish use, seems a pretty effective strategy. When your young, and small, play a female role to have guaranteed offspring, only once your big and at your most fit bother competing as a male for access to multiple mates.

A related question, associated with my own attempt to explain why existing hermaphrodite species don't seem to take as many varied forms; but which I'm not really sure I buy. Is it possible that the lack of sexual conflict (primarily, but not limited to, competition between males for mates, driven from the fact that males would have more drive to compete then hermaphrodites since it's their only reproductive strategy), would mean there was less of a drive towards adaptation in hermphrodite species? A sort of red-queen scenario, where your own species is the red queen?

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    $\begingroup$ I'm no biologist, but what is your definition of "complex"? Land snails and earthworms are hermaphrodites, which makes sense because they move rather slowly and therefore have fewer opportunities to find a mate. Do you mean "vertebrates"? If this is what you mean, the probable answer is that simultaneous hermaphroditism is something that hasn't existed in the verterbrate ancestral lineage for a very long time, if ever, and therefore it needs to "re-evolve." Given that vertebrates are highly mobile and therefore have frequent opportunities to mate, it seems unlikely to happen. $\endgroup$ – Level River St Jul 23 '15 at 21:09
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    $\begingroup$ "I can't think of any large (larger then a small rodent) hermaphrodite species, or any that we tend to think of as having complex structures; hermaphrodite species all seem pretty boring really." !!! There are snails much larger than mice. And "boring" is very opinion based. $\endgroup$ – anongoodnurse Jul 23 '15 at 22:56
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    $\begingroup$ I was really happy to see this question, cuz it's been bugging me for years too. (Like, I remember trying to get a satisfactory answer from my grade 12 bio teacher.) But it seems like the community in general isn't quite getting the intent, the way you phrased it (it makes perfect sense to me, but then, we're obviously in the same mindspace on this point already). Maybe try making it more concrete? $\endgroup$ – Owen_R Jul 24 '15 at 17:18
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    $\begingroup$ Like... sketch out an idea for how a species of birds could evolve hemaphroditism, maybe? The selective pressures you think could create and maintain it? $\endgroup$ – Owen_R Jul 24 '15 at 17:19
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    $\begingroup$ Abstractly, the possible answers to our question that I can think of are: {1 Our ideas of how pressure for hemaphroditism would be generated in species like this are wrong because...?} {2 Our ideas aren't necessarily wrong, but there would be stronger pressure against it because...?} {3 There would be a stable net pressure maintaining hermaphroditism, but there's a barrier against it initially re-evolving from a [antonym of hermaphroditic] population because...?} So write the question to try to focus on concrete examples of those possible explanations, I guess? $\endgroup$ – Owen_R Jul 24 '15 at 17:22

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