Does anyone know why it is that evolution favours creatures that have males and females, rather than creatures that have elements of both? See, I have an alien race in the novel I am currently writing and they have a combination of male and female genitals and I need to figure out how it was that evolution took this different path on their home planet - it'll help me get ideas about what their home planet would be like and also make the whole thing seem more credible. At first I thought, well maybe they don't have viruses on their planet so they don't need genetic diversity. But then I realised that it's asexual creatures that don't have genetic diversity. Hermaphrodites do have genetic diversity, they still have sex with each other, it's just that they can each perform either male or female role in the mating. So I wondered whether anyone with any biological knowledge could help me by answering why it was that hermaphroditic life-forms did not develop into complex life-forms. What evolutionary advantage do creatures with two sexes have over creatures that have both sexes combined?