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?
Komodo Dragons, some frogs, and a few other reptiles, amphibians, and fish can fertilize their own eggs, or become a member of the opposite sex. Becoming the opposite sex is more common, and is done when there are several animals of the same sex available. One or two become the missing sex and mate with the others. In the cases of self-fertilization, its done when no other animal is available of either sex. This does appear to be a throwback to when species were gaining different genders as the most advanced creatures we see it in are lizards that are almost living fossils. All the plumbing isn't as set as it is with mammals and birds. Some tape worms actually have fights while mating to see which one will carry the eggs. They use acid, spikes, and teeth to puncture their opponent and basically inject the sperm directly into them which makes the loser the egg bearer. This is really weird, but taking care of the egg requires a lot of energy so neither animal wants to do it, as it reduces their own ability to survive. So the aliens could be like the first example and just be hermaphrodites as an extra means of general species survival. We don't see this in the higher species on Earth, but an alien planet doesn't have to follow the same rules.
Believe me when I was reading the Discover article about it I was killing myself laughing. When I wasn't scratching my head in wonder.
ZOMG! It's Virgin Mary in scaly, slimy form! YESSSS!!! .... ...but anyway, I don't really have a solid answer for this question, I really just posted because a topic with a title like this begs to be posted in. However, as a stab in the dark, I might suggest that since all of life is about reproduction and working towards it, no matter how contrived your means may be, and herms could supposedly reproduce by themselves, I guess maybe we evolved as two seperate genders because...um...it's more fun to reproduce with two people instead of just by yourself (comparing an orgy with someone else to fapping when no one is home, eewblech.), so a species would be more likely to survive if they were more inclined to reproduce. but that's much more of a lame joke than it is an answer. However, maybe one of your problems is that you're taking this from a purely scientific, logical, Darwinian view. However, I've read a summary of the novel you intend to write, and it sounds much more fantasy-ish then it does like an attempt at realistic writing, so maybe you should try your luck and go with a fantasy answer, like involving the tampering of some greater, ancient being, or some ritual enacted by the herm aliens themselves. Or, you could be really cheeky and say it has to do with religion as we know it, but not preachy-like, just taking it from a Philip Pullman view (Which, if you've read His Dark Materials, you'll knew is a cold, detached, matter-of-fact, slightly harsh viewpoint on religion.), so you could say like "And God made the humans with two genders because Adam was greedy and requested a partner, and thus the vices were created. But [size=-1]<insert species here>[/size]kind was pure and true, and required only one creator." Incorporating religion into this would be extremely risky, but could also open up a whole new view on the events that transpire within your novel, with the whole "gating" idea, or whatever it is you're writing about. Whew. I don't know if that did anything at all for you, but that's just my bowl of cherries.
But hermaphrodites do have sex with each other (and certainly my alien race do), it's just that they can choose each time whether to play the male or female role in the mating. In biology hermaphroditic creatures do sometimes self fertilise, but only when there are no partners around. Usually they mate with each other but have to decide (often with some conflict, it turns out) which one will be female and which will be male this time. It's asexual creatures that totally self-fertilise - in that they don't even have males or females. They just lay eggs that give birth to genetically identical offspring (aphids do this, I believe).
