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How from a evolutionary standpoint hermaphrodite species can exist and have a good genetic diversity, with no self-impregnation?

But, this race visually looks more like females, they do have breasts This is how they look like in my setting

I think of two options:

Two individuals always impregnate each other, there is no “male” or “female” roles. But they have reproductive system, based on male and female biology, with no chance to an individual impregnate themself (how to justify that?)

Or there is a completely different sex, not based on human like reproductive sistem, no male or/and female organs. But how would that system looked like and worked, what biological justifications can be for that?

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    $\begingroup$ "should" is opinion based. Both solutions can work. Consider rephrasing this question in a reality-check style: tell us what you want and let people answer if it would work the way you think it would. If it will not, you can ask a follow up question about alternatives. $\endgroup$ – Mołot Nov 1 '18 at 11:31
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    $\begingroup$ As Molot says, I'd stick to asking how it works, not what should or shouldn't be. $\endgroup$ – Hyfnae Nov 1 '18 at 11:32
  • $\begingroup$ While hermaphroditic is a term you can use comfortably in biology, the term to use for actual human beings is intersex. I realize you're inventing human-like people. But in the real world, intersex folks exist (with endless variety in their genetics and bodies...though nearly all have been subjected to unnecessary surgeries to make them more "normal") and a lot of them have biological children. I think an entire species of non-gendered people is very cool. I just urge you to consider that you're in part talking about real human beings who might not want to be treated like science experiments. $\endgroup$ – Cyn says make Monica whole Nov 1 '18 at 14:59
  • $\begingroup$ @Cyn of course I understand that, and I know about intersex people, but I preferred to use term “hermaphrodite” because it is a question focused on biology, and was not really comfortable with using “intersex” because I didn’t know was is appropriate in this question (well, I am not a fluent English speaker, so I avoid using words, that I am not very familiar with, or with which meaning I am unsure) $\endgroup$ – MonLi Nov 1 '18 at 15:19
  • $\begingroup$ I hear you. My comment was also aimed at people answering you. $\endgroup$ – Cyn says make Monica whole Nov 1 '18 at 15:50
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There are some interesting "non-selfing" systems in plants - specifically primrose and buckwheat ("selfing" being jargon for self-fertilization). In these species all individuals are hermaphrodites, but there are two reproductive morphs. One has the male parts at the top of the flower and the female part at the bottom, and the other the reverse. Since these species are pollinated by individuals hopping from flower to flower, these individuals get pollen of each type spatially segregated on their bodies, which means that it preferentially gets distributed to the female parts of the opposite type. Something like this could allow for two "mating types" in a species with little to no differences in overt appearance.

Alternatively, the spatial segregation of the male and female parts could be the same in all organisms, making it completely impossible to mate to yourself. Or the activation of one sexual organ could require the touch of the other, or to dovetail with L. Dutch's reply, the presence of a "non-self" antibody or some other substance.

Source for plant stuff: https://www.sciencedirect.com/science/article/pii/S0960982214001213 Schwander, Tanja, Romain Libbrecht, and Laurent Keller. "Supergenes and complex phenotypes." Current Biology 24.7 (2014): R288-R294.

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no chance to an individual impregnate themself (how to justify that?)

Use an antibody like system: the surface proteins of the egg cell let enter only sperm cells which display non-self markers on their surface. Those with self markers are barren out.

This is how some plants prevent self pollination.

By the way, if they are mammals, it make sense that they have breasts: the newborn has to be fed.

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some animals, such as various fish & amphibians, are born primarily of one gender, and the dominant member of the population changes to the other gender when an opening presents itself.

with your species, the dominant female could turn male and then impregnate the females.

with the lower animal species, this transformation can occur in a matter of days or weeks.

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  • $\begingroup$ So fish and frogs have "gender"? What's the difference between a fish's sex and their gender? Can fish experience gender dysphoria? $\endgroup$ – AlexP Nov 1 '18 at 11:49
  • $\begingroup$ gender / sex. the sexual organs change, and do so functionally. the dominant male clownfish becomes female and bears young. exposure to pesticides can cause male frogs to become fully-functional females. $\endgroup$ – theRiley Nov 1 '18 at 11:53
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If there is no self-impregnation, then there's no chance of an individual giving birth to what is essentially a clone of themselves, thus they need to find a partner. L.Dutch gave a very good biological way to prevent self-impregnation.

Once you have that, they can reproduce exactly the same way we Humans do, which keeps genetic diversity constant unless their population drops below a certain level. Last number I've seen ask for about 10.000 people to maintain a correct level of genetic diversity and avoid some problems.

The main difference would be that two partners could impregnate each other and both be father and mother at the same time. Which would probably be quite normal in a society made up exclusively of those beings.

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