I'm creating a humanoid species, roughly based on humans. The difference is that not every opposite sex couple of fertile age could produce offspring.

The premise of my story is based of some men being incompatible with some women. And I mean genetically, if they mate they won't be able to produce children.

Is this possible?

And is there something similar in the nature, it doesn't have to be in humans.

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    Are you asking if the egg and sperm will successfully split, or whether the resulting embryo/fetus will survive until term? – RonJohn Dec 6 at 9:23
  • Is the point for some people to be incompatible, or for scientists to notice there's a specific reason for incompatibility that becomes a plot point to segregate humans? – Thymine Dec 6 at 10:28
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    @RonJohn It doesn't matter to me, as long as they can't have any children – liraxu Dec 6 at 11:28
  • @Thymine Actually they are not humans, I just need human like species where this incompatibilities happen often. – liraxu Dec 6 at 11:29
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    Something to keep in mind: one biological definition of a species is that all the members of it must be able to interbreed and produce fertile offspring. If you've got multiple groups unable to breed with each other within a species (each one limited to members of their one group), you probably don't have a single species anymore. If, however, it's something more like this: group A can breed with B and C but not D, while D can breed only with B or E, and E cannot breed with B but can with C, etc., well, that's an entirely different problem and one that I don't know any specific term for. – Palarran Dec 7 at 4:05
up vote 50 down vote accepted

Yes, this is possible and in fact already happens in humans causing apparent infertility in couples (alongside a host of other sexual reproducing organisms). Consider for instance variations in two genes encoding for proteins in the eggcoat, ZP2 and ZP3, and another gene encoding for a sperm protein that latches onto the eggcoat, C4BPA. Each of these three come in various, in themselves stable and viable, forms, but the end result is that a male with one form of C4BPA cannot produce offspring with women with particular forms of ZP2 and ZP3 because the sperm fails to latch onto the eggcoat because of shape differences of the proteins. See here for details.

That said, the woman and man may hedge their risks through things like polyandry (having sex with more than one man) or polygyny (having sex with more than one woman) and this in fact does happen in apparently strictly monogamous relationships. Other mechanisms are at play here too, see for instance The evolution of polyandry II: post-copulatory defences against genetic incompatibility.

Something similar is possible in humans due to a difference in Rh blood groups.

The Rh blood group system consists of 49 defined blood group antigens1, among which the five antigens D, C, c, E, and e are the most important.

[A]ntibodies to the Rh(D) and Rh(c) antigens confer significant risk of hemolytic disease of the fetus and newborn.

(from Wikipedia)

In short, the egg and sperm of Rh incompatible people can produce a living fetus, but antibodies of the mother attack the blood cells of the fetus which often leads to the death of the fetus if not treated.

Please note that in humans this mechanism takes effect only after the first pregnancy. The first child is born healthy, but after that most or all pregnancies end in miscarriage or the children dying shortly after birth.

Since you create your own species, you can adjust the mechanism so that even the first child of an incompatible couple dies.

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    This only kicks in with the second and subsequent pregnancy,. The first birth is when the mothers immune system gets exposed and sensitized to the fetal antigens but the baby is already out. The mothers immune system goes to work on the second pregnancy. So I think this does not fulfill the OP - this couple can have a baby. – Willk Dec 6 at 12:58
  • @Willk Damn, I forgot about that. You're absolutely right. – Elmy Dec 6 at 14:45
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    The first pregnancy does not have to be to term to trigger this. A miscarriage or even an ectopic pregnancy can be enough. In humans, there needs to be blood transfer between an Rh+ mother from an Rh- fetus/embryo. But in your species, maybe the sensitization comes from sperm and affects future fetuses. – Cyn Dec 6 at 15:59

See also the white-throated sparrow that had a gene mutation that has, effectively, created four genders. There are still the standard "male" and "female" sexes, but a separate gene also drives the nesting and young rearing behaviors.

Offspring are genetically viable with off-pair pairings. However, due to the behavior drivers, the offspring don't survive as the pair needs a matched set of opposites: aggressive and territorial vs. nest-watching. Two agressive types don't sit on the eggs. Two nest-watching types fail to fend off predators.

See this article for more details that I might've forgotten.

As a result, the behavior gene is starting to drive phenotypical differentiation, so you get α-males and β-males looking different so that they can be identified by their counterparts: the β-females and α-females respectively, which also have phenotypical expressions for the same reason. In a few million years it wouldn't be unexpected for α/α or β/β pairs to be genetically non-viable (in the same way that male/male and female/female pairs can't reproduce) in the way you're looking for.

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    This is amazing. – Kevin Krumwiede Dec 7 at 2:22
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    @KevinKrumwiede Don't you love it when you make up some weird biology and Nature says, "Actually..." – Draco18s Dec 7 at 4:21
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    The article you linked doesn't mention the alpha/beta pairings. Is that the right bird? – Kevin Krumwiede Dec 7 at 7:49
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    @Draco18s: Actually, after following this site for a while, it's more like "Nature: You call that weird? Nah... let me show you weird." – Matthieu M. Dec 7 at 8:29
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    @KevinKrumwiede You were right! Because I hadn't wanted to open a bunch of links I saw three or four titles referring to the ruff, but that wasn't the right bird. Thanks for the fact-check, I've corrected the links. – Draco18s Dec 7 at 23:05

Another way for this to happen is a ring species. Essentially you have a bunch of subspecies, each of which is similar enough to the one next to it to allow interbreeding, but the ones on the ends have diverged far enough from each other to make them incompatible.

Yes, that is possible. Rabbits can have a dwarfing gene, which leads to a smaller animal. Two of these genes are lethal.

See:

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    A given offspring only has a 25% chance of getting 2 of those genes. The other 75 % will be viable. – Willk Dec 6 at 13:00
  • @Willk You are absolutely right. I just also came to this and wanted to post it, too, but you were faster. – Julian Egner Dec 6 at 19:04
  • Horses have overo lethal white: en.wikipedia.org/wiki/Lethal_white_syndrome You could have two people be incompatible for ethical reasons if they knew that 1/4 offspring would die horribly. – user3067860 Dec 6 at 19:10
  • You would need to hvae two genes, wich are not compatible to each other - so one part of the species would have gene A and one part would have gene B, and if one induvidual would have A and B, it would die. But would these still be one species? – Julian Egner Dec 6 at 20:30

Absolutely possible. See reference: Homo Sapiens.

There are many genetic incompatibilities that prevent offspring, such as incompatible blood group, guaranteed-lethal recessive gene defects, etc..

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    Welcome to StackExchange World Building! Please take the tour and have a look at the help center. We encourage elaborate and self-explaining answers here. Although technically you did answer the original question "Is this possible?" your answer would benefit from explaining how these incompatibilities work or how often they occur in humans. There already is an answer explaining incompatible blood groups, but how do guaranteed-lethal recessive gene defects work? – Elmy Dec 6 at 12:39

And this is actually the way evolution works in creating new species. Eventually a group of individuals (a subspecies) emerges that is no longer able to reproduce with all other individuals of the species. And as over time the group of individuals they are not able to reproduce with becomes gradually larger the more you could speak of them as a new species.

This is something that happens every day. Slowly, on a small scale in every species. It is more of a question how likely it is, to be exactly at that point in history where a species is about to split into two no longer compatible sub species. It is not impossible, but also not likely to happen more than once in 100K+ years.

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