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.
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.
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.
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:
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..
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.
