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In my setting, every humanoid creature in our worlds mythology is part of a different species that branched off from humans millions of years ago. I'm trying to devise a way to prevent crossbreeding so I can realistically have multiple races without cross breeding and assimilation.
Question: In order to prevent the humanoids from mating with each other and creating viable offspring, what conditions can I create to prevent interspecies mating?
AFAIK the definition itself of specie is about the possibility to have fertile offspring.
If you state there are different species they "by definition" won't have fertile children.
If you have fertile offspring then the two divergent lines are called "races" (even if the term has been abused in the past, so it's very "politically incorrect" to use it).
If your humanoids drifted away enough (millions of years seem more than enough, as our species seems to have branched out from our cousins about 7My ago) there would be little to no chance to have any offspring (like trying to hybridize a man with a chimpanzee or a chimpanzee with a gorilla).
If they drifted less (~< 2My) then cross-breeding would be possible, but infertile (like mule or ligre).
Even less drift (~ 500Ky) would allow (to be verified) also fertile offspring.
You can have whatever suits your plot just trimming accordingly the amount of time the various communities remained separate.
Of course the ability of mating (in the sense of having sex together) has little to do with the effective results; again you can chose as it suits best your story.
The rule for a fertile animal hybrid is that the parent species must have the same chromosome count, allowing for a homoploid hybrid with a single chromosome set. Heteroploid or polyploid hybrids in animals are basically infertile and strongly discouraged, albeit not impossible, by mitosis failures during embryonic development.
So, atop physiological barriers, making it so that your humanoid species developed altered chromosome counts or other chromosomal incompatibilities (such as layout issues for key genes) early on would make hybrid speciation basically impossible.
This is pretty simple. For starters, you can drastically lower attraction between species by removing physical markers of attraction (brutish orc women or bearded dwarven women may not appeal to elven men, for example). Pheromones are also a major part of sexual appeal, they can lead to attraction or actively repulse members of the opposite sex. Different humanoids won't have the same attraction pheromones and may repulse other species. This wouldn't prevent conception though rape though.
Sperm has to be capacitated when it crosses the uterus and fallopian tubes to reach the egg. Failure to do this for a specific species will lead to infertility. Even if chromosomally compatible (assisted by a mad wizard/scientist, for example) in vivo cross-species conception won't happen naturally.
IgG antibodies cross the placenta. This is why women who are blood group D negative (A neg, B neg, etc) may develop antibodies to their D positive children (O pos, etc) after delivery of the first child or even during fetal-maternal hemorrhage during pregnancy. Easy to imagine similar antibodies towards other species, so even if a woman were to get pregnant, she would develop an immune reaction and terminate the fetus.
A particular mineral, vitamin, or other nutrient is deficient, leading to a failure to grow of a cross-species fetus. Human embryos need folic acid in order to develop the spinal cord, but perhaps orc females don't utilize folic acid at all, thus a human-orc embryo would fail to grow due to lack of this human essential vitamin. Again, this would allow for successful hybridization by magical or scientific means, but something that wouldn't happen in nature.
Physically, it could be because the different species have different methods of breeding/giving birth. I don't know what your humanoids are like, but different gestation periods, reproductive systems, rearing methods, and physical biology could play a part.
Possible examples:
1. Amphibious nixies mating with humans might try to raise their offspring underwater - and the newborns might be unable to survive without air.
2. Humans are pregnant for 9 months while elves may be pregnant for 2 years. Elven mothers would carry half-human babies for too long, and human mothers would give birth too early to half-elves.
3. Different species may have completely different blood types, resulting in *severe haemolytic disease* and *hydrops fetalis*. For more info on both conditions, see here: https://en.m.wikipedia.org/wiki/Blood_type
4. Some species might be predominantly intersex or hermaphroditic - or, if magical, capable of self-replication and therefore be biologically infertile.
5. Newborns might be fed in different ways. If a vampiric child was born to a human, it would either kill its mother (and then potentially starve to death, as it may only be compatible with its mothers' blood until it matures), or starve earlier as it would refuse to suckle.
On a less physical/biological level, certain cultural differences may be in place. If Group A and Group B live in close proximity, but perhaps distrust each others' cultures enough to make interspecies relationships taboo enough that resources, material goods, and emotional support would be withheld. Few already-weak interspecies babies would survive exile.
If you wanted a different kind of plot point, it could be that interspecies children are too powerful/destructive/etc. An ancient edict may demand that no cross-species child be allowed to exist - and the odd exceptions may set fire to villages/level towns/bring plagues to cities that eventually occasion their death as well.
Biological reasons are probably easier to enforce than social/cultural 'rules' (someone always wants to be different!), but a combination of the two might work as well.
You cant
The only thing that can prevent inter breeding is genetics ultimately saying these two sets of chromosomes are too incompatible as everyone else has said here.
There will always be humans or member of a specie who are 'special' enough to cross the line, take for instance the seal who went down on a penguin or other instances easily found on darker google searches.
Historically even governments have tried to implement policies to prevent interbreeding of human races only to result in failure, tragedy, and tons of human suffering.
But there is no amount of hate, disgust, science, disease, or fear that can prevent all attempts of inter-specie relationships.
1 Have you seen this? While this wouldn't prevent all crossbreeding, something along these line could slow assimilation. "Don't marry her, you will never have sons!" Modern human females and male Neandertals had trouble making babies. Here’s why
2 Worldwide there are many adaptations that help humans survive in different environments. The one I've read the most about is the high altitude mutation that allows Tibetans to make better use of oxygen than lowlanders. Other examples include the ability to process lactase or resistance to malaria. These folks are all human and can interbreed but in a fantasy story where the extremes are tweaked, the children of these matches are not going to thrive in the hostile environment.
