I ask because the world I'm building has eight distinct races of humans on one planet, as well as various non-human animals that don't currently exist in the real world.
I'd like for some forms of hybridization to be possible, with some resulting in sterile mule/liger-like hybrids, some resulting in completely stable hybrids like cross-breed dogs(or more accurately, humans of mixed ethnicity), and for other race combinations to not be possible at all. So how do I know when their biology has gone too far for hybrids to be viable?
The most similar threads I've seen about this are discussing the particulars of passing on alleles or the bare basics of evolution. None of the answered questions give a hint as to how to estimate what kinds of hybrids are possible based on genetic traits.
Obviously hybridization between Giants and Dwarves relies on the female being of the larger race, but what about other features? Do the tusks of Orcs require an extra chromosome or just some weird genes within the same number of chromosomes? Can a race of humanoids with ears as wide as their heads breed with a race whose ears are normal, and can the normal-eared humanoids breed with a race who has no ears? If Wargs are basically wolves that were bred to be the size of a horse and have saber teeth, could they breed with normal wolves or dogs?
How do you tell?