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?

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    $\begingroup$ Some romantic music and a good bottle of wine is the usual way...... $\endgroup$
    – Thorne
    Aug 14, 2019 at 23:32
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    $\begingroup$ And how can anybody other than yourself answer this question? If Wargs are a race of wolves (in the biological meaning of the word "race") they are of course interfertile with other wolves, including dogs. (And BTW, ligers do occassionally produce offspring. Even mule females do very occasionally produce offspring.) $\endgroup$
    – AlexP
    Aug 15, 2019 at 0:32
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    $\begingroup$ It's worth noting that one definition of "species" is basically "widest group of individuals able to have fertile offspring with one another"; if your hybrids are fertile (if your elf and orc couple can have grandchildren, in other words), then the parents could technically be considered the same species. And yes, this means that wolves and dogs could be considered the same species under such a definition, since fertile hybrids do occur. Keep this in mind when considering the terminology around speciation, because otherwise you could easily end up confused. $\endgroup$
    – Palarran
    Aug 15, 2019 at 1:10
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    $\begingroup$ Not sure why this was closed. I think everything you want to know can be found in the Wikipedia article for Reproductive Isolation. Specifically the Genetics section. The general rule you can use is that if two organisms have a common ancestor within a few million years then it is at least plausible that they can hybridize. Phenotypic traits like tusks, or different ears, or sizes can change in significantly less time, so no physical traits preclude hybridization unless they result in pre-zygotic isolation. So, it's really up to you. $\endgroup$ Aug 15, 2019 at 15:58
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    $\begingroup$ The world I'm building presents as fantasy because the people have low-level technology and science, but I think of the world primarily as a biology-focused scifi. I use words like "orc" in the question, instead of the made-up names of my fictional races, to make it relevant to other creators. Maybe that why it was put on hold for being too broad? IDK... $\endgroup$ Aug 15, 2019 at 17:58

1 Answer 1


In taxonomy, one of the specific attributes used to help determine if a given group of organisms within a genus should be considered a separate species is whether or not it can successfully interbreed with other similar individuals outside that group - as are discrete differences in morphology. Note, by the way, that in science, species is the smallest / lowest unit of taxonomy.

So in your fantasy example, your semi-humanoid dwarves, orcs, elves, giants et al might all belong to one over-arching humanoid genus, but if they cannot interbreed then they are separate species - if, for example, elves and humans can interbreed, then they may not in fact be separate species, although if their morphology is disparate enough and yet they can interbreed they may represent examples of taxonomic outliers - where speciation is driven by one over-riding factor but nonetheless interbreeding can occur - these do occur in the real world too.

That being the case, you get to determine some of this strictly on a narrative basis, though for your reader's sake you should apply some proportional logic as well as morphological logic to this; E.G. - wargs may have been bred from wolves, but may no longer be able to breed with them due to extremity of size differences of reproductive organs, or due to gross social behaviour mis-matches - perhaps in the course of typical wolf mating behaviour, the Warg will simply attack and then eat the prospective mate out of simple irritation.

I think this question may get Vote-To-Closed for being Primarily Narrative Based - I suspect you might want to bone up a bit on real-world taxonomy and speciation sot hat you have a better grasp of the basics for writing the relevant portions of your story.

Hope that helps.

  • $\begingroup$ I don't think this is a narrative-based question, I'm asking how to estimate whether something is possible based on physical criteria. $\endgroup$ Aug 14, 2019 at 23:28
  • $\begingroup$ And that old definition of species is quite muddy as it doesn't apply cleanly to a lot of situations like ring species for example, but that's beside the point. Maybe I should have been more liberal with the word 'race' $\endgroup$ Aug 14, 2019 at 23:30
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    $\begingroup$ @MaddockEmerson the physical characteristics you list don't actually matter. If somebody has large ears or not does not determine whether they can produce offspring with another individual - being of the same species does. It's possible that the same species have quite varying appearance - take dogs for example, a basset hound, a stattfordshire terrier are quite widely different but are all dogs. The former even has quite big ears. $\endgroup$
    – VLAZ
    Aug 15, 2019 at 6:54
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    $\begingroup$ "A given group of organisms within a genus should be considered a separate species [if] it can successfully interbreed with other similar individuals outside that group": nope, that's not it. The criterion is whether it actually does (or potentially would in the absence of natural barriers) interbreed with other populations. What counts is genetic separation, not the mechanism for genetic separation. "Species is the smallest / lowest unit of taxonomy": subspecies (in zoology) and varieties (in botany) are fully formalized and accepted taxonomic ranks, with scientific names. $\endgroup$
    – AlexP
    Aug 15, 2019 at 13:00
  • $\begingroup$ @AlexP - I stand happily corrected - thanks for the clarifying comment! $\endgroup$ Aug 15, 2019 at 15:17

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