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How should I justify different intelligent species being able to interbreed (and potentially produce healthy and fertile offspring) in a fantasy setting, especially if I want it to not just boil down to "The author said so"?

Considering some real-life examples, like humans with dwarves and elves it seems feasible, as one could just say that they're closely related, like how Neanderthals interbred with modern man, closely enough that it's not an issue, but what about when that's not the case?

Or when it's between a human and something harder to justify a relatively close relation with, like a troll or ogre/orc or goblin/merfolk or something?

Or what about in situations like human/dragon hybrids (assuming that shapeshifting doesn't change you genetically into the species you're becoming), or human/merfolk? I was actually inspired to ask by thus by reading on TV Tropes about D&D's examples of hybrids.

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    $\begingroup$ @BinaryWorrier I don't think that question is a duplicate. That question is about science fiction while this question is about fantasy. The solutions which are applicable to one of these genres aren't necessarily applicable to the other. $\endgroup$ – Philipp Jan 10 '18 at 12:09
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    $\begingroup$ Why would you want to justify it? You decide how reproduction works in your fantasy world. $\endgroup$ – Cyrus Jan 10 '18 at 12:17
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    $\begingroup$ you should probably rework your phrasing: dwarves and elves are all but real life examples (last time I checked neither of the two was living in my garden). Moreover, if your readers can accept the existence of dwarves and elves because the author says so, why cannot they accept the interbreeding? $\endgroup$ – L.Dutch - Reinstate Monica♦ Jan 10 '18 at 12:18
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    $\begingroup$ Voted to close. "How should I..." makes this too broad, and explicitly asks other people to come up with the basis for your work instead of doing the legwork yourself. An acceptable question would be "I plan on making species X and species Y interbreedable, to meet a plot point. I've foreseen problems A, B and C arising from this. What other issues might I (or the characters/world/science) face?" $\endgroup$ – Graham Jan 10 '18 at 13:25
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    $\begingroup$ I don't see a strong reason to close this question, so I'm voting to remain open. But you should know that D&D's hybrids are 50% handwaved just for the fun/cool factor. If you're really daring, you can take a look at the Book of Erotic Fantasy to see the underpinnings of how the hybrids work. I wouldn't recommend carrying that book around in public, though. $\endgroup$ – Frostfyre Jan 10 '18 at 13:27
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Magic.

Don't have procreation in your universe work on principles of modern genetics. Instead get inspired by how older cultures imagined procreation to work. These models were usually far more esoteric.

For example: When two creatures mate, don't think of it as a mating of their genes, think about it as a mating of their souls. When two creatures have sex really, really, really like each other, their souls connect to each other and a new soul is created in the reproductive system of the female. So the chance to crossbreed between creatures depends on their psychological similarity, not their physiological similarity. This model also creates some interesting plotlines for traditional marriages. An arranged political marriage will have a low chance to result in children unless the partners find a way to connect to each other on an emotional level. It might also open up the possibility for lesbian couples to have children, if you want to. You could even allow male creatures giving birth to children under some circumstances. There are some examples from Greek mythology for this (Zeus gave birth to Athena through his forehead).

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  • $\begingroup$ @L.Dutch I don't get how this is related to bitcoins. I know some jokes lose their comical effect if you have to explain, but I'm dying to get this one. $\endgroup$ – Renan Jan 10 '18 at 13:09
  • $\begingroup$ @Renan, if it is only the genuine liking to allow reproduction... $\endgroup$ – L.Dutch - Reinstate Monica♦ Jan 10 '18 at 13:48
  • $\begingroup$ @L.Dutch then only a handful of people are going to breed. I still don't get it and I feel I'm more naïve than I believed myself to be for still not getting it XD $\endgroup$ – Renan Jan 10 '18 at 14:03
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    $\begingroup$ Reminds me of Asari from Mass Effect $\endgroup$ – phflack Jan 10 '18 at 14:06
  • $\begingroup$ I like this answer! @Preg-Fan, For one example of how it might work out, that also involves genetics, look at the Asari in Mass Effect. All-female race, reproduces through a hybrid psychological/biological bonding that results in an Asari, but with some (mostly tempermental) traits from the non-Asari partner. And pure Asari actually aren't a good idea. I'm not sure if any other interspecies relations can produce offspring in-universe. $\endgroup$ – Michael Ekstrand Jan 10 '18 at 15:12
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A slightly more sciency approach. Let's start with the tenet that a species is often defined as the largest group of organisms in which two individuals can produce fertile offspring, typically by sexual reproduction (that's wikipedia take on species). So:

  1. They are not different species after all. Just like dogs, despite the variety of looks, belong to the same species and can interbreed. dogs

  2. They are different species. Then, either the offspring is a sterile hybrid, which could add some particular flavor to the story (think donkey and horse mating), or the reproduction does not follow the tenets of biology. For the second case, you could imagine that reproduction happens by infusing a life-spark in an adequate medium, e.g. by moulding a special type of holy clay and infusing it with one drop of blood from each of the parents (borrowed from the Prometheus myth). The clay will then grow following the inclinations and looks of the mix of the parents.

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  • $\begingroup$ Yeah, that works for humans and dwarfs/elves, but what about orcs and trolls? $\endgroup$ – Preg-Fan Jan 12 '18 at 1:49
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Mythologically speaking, there's precedents to draw on to establish a world.

Zeus was notoriously fecund, birthing demigods left right and centre as various mortals caught his fancy. You might draw on that for some half-humans - in the times of yore the minotaurs were the offspring of a god in bull form and a human, hence they're essentially humans in spite of appearances. Perhaps an elf is really a half-tree human.

Some shapeshifters can take the form of humans: selkies, legend has it, had a cap that allowed them to turn into seals. Some fishermen would steal their caps and hide them, so they could take them for brides. Thus, their children would be part selkie.

Occasionally an unfortunate mortal would be transformed by magic. For example, one Chinese myth concerns a little boy who swallowed a magical pearl and became so thirsty he drank down a river, transforming into a dragon. In the greek tradition, Arachne was transformed after offending the gods. Perhaps, even after the magic, they are in a way human.

Another myth goes in the opposite direction: A snake falls in love with a man, and uses magic to transform herself into a human to court him - aided by a fellow snake who transforms herself too and opposed by a jealous terrapin in the guise of a monk. Over generations, perhaps the truth of their animal heritage may slip through.

Finally, there is the idea that some paleolithic depictions of animal-headed humans in truth depict shamans trying to gain the power of animals - it's certainly plausible in your world the non-human races are the product of success in this endeavour.

Hopefully these concepts will give you some useful directions for how interspecies breeding might be justified from a mythical point of view

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Scientifically, you can't, but this is not strict science, so maybe there is a pseudo-scientific approach.

Suppose all your intelligent species were basically one species, but with genetic variations? Each of your intelligent species would then effectively become different races with the potential for interbreeding.

A particular combination of genes would give you classic dwarf features, another combination would give you elves, and so on.

You would need to work out what those genetic traits were, and come up with an explanation why races would usually, or at least historically, not interbreed but that does not seem a difficult task.

Then, to work out what an elf/dwarf hybrid would look like, for example, you mix and match those genetic traits.

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Our real-life DNA-based protein-encoding stuff is a finely tuned and interwoven system that simply doesn't produce viable offspring if you mix DNA that's too different, because it only takes one tiny chemical process to fail somewhere for the whole organism to fail.

If you want to explain fantasy creatures in those terms, you're painting yourself into a very small and uncomfortable corner. The closest you could get is saying that the basics of life are much more universal on your world and creatures have additional "DNA" on top of the basics that modify the template in a mostly non-breaking way. For example, the "DNA" for wings would add an extra pair of limbs to the template and change them to be wing-like. If the creature also has scales DNA, the wings will be scaled, if it has feathers they'll be feathered.

Interbreeding would mean the offspring gets a mix of properties from the add-on "DNA" and if those don't conflict too much it will be viable. If it gets wings "DNA", it will have wings. They might not work very well, but they'll grow.

However, I would take a simpler non-scientific explanation that doesn't cause readers to go "Hey, biology doesn't work like that!" all the time. Something like this:

All living beings come from the same source/essence, so they can interbreed if it's physically possible. They will then have properties from both parents' essence in some mix.

This way the form is secondary to the living essence. All that's needed is a mix of two parents' essence and a compatible place for it to develop.

You can dress this theory up to fit other aspects of your world. Maybe all life starts as the snot spread when the primal deity Achoo smells the air on a new planet and sneezes.

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