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One species has two genders, like humans, the other has three(essentially male, female, and one without breeding capabilities that first appears as a genetic quirk but quickly becomes more prominent until it is equal to the other two). It is in an advanced society with two different regions, one for each species, on opposite sides of a small planet.

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    $\begingroup$ This is an interesting question but to help focus the answers - Why do you think it wouldn't be? What stops it from being viable? $\endgroup$ – Tim B Oct 28 '18 at 9:23
  • $\begingroup$ Hello @EnglandsGirl1818 and welcome to Worldbuilding! In order to make your question easier to answer, it would help, if you could clarify what you mean by "quickly becomes more prominent until it is equal to the other two". $\endgroup$ – Alex2006 Oct 28 '18 at 9:25
  • $\begingroup$ I'm not sure if it would work biologically, but there is a physical difference between the two species. One has purple skin and silver hair, while the other is essentially human. Would humans slowly develop purplish skin or would the other species change in response as they interbreed? $\endgroup$ – EnglandsGirl1818 Oct 28 '18 at 9:27
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    $\begingroup$ So basically it all comes down to, can the offspring reproduce. You should decide that; we can't decide it for you and it would make the question off topic. If they can't, the answer is what will you do with mules; if they can, the answer is what will you do with the various bastard subspecies that will arise and will the "purebreds" have any advantage. $\endgroup$ – Mr Lister Oct 28 '18 at 12:34
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    $\begingroup$ If they can interbreed, doesn't that technically make them the same species, at least if the offspring is also fertile? $\endgroup$ – HelloGoodbye Oct 29 '18 at 10:26
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Would a society with two different species that can interbreed be viable?

Sure it's possible, by a number of processes the most obvious being hybridisation. The process by which two distinct species combine to form mostly infertile offspring (Vis-a-vis mule)- but just occasionally they are viable.

There is evidence of hybridisation between modern humans and other species of the genus Homo. In 2010, the Neanderthal genome project showed that 1–4% of DNA from all people living today, apart from most Sub-Saharan Africans, is of Neanderthal heritage.

But you can't expect them to be a perfect halfway house:

Hybrids are not always intermediates between their parents....but can show hybrid vigour, often growing larger or taller than either parent.

Another intriguing way would be through chimerism:

A woman was found to have blood containing two different blood types. Apparently this resulted from her twin brother's cells living in her body.

However it may be more obvious than this as it could occur in your creature's skin, giving a mottled blend of the two skin types you mention above by mosaicism.

Culturally, would either society accept a crossbreed? See Nazi Eugenics for an idea of what can happen. Would the prevailing cultural atitudes drive these unique creatures to the fringes, desperate, and in hiding - would they have a political voice? Allies in the general populations? Would some be driven to violence? Only you can decide.

Here's further reading to fuel your immagination:

https://en.wikipedia.org/wiki/Polyploidy

https://en.wikipedia.org/wiki/Horizontal_gene_transfer#Plants_to_animals

https://en.wikipedia.org/wiki/Endosymbiont

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In biology a species is defined as

"a group of living organisms consisting of similar individuals capable of exchanging genes or interbreeding" or "the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction."

Therefore, if they can interbreed, they are by definition basically not different species but only two very different looking races of the same species i.e. variants of the same species. This does not mean that they cannot have completely different physical appearances or capabilities; as an example consider how different a chihuahua is compared to a St. Bernhard, but they are basically both dogs and can interbreed.

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    $\begingroup$ Races do not exist in science and Homo Neanderthalensis and Homo Sapiens were able to inbreed, even though they were different species of humans. $\endgroup$ – user56555 Oct 28 '18 at 10:01
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    $\begingroup$ @EnglandsGirl1818: Humans now don't have two genders, regardless of whether you mean gender in the sociological context or sex in the biological context. In both cases we have a bimodal system in which we label one hump "male" and the other hump "female" and the vast majority of the population comes close enough to one of them for it to basically work, colloqially. It's entirely plausible you could tweak the numbers so the bimodal system is less humpy and more spread out, there's no sci-fi there and no reason why it wouldn't just work, biologically -or- socially. $\endgroup$ – Phoshi Oct 28 '18 at 19:18
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    $\begingroup$ The mallard duck can breed with multiple species of duck that can't breed with each other. In actuality it's been concluded that the clear cut concept of species does not adequately describe the interbreeding situation. There are different species that can breed with each other. $\endgroup$ – Matt Samuel Oct 28 '18 at 23:26
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    $\begingroup$ @Eries: Races absolutely do “exist in science”, as taxonomic groupings below the level of species — it’s just that doesn’t line up at all with the usual meaning of race for humans, which is far more socially constructed than biologically based. $\endgroup$ – Peter LeFanu Lumsdaine Oct 29 '18 at 0:20
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    $\begingroup$ @Phoshi That is a fairly misleading thing to say. Traits like height are distributed bimodally, but that does not mean that sex itself is bimodal. There are two types of gametes, egg and sperm cells, and there is no human on this planet that produces something inbetween. Gametes define sex. If you're not willing to make a categorical distinction between sexes, then you can basically not make any categorical distinctions in biology at all. You could say that tigers and geckos aren't categorically different, just "bimodal," but that would just be a point of confusion, not clarity. $\endgroup$ – Eff Oct 29 '18 at 7:37
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Short answer: Sure. Why not?

Longer answer: Horses exist. Donkeys exist. Horses and Donkeys can interbreed. Do donkeys become more horselike over time? No. Why? The offspring of an inter species breeding are typically infertile, and so can’t pass on the traits of either species. They can mess with breeding patterns by ‘stealing’ potential mates, but otherwise they have no further genetic impact.

If the two species can interbreed and produce fertile offspring then Alex2006’s answer applies: they are not two distinct species. You can get fuzzy on the whys and wherefores of when you choose to ‘split’ the species, but that’s mostly an arbitrary ‘because I said so’.

As for the social implications of whether two sapient species can coexist peacefully without the word ‘genocide’ getting used a lot: we can’t say much. But from a biological perspective if you simply have interspecies children be infertile then your problem completely goes away.

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    $\begingroup$ The definition of "species" is not nearly as clear-cut as we'd like it to be. There are plenty of organisms acknowledged as different species that can interbreed and produce fertile offspring. See en.wikipedia.org/wiki/Species_problem for an overview. $\endgroup$ – Phasma Felis Oct 29 '18 at 2:48
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    $\begingroup$ An interesting variation to avoid the "one species" issue would be a redundant chromosome pair: Humans have 23 pairs, so let's say that the other species has 24. The 24th pair is typically ignored, unless both carry a recessive trait (resulting in an offspring of the "3rd gender", regardless of whether their 23rd pair is XX or XY) and hybrids with 23 pairs and a "floater" are handwaved as both viable and fertile. Their offspring then have a 50% chance to produce another hybrid and either 50% to produce the same as their partner, or 25% for each species if their partner is a hybrid $\endgroup$ – Chronocidal Oct 29 '18 at 13:49
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It depends on too many variables.

As alex2006 pointed out, by definition you only have one species. Let's call their makeup as XY and XX for the group 1, and xy and xx for group 2. Equal symbols mean female phenotype, different symbols mean male phenotype.

The "third gender" could arise as a variant of Klinefelter's syndrome where xxy "males" do not have any impairment at all, except for sterility, very low sexual drive and androgynous appearance.

A XY male and a xx female may interbreed and have Xx female or xY male children. Possibly xxY children too.

What happens to Xxy, XXy and xxY children? It all depends on the exact nature and interaction of the sexual chromosomes.

  • the combination is immediately fatal: the foetus does not develop and you don't even get a pregnancy. To all intents and purposes, "Group B" only breeds true. An intermixing of the groups will then lead to a diminishing of the third gender group, as less of them will be born.
  • the combination is later fatal: the foetus is born dead or it even damages the mother, or makes it sterile or selectively sterile (for example by a mechanism akin to mother-foetus immune incompatibility); the xx mother will from now on only be fertile with pure xy males. This will easily lead to resentment and conflict.
  • the combination is harmful but not fatal: interbreeding has a high chance of children being born handicapped, or worse. Either group may blame this on the other.
  • nothing much happens: the two groups can interbreed and the "third gender" demography might change, or not. The other physical characteristics will naturally blend, and there's no biological reason for racism to develop (this still doesn't mean it couldn't).

There are several other possibilities with the same, as well as wildly different, outcomes. Just for kicks, imagine that XXy children are born with some marked advantage (longer lifespan, higher intelligence...) and decide they're the new Ubermensch?

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  • $\begingroup$ The definition of "species" is not nearly as clear-cut as we'd like it to be. There are plenty of organisms acknowledged as different species that can interbreed and produce fertile offspring. See en.wikipedia.org/wiki/Species_problem for an overview. $\endgroup$ – Phasma Felis Oct 29 '18 at 2:49
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I understand "viable" in your question as "capable of long-term existence" with the biggest threat being permanent hybridization and ultimately unification into one species.

Hooded crows and carrion crows are related closely enough to interbreed. Their ranges overlap, so the two species meet. For years they were thought to be subspecies of one species and hybridization to occur widely. However, it turned out it's not so. They're classified as distinct species now. What's most important for you, they can crossbreed, but they choose not to. Hybridization is rare.

That's pretty much your world. The two groups were one species very long time ago. They were separated (eg by continental drift) and drifted apart, genetically. Now (thanks to technology?) they can meet again.

IMHO your hybrids should be at disadvantage. In our world, hybrids usually are at disadvantage, sometimes completely sterile, sometimes just having slightly more trouble breeding. A hybrid can have an advantage personally, eg. combine physical strength on one side with intelligence of another. But, for the species as a whole it's reproduction that matters the most. Even small reproductive disadvantage is enough for purebloods to always outnumber hybrids, pushing them into outskirts and maintaining distinction between two societies. That's enough to make your society maintain status quo indefinitely, what I interpret as "viable".

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