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I have a sentient race that is similar to humans in appearance as well as culture. The culture they embrace currently is the 'arranged marriage period' where such a union may happen for wealth, political reasons or under specific customs that the people follow.

Of course in human history it was found inbreeding can cause genetic problems, the lack of gene variety increasing the chances of rare conditions and health problems for the child. Even if these problems were to be 'solved' there would still be the social and psychological backlash that human society has bred into us.

But if this alien species never had the genetic downside would the anti-incest stigma be there or ever develop? Would there be a heavy increase of inbreeding or would it seemingly normalise in comparison to other types of breeding?

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    $\begingroup$ You can't say that there are "no genetic downsides". Any trait can be either positive or negative depending on the environment. Assuming Earth-like biology (but you'd need something similar anyway, if not identical), genetic diversity is what allows a species to adapt to a changing environment, as previously neutral or perhaps even negative traits become positive. $\endgroup$ – a CVn Aug 9 '16 at 12:14
  • $\begingroup$ @MichaelKjörling : Very good point :) $\endgroup$ – King of Snakes Aug 9 '16 at 12:15
  • $\begingroup$ @Michael Kjörling: A mutation that interferes with the proper development of organs would be negative regardless of the environment. I think what you mean is situations like the black moths that outcompeted white moths when the white trees they lived on were stained black by pollution. $\endgroup$ – Anonymous Aug 9 '16 at 12:31
  • $\begingroup$ This is somewhat evolutionary based, methinks. Check en.wikipedia.org/wiki/Inbreeding_depression for what I mean. If the populous can 'self propagate', then the depression is more likely to occur if there is too varied a 'genetic pool' to select from. So this question is a bit ambiguous. $\endgroup$ – Fayth85 Aug 9 '16 at 13:53
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    $\begingroup$ I highly recommend reading 'The Selfish Gene', as the grasp of genetics here is a bit facile. Thinking of genes as having 'upsides' and 'downsides' is a narrow viewpoint and doesn't help one to reason about genetics very easily. $\endgroup$ – Nathaniel Ford Aug 9 '16 at 22:46
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Such a society will probably evolve a caste system (like in India). Given enough time, the castes will become different species that cannot interbreed any longer.

Note also that in history, there was extreme inbreeding in royal families to keep the "royal blood" pure (e.g., in Ancient Egypt or Medieval/Early modern Spain). It ended in a genetic depression most of the cases.

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    $\begingroup$ Weathy families would inbreed to avoid having to share resources or money through marriage, separating themselves from soceity. Indeed, this is a form of articial especiation where populations become seperate and do not interbreed. Eventually, after many generations the populations would be genetically different since they have experienced completely independant natural selection or evolution. This would prevent them from successfully breeding and classify them as a different species. Of course, this is assuming that the family continued inbreeding for hundreds of generations. $\endgroup$ – Aric Aug 9 '16 at 12:15
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How long has the "no genetic downsides" aspect of their biology existed? It seems virtually impossible that their entire evolutionary cycle had this quality, how else could they have evolved from simpler organisms in the first place? Or are they artificially engineered this way?

As a primitive culture, assuming they had a hunter/gatherer phase, the need to interbreed with other groups would be necessary to prevent small tribes from spinning off into their own separate species. Unless their planet had several near total extinction events that kept culling these other tribes, eventually allowing just one tribe to quickly flourish across the planet, it seems more likely that only with advanced technology could they realistically remove the negative aspects of HABITUAL incest (1-2 generations of incest isn't really a problem) within a small subgroup of the population.

Humans have several biological mechanisms designed to prevent incest. Pheromones that drive sexual attraction typically "turn off" closely related individuals in most cases. Your aliens probably have a similar mechanism from when they needed more genetic diversity. This would have to be overcome for widespread incest relationships to flourish, I would think.

Socially marriage and children, at least amongst property owners, nobility, and the like, was a mechanisms to cement partnerships, alliances, and resource sharing agreements. Incest relationships won't drive these things, it would just ensure continuity of control over resources a family ALREADY has, so I suspect in an advanced society there would still be plenty of reasons to have outside relationships. So under most circumstances I don't think incest as a normal relationship event would develop.

BUT, if there was some unique quality/ability that had to be maintained (like the Targaryan dragon riding ability from GRRM's "A Song of Ice and Fire") then that WOULD drive incest in order to cultivate that genetic ability and limit it's spread. Perhaps certain technological devices were keyed to specific genetic sequences (and the ability to change those keyed sequences was then lost), so incest was used to ensure those sequences were passed on (as opposed to having to test all the offspring across several families, hoping the sequence pops up).

Of course institutionalized incest has other problems. Partners are necessary, but siblings are going to be staggered in age and gender. Even bringing in first cousins (and presumably the parents) you will have large gaps in age between partners and potentially surplus members of one of the genders. What to do with this surplus? Run them off to a celibate priesthood? Marry them off to outsiders (and risk losing control of the family genetics?). Seems like a source of rebellion to the ones with family partners. Of course if the family is genetically homogenous enough, perhaps they don't partner at all, just mate when and with whom they like since they may not feel a strong personal desire to parent a child so long as SOMEONE in the family does it (an extreme interpretation of the "it takes a village..." concept).

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  • $\begingroup$ So this state of 'no downsides' would only make sense as a "final evolution" of sorts, though this assumes that the species lives in an environment where any subset of genes can survive easily. $\endgroup$ – CEObrainz Aug 11 '16 at 17:56
  • $\begingroup$ Yes. So once they master genetic manipulation and remove any negative genes, incest wouldn't have a bad outcome because there are no negative genes that incest would increase in incidence (like hemophilia in the European royal bloodline). The ultimate outcome of this would be a race of genetically homogenous pseudo-clones with very little genetic variation between members as they selected for all the genes they like and cut out all the ones they don't. Superficial things like hair or eye color could be left in, they account for a miniscule amount of genetic variation. $\endgroup$ – Jason K Aug 11 '16 at 18:02
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A species like this would have to have an enormous capacity for inherent genetic diversity. This would constitute greater genetic diversity between any two individual members of any given family. Assume this species has an oversized genome with huge amounts of genetic redundancy. This will remove the usual harmful effects of inbreeding from consideration. It will also stop any stigma against inbreeding arises. This would be a normal form of breeding.

Interestingly in terms of compatibility for marriage siblings are remarkably suited to each other. Having children marry and breed would work out quite well. Cousins marrying would be good too.

Cultures where marriage is an institution for political alliances, wealth, and other social reasons is and has been quite common historically. Our Western custom of marrying for love is the exception to the rule. Arranged marriages can and have been successful. The marriage culture of your sentient race isn't at all unusual.

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  • $\begingroup$ That's interesting. Although my final design will be different I think that the idea of having a larger gene pool where only a small amount are selected is certainly worth thinking about. $\endgroup$ – CEObrainz Aug 11 '16 at 17:53
  • $\begingroup$ There are many species on Earth with more complex genomes than humans. An inherently richer, more diverse and complex genome is the easy way to avoid the adverse effects of inbreeding. You are free to choose whether you uses any ideas suggested here. Thanks for considering my idea worth thinking about, that's the nicest compliment I've had today. $\endgroup$ – a4android Aug 13 '16 at 4:47
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Another problem with inbreeding is divorce. (Presuming there is such a thing as divorce or messy breakups on this alien planet.)

A family is one of the basic units of a society. If two family members start a relationship and then have a messy breakup, this could potentially divide the family.

There may be stigmata about inbreeding/incest to keep families together. They might not recognize it as that. They might just say its bad luck to date within the family.

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  • $\begingroup$ I think this argument doesn't make this society non-viable. Family is already full of drama, it is not that important that you have kid with your sister. The context would certainly soften the break up. $\endgroup$ – Rigop Aug 9 '16 at 13:37
  • $\begingroup$ It seems far more probable that divorce and breakup would be unthinkable and unexistant in this civilization. $\endgroup$ – Tryss Aug 9 '16 at 17:53
  • $\begingroup$ I think divorce would be a problem no matter where you go, unless the practice was unheard of for a race of people. $\endgroup$ – CEObrainz Aug 11 '16 at 17:47

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