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There are quite a few known drawbacks to incest. However, incest can be helpful when trying to help a desired trait spread.

Incest does happen in nature to some degree but it's not exactly actively sought out.

How (if at all) could selective incest become a beneficial part of the development of a species?

This concerns both an intelligent species, knowingly commiting incest or using it on domesticated animals (but only for practical purposes) as well as non intelligent species doing it out of instinct in appropriate scenarios. These situations could include when an individual shows very strong genetics or the environment changes rapidly and an individual seems more adapted to the new environment than others.

I'm looking at this question from a purely genetic perspective, the social implications of incest being common within an intelligent species should be considered out of scope and kept out of the answers unless necessary. Likewise, inbreeding for aesthetical purposes is not useful and not on topic. Discussing drawbacks should only be done to illustrate how these drawbacks can be overcome or at least outweighed.

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  • $\begingroup$ Does this allow for made-up species where incest could be made to be beneficial, whether they are natural or artificial? $\endgroup$ – mechalynx Oct 14 '14 at 13:08
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    $\begingroup$ @githubphagocyte I think the actual question is "can eugenics be useful for a species (fictional or not) when it includes or is based on inbreeding?" or "can inbreeding arise as a natural tendency that is beneficial in a species". I don't think the question cares about the downsides, which it already cites, especially the psychological ones. $\endgroup$ – mechalynx Oct 14 '14 at 13:21
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    $\begingroup$ Why is the title censored? $\endgroup$ – Epiglottal Axolotl Oct 14 '14 at 15:51
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    $\begingroup$ Why is "incest" a censored word for the hotlist Upvote if you want to find out I guess? $\endgroup$ – Mourdos Oct 14 '14 at 16:01
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    $\begingroup$ I can't believe nobody brought up Kwisatz Haderach yet!!! $\endgroup$ – user4239 Oct 15 '14 at 3:21
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If they're intelligent polyploids, their civilization may crumble if they don't breed with siblings

Incest is a form of inbreeding, which as we know is deleterious. This effect is termed "inbreeding depression" but many plants are less susceptible to inbreeding depression than animals. This is probably because of "polyploidy", a condition where the genome contains more than two copies of each chromosome. This creates a much wider range of possible combinations and a much smaller chance of being stuck with deleterious recessive traits.

This is seen in for instance apples, which farmers propagate by grafting. This is because a tree growing from an apple seed, even one fertilized by two genetically identical trees, will turn out quite different from its parents. Often reverting to small, sour apples. The protection offered by polyploidy is so strong that polyploid plants can opt for widespread self-fertilization. This is less common in non-polyploid plants, which more readily suffer inbreeding depression when self-fertilizing. An example is the common garden plant chives, which is polyploid, self-fertilize and where you can find lots of different chromosome numbers, within the same species!.

So in short, a polyploid species would suffer less from inbreeding and therefore may not have evolved the instincts that prohibit incest in terrestrial animals. Now with inbreeding depression out of the way, incest is actually an advantage. This is because you want to propagate your genes, but your offspring only gets 50% of your genes, the other half coming from your partner. But if your partner is a sibling, about half its genes are also yours, so you get 75% of your genes into your offspring. You could imagine that this species is also hermaphroditic and occasionally self-fertilize. (not the same as cloning)

plot-wise, you can also play on the increased range of outcomes in a polyploid mating. Perhaps the culture in the story is a collection of families with similar traits (phenotypes) such as intelligence, but which have arrived at the same traits independently, by different genetic routes (genotypes). When non-related individuals in this culture mate, their offspring revert to an ancestral phenotype like brute cavemen, or into unpredictable new phenotypes like the first child of a completely new race, ones that eat different types of food, or something that to earthlings would look like a completely new species. So incest is a necessity for the continued existence of their civilization (or so they think).

An additional plot device is the slow but persistent churning of inbreeding depression which can only be alleviated by outbreeding, but which has such unpredictable consequences in this species. Play it safe and lose in the end or take the leap into the unknown?

For more on polyploidy, there's a wonderful article on how the different types of apples arose from exceptional individual trees, that stood out in forests of less remarkable seed-planted ones, with a rise and fall of dynasties of apple types.

Some academic references on polyploidy vs inbreeding: http://dspace.library.cornell.edu/handle/1813/9410 http://www3.botany.ubc.ca/rieseberglab/plantevol/Sample_Project_Proposals.pdf

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Look up "linebreeding". When one animal (especially a male) shows an especially beneficial trait, farmers and ranchers will often breed that animal back to his own daughters and/or grand-daughters.

The goal of linebreeding is not focusing on inbreeding for its own sake; it is focusing on "getting as many copies of the genes from this prize animal into the herd" and "maximizing the chance of producing some beneficial trait that relies on double-recessive alleles".

Inbreeding always increases the chance of unfavorable mutations. A lot of mutations essentially boil down to: this broken gene does not product protein Z properly. One copy of the mutation means you get half as much of protein Z as others, which may be fine. Two copies of the mutation means you don't get any Z, which can be lethal.

Inbreeding increases the chance that a parent with a single copy of a broken gene could pass that copy onto a child/grandchild twice, thus causing problems. There's no way around this limitation.

Furthermore, diversity is often vital to the survival of a herd. Parasites and diseases will occasionally be more deadly to some animals than others, due to a fluke of genetics. The more inbred the population, the greater chance a single disease could wipe out the whole herd.

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Incest is a risk. For humans, we do not accept that risk for our own species, but we do accept that risk for domesticated animals. I say "we" to summarise the current legal approach in most countries. Obviously individual humans differ in their attitude to what rights animals should have.

Domesticated animals

The benefit of inbreeding is that it gives more fine control over the selection of desired traits and exclusion of undesired traits, so that artificial selection can be used more quickly to change the nature of a domesticated animal. For a species that accepts the frequent birth of animals that have avoidable genetic diseases, shortened lifespans, and increased suffering, inbreeding gives a shortcut to faster manipulation of genetics without need for technology such as genetic modification or even being able to read a genome.

I would expect inbreeding of domesticated animals to be a common part of the history of most intelligent species, used to gain an advantage until technology allows them less damaging alternatives. Some species may stop the practice early due to ethical objections, while other species may stop later, when technology renders it redundant. In both cases I would expect inbreeding to play a major part of the early history of domestication.

Intelligent species

A similar pattern is likely to be followed for an intelligent species actively choosing inbreeding of its own species.

  • Personal choice
    Human history includes inbreeding where people have perceived that their own family is superior to other humans, and chosen to breed with close relatives in an insular breeding group. This is most notable in ancient royal families.

  • Slavery
    Human history also includes the enslavement of humans by other members of their own species - humans being used as domesticated animals. For this reason the same applies as in the paragraphs on domesticated animals. Many intelligent species are likely to use domesticated animals, and those which do not have some protection against treating their own species in the same way are likely to have a period of using their own species as slave labour, including breeding their own species and forcing inbreeding for traits. This period will last until the species takes steps to prevent it for ethical reasons, or in a species that lacks an ethical objection it may continue until it becomes unnecessary due to technology, or until it is ended by uprising.

Advances in technology do not necessarily lead to advances in ethics, so for some intelligent species slavery and the associated inbreeding for traits may persist, eventually leading to a new species.

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    $\begingroup$ Even when slavery is rendered useless through technology it may still persist as a form of status - people like having other people they can order around. $\endgroup$ – Tim B Oct 14 '14 at 14:18
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    $\begingroup$ What is the link between slavery and inbreeding? you don't mention any link between those 2 except that it's done for the same reason as inbreeding domestic animals. $\endgroup$ – Nzall Oct 14 '14 at 14:20
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    $\begingroup$ @NateKerkhofs it's too much effort/cost to bring in a new broodmare from outside, compare how puppy mills work, one female dog pumping out puppies as fast as she can, probably from her brother/close cousin $\endgroup$ – ratchet freak Oct 14 '14 at 14:35
  • $\begingroup$ @ratchetfreak You're assuming that slavers keep families together, or that they only keep 1 family at a time. usually, a slaver has a number of families to breed with. And a slave owner usually buys slaves based on qualities, not relationship. In fact, sibling/cousin protectiveness might be counterproductive to keeping siblings together, depending on how violent the owner is when dealing with his slaves. $\endgroup$ – Nzall Oct 14 '14 at 14:41
  • $\begingroup$ I'd suggest removing the slavery paragraph, since it doesn't really add anything useful to the answer. $\endgroup$ – Bobson Oct 14 '14 at 18:24
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The main biological reason against inbreeding is that, since close relatives have highly similar DNA, it means that the DNA of the child is almost exactly the same as that of the parents. This means that any genetic defects in the family are likely to remain in the family, including those responsible for hereditary illnesses and pathogen vulnerabilities.

However, a sufficiently advanced race could have found a way to induce certain genetic changes during the gestation period, meaning that 2 relatives can produce offspring while still giving birth to a sufficiently different offspring. Already, humanity is getting there, with in-womb genetic testing and use of viral-based delivery methods for genetic code.

So once the genetics are out of the picture, the only hindrance is the social taboo. Already, there are communities like certain rural areas of Southern US states like Alabama where first cousins can marry and procreate. And even in western Europe, it's estimated that 1% of new marriages is between first cousins. It wouldn't be the first time that social taboos get broken, as happened with interracial marriages in the 1960's-70's and gay marriage in the past decade. In fact, back during the classical eras of the Romans, the Greek and the Ancient Egyptians, there was no such taboo. For example, Tutanchamon was married to a half-sibling.

There are not really any benefits to large-scale incest, though, especially in a science-based world. A genetic trait would be distributed far better through an air- or waterborne pathogen. In fact, incest would be counterproductive to spreading a trait, because you want to spread the trait to as many family trees as possible.

However, on a smaller scale, incest can enhance traits found in a small subgroup of a society. The only reason this would be acceptable, though, is if there is only one family with this genetic trait, which is rarely the case. usually, due to the natural resistance against inbreeding, any genetic traits formed through chance mutations during conception are present in multiple families.

Finally, fringe groups in society might naturally distrust people that aren't part of their small group, and would eventually have to start performing incest to keep the group population big enough.


Another reason that incest might be performed is for adult entertainment purposes, but those almost never result in offspring. In fact, as long as there is no offspring from the act, incest might not be viewed as a big social taboo.

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  • $\begingroup$ Not a bad answer, but mostly made irrelevant when the OP added that they were looking for purely genetic reasons. $\endgroup$ – Bobson Oct 14 '14 at 18:13
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I don't think that could have happen in a real world, as freak events are just too rare. On the other hand, as you consider a fictional species, we could make it so that it will be beneficial.

First, please note that the reason for both inbreeding and avoiding it is the same: the genetic advantages and disadvantages of them. So, to make it beneficial, we have to create something that will outweigh the other costs.

For example, imagine a species with psychic powers. If these are very rare, but on the other hand very powerful, it might make sense to try to strengthen them. Still, if there are other specimens holding similar powers, it will make even more sense to stop inbreeding.

So to make it work, it has to be rare, but not very rare (as it would just disappear). Also it has to clearly outweigh other costs, and there has to be some big disadvantages for non-related specimens to get together, e.g.

  • there are cities run by families that fight each other very fiercely,
  • powers of different families interact very badly (e.g. no such two specimens can met without one dying, or, the offspring would be insane or with no powers, or with too dangerous/unstable powers).

I hope this helps ;-)

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You also get the benefits of keeping good traits. Which may be a function of intelligence.

Or, why do humans only have 23 pairs of chromosomes when our ancestors did not?

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  • $\begingroup$ Humans normally have 23 pairs of chromosomes. $\endgroup$ – Ross Presser Jun 25 '18 at 16:11
  • $\begingroup$ @RossPresser Thanks for the correction. Doesn't change the gist of the question. $\endgroup$ – user3082 Jun 26 '18 at 6:48

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