Alright, this eill require explaining why inbreeding is bad on a genetic level first
On a really simplified level, the amount of genetics you share with a relative halfs with each degree of separation (or 1/2^n where N is the degrees of separation)
You are 0 generations removed from yourself, so you share 1/1 or 100% of your genetics with yourself.
You and a sibling share an ancestor 1 generation removed so you share 50% of your genetics, but you also share 50% of genetics with your parents
You share 25% of your genetics with your aunts and uncles (You share 50% of the genetics with your parents and they share 50% of their genetics with their siblings)
You share 12.5% of your genetics with your first cousin because they share 50% of their genetics with their parents who you are 25% genetically identical to, and as you pull back the generation of the shated common ancestor and become 2nd, 3rd, 4th etc cousins that number keeps halfling
This fact is how things like 23 and me can tell you how you’re related to someone with enough data on people surrounding you
The closer to 1 that number is, the more like any recessive genetic disorders contained in that shared genetic code become more likely to appear
Second the closer related to someone you are, the more genetically similar your kids will be. If a pathogen develops that exploits a genetic flaw, the more similar a given population is genetically the more capable the disease is of destroying the entire population of (this is actually a very big problem regarding food monocultures)
Now, those are the reasons that incest is bad from a genetics perspective, but here’s some information that helps mitigate that somewhat
DNA is a fixed length, and most humans are going to share a LOT of DNA because all of our bodies work fundamentally the same way. Similar to how even 2 copies of Windows with heavy modifications are going to share a lot of source code.
So, there is a point where you are no more genetically related to a particularly distant cousin than you are any random person
That’s the key to the puzzle right there, long term stable population groups in highly isolated locations started with enough people that they could avoid incest being an inevitability until after that threshold was met
Give it a long enough time and recessive traits will still be expressed more often because people will have those genes at higher than average rates, but genetic diversity will still be strong enough that a plague won’t instantly kill them faster than any random population for genetic reasons
Tldr: you need a minimum population of somewhere between 100-500 to prevent inbreeding in any animal population (exact figure unclear).
From a practical point of view, a lot of it is talking with family to check who’s related to who (“hi gran. Do you know X!” “Of course, they’re your 2nd cousin. Why?” “…… nevermind. Doesn’t matter. I need to make a phone call”), and now specific technology that has been developed to check these things.
Geeky science dna bit:
There is a concept of “minimal viable population (MVP). It’s the smallest population of any animal that allows inbreeding to be avoided and to reduce something called genetic drift. Genetic drift is to do with changes in the variation of different genes in a population so you can ignore it here, I just mention it because the “rule” I’m going to talk about has a second number that you might wonder what it is
The is a rule of 50/500 that says you need a minimum of 50 people to prevent inbreeding (the 500 is to do with genetic drift). At 50 people/animals you can avoid inbreeding potentially for many generations. However, ultimately there will be inbreeding if the population is that low.
This rule has been further explored and the evidence now points to needing a bigger population for most species. Figures like 100/1000 or even needing a minimum of 500-1000/5000-10,000 people to continue a population indefinitely without major issues.
There will be point however where there will be some overlap in families, but from a human view point, at which point do you decide the connections are ok? What if you found out you shared great-great-great-great-great grandparents with your partner and are there for 6th cousins (share about 0.1% dna as a result of that connection)? What about if it was 5th, 4th or 3rd cousins?
Actual practical stuff that humans use:
So the above is just going with the genetic and numbers, and working on the idea that new people don’t come into a population.
In practice, people move, new people will move to some of these places sometimes.
But the biggest factor with humans now is just us talking and being social animals. We likely know many of our cousins and just through word of mouth can find out if we have other connections. If you’re from a smaller community where this may be an issue, you might specifically ask family members if you are related to someone your interested in.
I have a friend who is from a Caribbean country that although it doesn’t have a small overall population, they were from a smallish recognisable genetic group within that island. They had met someone they liked and been on a few dates, and then the thought came to their head that maybe they are related (they actually met in the uk rather than back home). My friend asked their gran who let the know, as far as she knew they were related. My friend found out later, that his now wife has done this same thing, asking her gran!
You then get modern technology and some areas have developed apps that you can check.
You then also get areas where the population is so low that it’s not going to be viable long term. The Pitcairn Islands currently has less than 50 people. The 2020 census said there were only 7 people less than 20years old (only 2 of them less than 10years old). The government tried to attract new people to the island but with no real success. In the longterm, the population isn’t sustainable unless something changes
This is what I heard in history lessons many a year ago, but whenever visitors came to remote villages in Greenland (about the time of the Danish colonization/occupation, it was a requirement to sleep with someone local). The classic boats were used to visit neighbouring villages during summer when such travels were possible for related (or, I suppose, specifically unrelated) reasons.
Whether this holds true or the stories were some late part of “white man’s burden”-style racism, I would not know, as I do not remember the name of the school book, and I haven’t been able to verify this description searching now.
At least in the case of Pitcairn, they don’t. Historically there has been a lot of inbreeding. ([https://www.nytimes.com/1936/11/08/archives/pitcairn-refutes-inbreeding-harm-biologists-study-of-island-records.html](https://www.nytimes.com/1936/11/08/archives/pitcairn-refutes-inbreeding-harm-biologists-study-of-island-records.html) / [https://content.time.com/time/subscriber/article/0,33009,883297,00.html](https://content.time.com/time/subscriber/article/0,33009,883297,00.html) etc)
I read something once (fact or fiction can’t remember) that said this was a recognised problem by village elders and chiefs in the “pre modern” world. Therefore when travellers passed through their lands (often European explorers) they would actually offer them a woman in the hope that they would get pregnant and that would diversify / enrichen their gene pool. Maybe this is nonsense, they wouldn’t have understood genes I’m sure, but it makes sense.
An animal conservation there’s something called the rule of 5s, or the 50/500 rule. It’s a rule of thumb used to tell whether a population can support itself. The minimal viable population to avoid the harmful effects of inbreeding is only 50 individuals by this rule. 500 is the minimum to avoid intense amounts of genetic drift.
While this isn’t a conservation case (by God, by most metrics we have too many people) most human populations have much more than 50 people.
As for the Pitcairn islands in particular, their population isn’t even growing much. Iirc they had one child birthed in 2003. It was the first person born there in 17 years. Suffice it to say, they’re not gonna have enough time for inbreeding to take place.
Isolated communities normally had some level of inbreeding. There are plently of communities connected by easy transport links to nearby populations that are quite inbred. However there are a few things that can counteract it: exogamy; meaning that marriages within the island, community or tribe are taboo and people have to go outside to find an acceptable partner; this could necessitate quite long, risky journeys and there was often a semi-reciprocal relationship, i.e our men traditionally find wives in the island 130 miles to the West and their men come here to marry our daughters, which in general gave some companionship and protection to whichever partner got transplanted but did also decrease the effectiveness against inbreeding.
Another factor was trade, when explorers or anthropologists started studying Pacific islanders, for example, they found that an awful lot of effort and risk went into trading goods between islands, far more than the actual value and utility of the goods could justify, the explanation is that the cultural exchanges of stories, ideas, people and genetic material fostered by the trade were much more important than the trade itself.
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