Natural selection. Naturally, the other natives will not select the genetically defective for breeding. This makes it difficult to grow the population.
Inbreeding has a large chance of genetic defects, not 100%. A smaller population makes it easier to find food, shelter, and potable water. In a sense, this is nature taking care of the ecosystem.
Inbreeding is bad but it doesn’t take that much “distance” to mostly eliminate the threat, second cousins doing it is already rather safe. With population hard to estimate but some putting it at 400 people, you can see how it’s enough.
It’s estimated a population as small as about 50 is enough to avoid the worst effects of inbreeding with some planning. There’s way more of them than that.
inbreeding does not Guarantee genetic problems. No. Inbreeding only brings about genetic problems when those genes already exist hidden in a population. It makes it more likely those genes will reveal themselves. But if there aren’t any issues in the population, then inbreeding is not that big a deal.
Also, while the population is small, it is not that dire. We estimate there are probably a couple hundred people on the island. Yes everyone would be related in some way, but thats no different than smaller tribes of the past. Assuming they have similar traditions to other places around the world, siblings wouldn’t inbreed, only cousins. and you only share about 12.5% of your dna with first cousins. potentially less. And with a couple hundred people its unlikely everyone is having kids with their first cousin.
Also, consider this, even if some bad genes are present maybe those kids just, die. If you have 4 kids and 1 is born with some genetic disorder, you still have 3 kids for the next generation.
> The exact population of North Sentinel Island is unknown, but estimates range from as few as 35 to as many as 500 individuals. Most estimates fall within the range of 50 to 200 people.
> > The “50/500 rule” in conservation biology suggests that a minimum effective population size (Ne) of 50 is needed to avoid inbreeding depression, and 500 is needed to maintain long-term evolutionary potential. While a population size of 5000 is not part of the 50/500 rule, it is a larger threshold often used in conservation to ensure long-term viability and resilience, particularly for species facing high extinction risks or environmental changes.
So the population of North Sentinel Island is probably large enough to avoid inbreeding, but not genetic drift.
In addition to the other comments about how even small populations can be pretty safe from the negative effects of inbreeding:
A very common effect of inbreeding is an increase in deleterious recessive genes becoming more common. Like sickle cell anemia and hemophilia, if there are none of those recessive genes in a population the effects of inbreeding can be reduced.
They have only been isolated for a few hundred years, not that long in genetic terms. Technically they have been contacted and used to trade with other islands until Europeans stole some of their kids and they decided to be done with the wider world
When the populations of the tribes of treblor declined through constant war and their isolation made them susceptible to inbreeding, Icarium, the maker of time, who had once lived among the ancestors of the Teblor of the Laederon plateau gave them the Laws of Isolation that were used to create their clans.
I would assume similar laws are born of necessity among such tribes through trial and error.
It’s not as if North Sentinel has been fully isolated for many centuries. It is 15 miles from other islands, which is quite manageable with a simple raft (if you’re careful to wait for good weather). Linguistically and culturally, we can tell they had various contact over the ages.
The term “uncontacted” with a lot of these tribes usually means uncontacted by westerners. To take the North Sentinel Island tribe for example they occasionally trade w/ other tribes in the area, and I believe that’s typically how we ensure they don’t need any aid.
> Wouldn’t inbreeding and tiny gene pool & genetic diversity have wiped them out long ago?
Inbreeding doesn’t cause genetic errors, it just makes it more likely for two copies of the same a recessive gene to appear in the same person. Those “bad genes” already exist inside you.
As an example say “A” is the good copy of a gene and “B” is the bad copy. If you have two copies of “B” then you die. Say you start with everyone having one copy of B, so the percentage of B genes is 50%. What then happens when two people have kids?
If two “AB” people have four kids then on average the kids genes would be AA, AB, BA, BB. Note the percentage of A and B genes is still 50%: so if B didn’t do anything, then the average just stays the same each generation.
However, the kid with “BB” dies, leaving AA, AB, AB. So now you count up the B genes, and you get that they make up 1/3rd of the genes, not half. So because the recessive B gene got expressed, that kid died, but now there are less B genes left in the gene pool.
So you’re right about inbreeding reducing genetic diversity, however keep in mind that the bad recessive genes are part of that “diversity”: in the above example if you keep inbreeding a small group eventually the B gene just dies out, because every time someone gets BB by the roll of the dice they get eliminated and don’t have offspring. Eventually everyone would only have the A variant of the gene.
So the “loss of diversity” and the “birth defects from recessive genes” aren’t really stacking up together, in fact one sort of counteracts the other.
That’s why farmers can inbreed livestock without anything weird happening: those farm animals are already inbred so most recessive genes that could cause those types of birth defects have already been weeded out.
It’s grossly exaggerated in popular culture. It takes very close relation (siblings, parent/child, Uncle/neice) over multiple generations to get those kind of negative outcomes.
For first cousins the rate of problems is higher than the strangers, so we mostly avoid it, but in the context of pre-modern child mortality the difference is negligible.
We say you get 50% of your DNA from each parent, which is true, but you aren’t actually getting 25% from each of your grandparents. When you prepare an egg or sperm your two sets of parental DNA are recombined with each other in large fragmented sections. The egg you came from might be 66% Grandma 33% Grandpa. Your sibling’s egg might be the reverse, and they average together to 50%. The fractions you have in common with siblings may be less than you think.
I forgot the name of the YouTube channel, but the degree of separation on each generation is not a flat 2x factor like we TLDR it in highschool. Past 6 or 7 generations you have no genetic relationship to most of your ancestors, and a comparative handful ancestors that contributed a higher than average fraction.
By the time your son/daughter marries their cousin and a child is born, their child has a < 12.5% genetic similarity with their nearest common ancestor, their great grandparents. From a genetic standpoint that’s not a major concern. It takes the entire family tree marrying cousins for generations, and even then you just end up with the European Royalty, who all jokes aside are pretty normal. We tell the story of hemophilia, but in all seriousness that’s a particularly common heritable disease because it’s X linked, only one parent has to have a copy to produce a hemophilic son. That’s going to stick around and present in the family even with plenty of fresh blood, because literally all of the Male Hemophiliac’s daughters will carry the gene.
Comments
Natural selection. Naturally, the other natives will not select the genetically defective for breeding. This makes it difficult to grow the population.
Inbreeding has a large chance of genetic defects, not 100%. A smaller population makes it easier to find food, shelter, and potable water. In a sense, this is nature taking care of the ecosystem.
Inbreeding is bad but it doesn’t take that much “distance” to mostly eliminate the threat, second cousins doing it is already rather safe. With population hard to estimate but some putting it at 400 people, you can see how it’s enough.
It’s estimated a population as small as about 50 is enough to avoid the worst effects of inbreeding with some planning. There’s way more of them than that.
inbreeding does not Guarantee genetic problems. No. Inbreeding only brings about genetic problems when those genes already exist hidden in a population. It makes it more likely those genes will reveal themselves. But if there aren’t any issues in the population, then inbreeding is not that big a deal.
Also, while the population is small, it is not that dire. We estimate there are probably a couple hundred people on the island. Yes everyone would be related in some way, but thats no different than smaller tribes of the past. Assuming they have similar traditions to other places around the world, siblings wouldn’t inbreed, only cousins. and you only share about 12.5% of your dna with first cousins. potentially less. And with a couple hundred people its unlikely everyone is having kids with their first cousin.
Also, consider this, even if some bad genes are present maybe those kids just, die. If you have 4 kids and 1 is born with some genetic disorder, you still have 3 kids for the next generation.
> The exact population of North Sentinel Island is unknown, but estimates range from as few as 35 to as many as 500 individuals. Most estimates fall within the range of 50 to 200 people.
> > The “50/500 rule” in conservation biology suggests that a minimum effective population size (Ne) of 50 is needed to avoid inbreeding depression, and 500 is needed to maintain long-term evolutionary potential. While a population size of 5000 is not part of the 50/500 rule, it is a larger threshold often used in conservation to ensure long-term viability and resilience, particularly for species facing high extinction risks or environmental changes.
So the population of North Sentinel Island is probably large enough to avoid inbreeding, but not genetic drift.
In addition to the other comments about how even small populations can be pretty safe from the negative effects of inbreeding:
A very common effect of inbreeding is an increase in deleterious recessive genes becoming more common. Like sickle cell anemia and hemophilia, if there are none of those recessive genes in a population the effects of inbreeding can be reduced.
They have only been isolated for a few hundred years, not that long in genetic terms. Technically they have been contacted and used to trade with other islands until Europeans stole some of their kids and they decided to be done with the wider world
When the populations of the tribes of treblor declined through constant war and their isolation made them susceptible to inbreeding, Icarium, the maker of time, who had once lived among the ancestors of the Teblor of the Laederon plateau gave them the Laws of Isolation that were used to create their clans.
I would assume similar laws are born of necessity among such tribes through trial and error.
Well first of all – most of them didn’t survive all these years.
It’s not as if North Sentinel has been fully isolated for many centuries. It is 15 miles from other islands, which is quite manageable with a simple raft (if you’re careful to wait for good weather). Linguistically and culturally, we can tell they had various contact over the ages.
Inbreeding is only a problem if there are negative recessive traits in the population
The term “uncontacted” with a lot of these tribes usually means uncontacted by westerners. To take the North Sentinel Island tribe for example they occasionally trade w/ other tribes in the area, and I believe that’s typically how we ensure they don’t need any aid.
> Wouldn’t inbreeding and tiny gene pool & genetic diversity have wiped them out long ago?
Inbreeding doesn’t cause genetic errors, it just makes it more likely for two copies of the same a recessive gene to appear in the same person. Those “bad genes” already exist inside you.
As an example say “A” is the good copy of a gene and “B” is the bad copy. If you have two copies of “B” then you die. Say you start with everyone having one copy of B, so the percentage of B genes is 50%. What then happens when two people have kids?
If two “AB” people have four kids then on average the kids genes would be AA, AB, BA, BB. Note the percentage of A and B genes is still 50%: so if B didn’t do anything, then the average just stays the same each generation.
However, the kid with “BB” dies, leaving AA, AB, AB. So now you count up the B genes, and you get that they make up 1/3rd of the genes, not half. So because the recessive B gene got expressed, that kid died, but now there are less B genes left in the gene pool.
So you’re right about inbreeding reducing genetic diversity, however keep in mind that the bad recessive genes are part of that “diversity”: in the above example if you keep inbreeding a small group eventually the B gene just dies out, because every time someone gets BB by the roll of the dice they get eliminated and don’t have offspring. Eventually everyone would only have the A variant of the gene.
So the “loss of diversity” and the “birth defects from recessive genes” aren’t really stacking up together, in fact one sort of counteracts the other.
That’s why farmers can inbreed livestock without anything weird happening: those farm animals are already inbred so most recessive genes that could cause those types of birth defects have already been weeded out.
It’s grossly exaggerated in popular culture. It takes very close relation (siblings, parent/child, Uncle/neice) over multiple generations to get those kind of negative outcomes.
For first cousins the rate of problems is higher than the strangers, so we mostly avoid it, but in the context of pre-modern child mortality the difference is negligible.
We say you get 50% of your DNA from each parent, which is true, but you aren’t actually getting 25% from each of your grandparents. When you prepare an egg or sperm your two sets of parental DNA are recombined with each other in large fragmented sections. The egg you came from might be 66% Grandma 33% Grandpa. Your sibling’s egg might be the reverse, and they average together to 50%. The fractions you have in common with siblings may be less than you think.
I forgot the name of the YouTube channel, but the degree of separation on each generation is not a flat 2x factor like we TLDR it in highschool. Past 6 or 7 generations you have no genetic relationship to most of your ancestors, and a comparative handful ancestors that contributed a higher than average fraction.
By the time your son/daughter marries their cousin and a child is born, their child has a < 12.5% genetic similarity with their nearest common ancestor, their great grandparents. From a genetic standpoint that’s not a major concern. It takes the entire family tree marrying cousins for generations, and even then you just end up with the European Royalty, who all jokes aside are pretty normal. We tell the story of hemophilia, but in all seriousness that’s a particularly common heritable disease because it’s X linked, only one parent has to have a copy to produce a hemophilic son. That’s going to stick around and present in the family even with plenty of fresh blood, because literally all of the Male Hemophiliac’s daughters will carry the gene.