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Lowest possible population without harmful inbreeding.

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Daifunka Since: Sep, 2014
#1: Sep 28th 2014 at 11:00:27 AM

Hello Ladies and Gentlemen of TV Tropes!

As I'm new, excuse me any plunders I make, because the layout of this site is unique for me.

Imagine an island. Island that has its own government, defenses and so on. They are self-managing. But just how small can population be before inbreeding begins? I'd like to know the lowest level before genetic deficits start kicking in.

Please give answers to

a) without any population influx

b) occasional new people in small groups enter the community from continent.

edited 28th Sep '14 11:00:46 AM by Daifunka

SeptimusHeap from Switzerland (Edited uphill both ways) Relationship Status: Mu
#2: Sep 28th 2014 at 12:58:10 PM

Greetings,

apparently it's in the ballpark of a few ten thousands of people.

"For a successful technology, reality must take precedence over public relations, for Nature cannot be fooled." - Richard Feynman
MattStriker Since: Jun, 2012
#3: Sep 28th 2014 at 1:44:43 PM

That's hard to say without a timeframe. Short-term? You can get away with around 1000 for quite some time before problems show up. Long-term? Probably in the 10000s if you want an effectively indefinitely sustainable population.

Reality is for those who lack imagination.
Slysheen Professional Recluse from My nerd cave Since: Sep, 2014 Relationship Status: Shipping fictional characters
Professional Recluse
#4: Sep 28th 2014 at 1:58:09 PM

Yup 10,000 plus, and if it were the lower end it would require breeding supervision, marriages arranged by the state would likely be the norm as well as mandatory birth control to prevent unknowns wrecking the equation.

Stoned hippie without the stoned. Or the hippie. My AO3 Page, grab a chair and relax.
DeusDenuo Since: Nov, 2010 Relationship Status: Gonna take a lot to drag me away from you
#5: Sep 28th 2014 at 6:11:07 PM

Be more specific about what you're trying to accomplish with this information. You may be annoyed to find out, after quite a bit of back-and-forth, that you were asking the wrong questions the entire time.

For example, is this population even human? You could explain (handwave) any small population as being 'genetically free of harmful mutations' through gene splicing or something.

I'd also say that it depends on the type of area this island is. A place as rough as Australia would very quickly thin out the population, for example, and would require a larger initial population.

(Building on what Slysheen said... Religious rules and tradition could take the place of government/state population control, if the civilization level is too low or too early.)

Daifunka Since: Sep, 2014
#6: Sep 29th 2014 at 11:20:38 AM

@ Septimus Heap, Matt Striker, Slysheen

Thank you very much for your opinions! They are very valuable for my work.

@ Deus Denuo

Thank you as well! Now you have some interesting points indeed...

The population is human. But in a way, I guess I have been bound by norms of real world despite having magic in this story. The first generations would be ordinary people (if you can count some of them being magic users as ordinary) But they have their own methods do control that births are as safe as possible. Certain magic to keep birth defects as low as possible as testing if parents don't have some bad genetic matches.

I didn't think of it, but such control and magical safe net, would it somehow cause next generations slowly develop more resilience of sorts towards genetic defects?

The island itself is not too big. A bit smaller than Ireland. It was picked out for good climate and then changed by the first teams to be safer for the refugees from continental wars (humans who escaped war between several human kingdoms and high elf empire - the frail balance that had lasted for centuries broke because some politicians thought they can push their will towards dangerous race)

They gathered those who could be trusted to not bring old political tensions from continent and then fortified the place hoping to survive long enough to find out a way to fight back or maybe elves would get bored and not come after this small group.

So the place itself was picked out and changed to be as safe as possible. But maybe I could handwave some of this with magic. Too close inbreeding is harmful, but if next generations grow more resilient/have safer ways to block harmful genes, then instead of waiting for 5th generations cousins, they could breed already with 2nd generation cousins.

You gave me something to think. I thank you!

DeusDenuo Since: Nov, 2010 Relationship Status: Gonna take a lot to drag me away from you
#7: Sep 30th 2014 at 12:39:07 PM

...No, I don't expect they'd develop a resilience to something they aren't being exposed to. (Your immune system doesn't get stronger because you're not being exposed to diseases, but I may have misunderstood your intent behind that sentence.)

Something like Iceland, then? Isolated, lots of geothermal and fishing, tolerable climate, pretty landscapes.

Otherwise, here. This looked useful.

It's good to know that they've had some time to prepare (and aren't having to build their new home from scratch) rather than be 'clothes on their back' war refugees. You've specified that they're united in their desire to not bring old strife and conflict with them, but this raises another question: are the newcomers all from the same country/background, or are they from different places.

Also, how did they get there? The means of transportation dictates how many people would end up there, and whether they need significant inbreeding control depends or not depends more on the initial number. 5 men and 5 women (reasonably young and fertile) would need considerable record-keeping to prevent inbreeding, 50/50 less so and 500/500 considerably less so - in addition to however many people won't be producing offspring.

Samaldin Since: Oct, 2014 Relationship Status: THIS CONCEPT OF 'WUV' CONFUSES AND INFURIATES US!
#8: Oct 3rd 2014 at 4:10:03 PM

i cant remember where i read it but the number is surprisingly low. In the short term (i think it was about 6 generations or so) 500 unrelated (so absolutly no brothers/sisters, cousins etc) people are enough. For a population to be stable with no time limet around 5000 unrelated people are needed.

MajorTom Eye'm the cutest! Since: Dec, 2009 Relationship Status: Barbecuing
Eye'm the cutest!
#9: Oct 3rd 2014 at 10:10:07 PM

^ Both of which are the lower limits for the worst/most likely outcomes of the Toba Catastrophe Theory.

In short, humanity has already encountered such a thing once before.

"Allah may guide their bullets, but Jesus helps those who aim down the sights."
Bloodsquirrel Since: May, 2011
#10: Oct 6th 2014 at 1:07:30 PM

I didn't think of it, but such control and magical safe net, would it somehow cause next generations slowly develop more resilience of sorts towards genetic defects?

The problem with an inbreeding population is that, over time, genetic drift is going to cause the population to increasingly become genetically homogenized. The birth defects caused by inbreeding are the result of genes that are harmfully but recessive, and as such are unlikely to appear in a large population. But with an inbreeding population you're going to reach the point where half the population has the gene, making the appearance of the defect extremely common.

What you need to avoid problems is a population large enough to generate genetic diversity as fast as it is lost through genetic drift. You've also got to count for natural disasters, fighting (external or internal), and other things temporarily reducing the population to lower levels, making the problem worse even after they breed back up.

TheBorderPrince Just passing by... from my secret base Since: Mar, 2010
Just passing by...
#11: Oct 8th 2014 at 7:32:45 AM

[up]

I agree with Bloodsquirrel. Any major disaster(s) that kill off a large number of people would be disasturous for your island. It doesn't matter if it is a plauge, natural disaster, war or magic disaster. Anything that kills a large number of people would be REALLY, REALLY bad... If the population goes below 5000 fertile couples is things not looking good for you...

It seems like the immigration to the island should be required to be quite high, altough that brings up the little issue of immigrants bringing in new diseases which the population lacks immunity towards. Think the about what happened to the Indians once Columbus showed up. Not good for an island with an as small population as this to be hit with an as fatal epidemic...

Of course, you can be lazy and handwave things with "it's magic", "they are half-elves" or something like that.

I reject your reality and substitute my own!!!
Wadnelis Since: Apr, 2014
#12: Oct 18th 2014 at 11:37:15 AM

So I read this thread and got a question - inbreeding causes recessive bad genes become dominant. I don't really understand the mechanics so it probably sounds really stupid, but if science or magic is developed enough, would it be possible to just... choose the better genes in human and only let those take action in baby making?

MattStriker Since: Jun, 2012
#13: Oct 18th 2014 at 12:09:26 PM

The mechanics are actually really simple (well, the basics are. The devil's in the details).

We have two copies of every chromosome, one from each parent.

A dominant trait will be expressed even if only one copy exists. A recessive trait has to exist in both copies of the gene in order to be expressed.

The problem in inbreeding is that the odds of negative recessive traits being supplied from both parents and thus showing up together go up significantly.

Of course, in reality it's not quite as binary (many so-called recessive traits do have a measurable effect even when they occur singly, for example), but it's a good guideline to work with.

As for using science or magic to intervene there...if it is possible to identify whether a sperm or egg cell carries an undesirable trait without damaging its viability, it is possible to perform in-vitro fertilization with two healthy copies of the troublesome gene.

However, since getting a closer look at the genetic material in a cell involves taking it apart, that's a bit tricky.

On top of that, you'd have to monitor countless genes all at once and sort through millions of sperm until you find one that doesn't have any of the traits you want to avoid.

Reality is for those who lack imagination.
Belisaurius Artisan of Auspicious Artifacts from Big Blue Nowhere Since: Feb, 2010 Relationship Status: Having tea with Cthulhu
Artisan of Auspicious Artifacts
#14: Oct 18th 2014 at 1:00:18 PM

It should be possible to screen individuals for damaged genes and pair them with partners with complementary sets. Granted, this edges towards eugenics but as long as you don't forbid people of having children it's still ethically sound.

If we assume that each couple has two children then it would take a minimum of ten generations for a damaged gene to spread to every one of the thousand breeding couples on the island. It's more likely that it would take 20 generations or more assuming that evolutionary pressures don't eliminate it entirely.

joeyjojo Happy New Year! from South Sydney: go the bunnies! Since: Jan, 2001
Happy New Year!
#15: Oct 18th 2014 at 2:29:40 PM

It should be possible to screen individuals for damaged genes and pair them with partners with complementary sets. Granted, this edges towards eugenics but as long as you don't forbid people of having children it's still ethically sound.

Well since society is has resorted inbreeding i think we're past ethics[lol] but yeah.

But yeah even with today tech, it's retentively easy to have 'designer babies', by genetically screening of embryos and eggs prior to implantation with IVF.

It should noted that despite what Liquid Snake thought recessive traits aren't necessarily more harmful. They're just more likely to be harmful because they are not heavily exposed to the effects of naturally section as dominant traits are. Recessive traits can often be quite desirable and only preserved by inbreeding. Hence why getting the gold Chocobos in final fantasy involves breaking the laws of man and godtongue

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DeusDenuo Since: Nov, 2010 Relationship Status: Gonna take a lot to drag me away from you
#16: Oct 21st 2014 at 10:21:26 AM

Or you could just pull a Gattaca and make Designer Baby screening and/or genetic manipulation the norm.

MajorTom Eye'm the cutest! Since: Dec, 2009 Relationship Status: Barbecuing
Eye'm the cutest!
#17: Oct 22nd 2014 at 8:13:16 PM

^ Well the thing is, to avoid inbreeding you'd need a sustainable population. Gattaca Babies doesn't really lean towards you having this naturally.

Plus there's one other problem with that. Even with genetic engineering genes mutate. That's how recessive genes emerge in the first place. All it takes is one mutation in one generation to introduce something to a population provided said individual with mutation procreates. In an inbreeding scenario one generation quickly becomes three quickly becomes seven quickly becomes ten and what was once one individual in the first generation can quickly become half of all population by the tenth.

"Allah may guide their bullets, but Jesus helps those who aim down the sights."
MattStriker Since: Jun, 2012
#18: Oct 23rd 2014 at 2:29:59 AM

Constant monitoring. Anything that deviates from the norm is either fixed with targeted gene-therapy or...purged.

You know, this is getting more dystopian with every new post :P.

Reality is for those who lack imagination.
DeusDenuo Since: Nov, 2010 Relationship Status: Gonna take a lot to drag me away from you
#19: Oct 23rd 2014 at 10:51:24 AM

[up][up] Two points. First, if the idea is to avoid inbreeding, yes, you'd need a sizable population to work from, but that isn't the same as avoiding harmful inbreeding. If you just want to avoid harmful genetic abnormalities (the result of the physical act of inbreeding) the trick would be to monitor the egg(s) and sperm being used in fertilization, and alter them as necessary.

Every. Time. (Matt Striker, I agree. After 5 generations or so there shouldn't be a need for the monitoring process, at least as a legal restriction, and I can't imagine people uniformly wanting to keep it. It's definitely the sort of thing pulp-ready teen dystopia novels are written around, after all.)

Second, you seem to be assuming that genetic mutation is a necessarily bad thing; it isn't. The thing with genetic mutation(s) in humans is, it actually happens all the time and they only occasionally result in something that is actually 'harmful'. You probably even have a really unique mutation in you right now, as you're reading this! But the odds of it being passed on down through several generations are quite low overall, as it's the interplay of genes from our two parents when we're building ourselves inside our mothers (or test-tubes, as the case may be) that really matters.

Basically, a random genetic mutation would have to beat the heavily-stacked odds against it through several generations in order to be any sort of societal problem at all. The idea behind Designer Babies as it should play out here is to catch and remove expressed genetic disorders from the gene pool (at least until the population is big enough to survive or ignore them on its own), and let regular genetic mutation continue on as normal. It would require extensive genetic mapping of the population, however, in order to pull it off.

Wadnelis Since: Apr, 2014
#20: Oct 24th 2014 at 11:10:18 AM

I hope this isn't off the topic, but now I got a question. Those who have read Naruto, know that Konoha is made up from several ninja clans. And their members all share similar traits - Aburames look like each other's clone, Naras are are obviously related to one another and Uchiha are all... well, Uchiha looking.

The clans mating only with other clan members and keeping their abilities (especially Uchiha Sharigan) inside while outbreeding could result in Sharingan disappearing is what I think people in this thread means as inbreeding that isn't negative. It causes certain traits to surface and exist.

With Naruto's example, I've got two questions to anyone ready to answer:

1) How big do you think the clans in Konoha are? 100 people? 500 people? 2) They do keep their bloodlines undiluted, but what if Uchiha never were wiped out? Or Hyuuga - wouldn't these small numbers eventually cause harmful inbreeding? Would ninja world be heading towards slow extinction, when Hyuuga's fertility rates start dropping with decades passing?

Just using Naruto as best example from wide known sources. No offense to anyone who hates it. [nja]

MattStriker Since: Jun, 2012
#21: Oct 24th 2014 at 3:32:58 PM

You bet they'd be in trouble. If the Narutoverse operated on something resembling proper laws of nature as opposed to pure Rule of Cool, those ninja clans would have a lot of problems accumulating over the generations.

Come to think of it, that might explain the Uchihas all going off the deep end in one way or another...

Reality is for those who lack imagination.
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