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Minimum size of founder population


Irbis
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Hello

 

I've been thinking about human population genetics, or more precisely, what's the minimum human population that can continue on growing without extinction?

 

There are several human groups descended from a very small founding groups.

 

1. The Amish - all Amish living to day are descended from a small group of about 200 Swiss and South German founders who migrated to the US in XVIII century. Despite having to cope with high infant mortality and several nasty genetic disorders, they managed to grow to 200,000 through breeding alone - the total number of converts to their religion is estimated at about 100 and has probably never been even as large as it's now. The same also applies to other Anabaptist populations (Hutterites and Menonnites)

2. The population of Tristan Da Cunha is descended from 8 men and 7 women who arrived at the island between 1816 and 1908. Their descendants number 264 individuals now and don't display any significant inbreeding problems with an exception of a highly elevated risk of asthma (3 people from the founding population had it).

3. High levels of inbreeding was shown by nobles through recorded history. It resulted in an array of bizarre genetc disorders but after generations of selection most seemed to be ok. Cleopatra had only 6 great-great grandparents (instead of 16) and despite this, she was healthy as well as a skilled politician.

Edited by Irbis
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When considering supposedly isolated and small founding populations of human beings, it's a good idea to rigorously verify and establish on physical and genetic grounds the actual frequency of outbreeding and within-group crossbreeding over the generations before drawing hard conclusions.

 

The number of great great grandparents Cleopatra had is at least six, would be a safer presumption - for example. This: "all Amish living to day are descended from a small group of about 200 Swiss and South German founders who migrated to the US in XVIII century." is not safe unless it has been carefully checked via genetic survey and rigorous physical analysis.

 

In the end, though, it's going to depend a lot on luck - the exact composition of the group, and the circumstances of their first few hundred generations. What usually kills off genetically uniform groups is not their own failure to reproduce, but the arrival of a new disease or other novel common vulnerability before they have grown (slowly, if inbred) to a large enough size and geographical spread to take the hit. The endemic Reds of the Caribbean Islands were doing fine until the arrival of European disease.

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Amish have historically been a very closed population. They did not accept converts and were discouraged from intermarrying. Also their pattern of displaying significantly higher rates for certain genetic disorders (dwarfism and maple syrup urine disease) is consistent with being descended from a very small founding population.

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Amish have historically been a very closed population.
You might want to verify that physically, the exact degree. From a founding population of 200 over ten generations, as mentioned, it would not take much conversion, adoption, or other outbreeding/crossbreeding events to swing the numbers - and humans seem to be remarkably good at selecting and arranging robust genetic infusion, far beyond what chance alone would supply.

 

Not that the "historical" picture isn't possible, just that if you are drawing hard conclusions from it you want to nail it down.

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@ OP,

 

The idea the Amish evolved all from 200 people is not nearly true. The culture has had trouble with genetic inbreeding and is well known to pay non Amish men to stud their women (allegedly while others are present, and with a sheet over the girl).

 

However to add to the idea. What is wrong with genetic defects if you are repopulating? The weak will die and the strong will survive and radiation and natural DNA changes that occur in evolution. Eventually the families will move apart to their own settlements and branching off will lead to their own unique DNA based on their regions.

 

I would think one healthy young couple should be able to populate a planet parent/child or sibling/sibling couples look will see genetic mutations in about 25% of their offspring (you should never share 6.25% or more same genetic material if breeding).

 

So.. My answer is 2 people with good lifespans and circumstances.

 

Genetic mutations might be cruel however so I think the OP should revise the question to be how many would it take without suffering mutations.

 

I'm sure if you used the 6.25% rule you could do the math (any Math gurus here want to take a stab at it).

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@ OP,

 

The idea the Amish evolved all from 200 people is not nearly true. The culture has had trouble with genetic inbreeding and is well known to pay non Amish men to stud their women (allegedly while others are present, and with a sheet over the girl).

citations needed

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@ ydops,

 

It would make sense, but could be an urban legend. A father of a disfigured mutation would have a lot of motivation to not want a repeat occurrence.

 

IF there is proof via DNA that itself would not prove it was not an affair or rape, so I won't bother looking, but I've heard geneticist love the Amish and are mapping their DNA.

 

Until science maps their unique(ish) DNA we won't know for sure and even then the children could be a result of rape.

 

Even rape and affairs would have added DNA to the mix and statistically it must have occurred to them with some regularity over the centuries.

 

There are countless tales of it though, such as,

http://www.chacha.com/question/do-amish-families-find-people-to-impregnate-their-daughters-to-stop-inbreeding

https://answers.yahoo.com/question/index;_ylt=AwrBTvlSBshTOV0AWYbrFAx.;_ylu=X3oDMTE0bjVpZnFoBHNlYwNzcgRwb3MDMQRjb2xvA2JmMQR2dGlkA01TWUNBMDJfMQ--?qid=20070116112830AAbJLRf

 

So I will withdraw the statement of it as fact, but prevention of DNA from outsiders seems like a statistical impossibility based on instances of rape or affairs over the centuries.

Edited by barfbag
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@ OP,

 

The idea the Amish evolved all from 200 people is not nearly true. The culture has had trouble with genetic inbreeding and is well known to pay non Amish men to stud their women (allegedly while others are present, and with a sheet over the girl).

 

However to add to the idea. What is wrong with genetic defects if you are repopulating? The weak will die and the strong will survive and radiation and natural DNA changes that occur in evolution. Eventually the families will move apart to their own settlements and branching off will lead to their own unique DNA based on their regions.

 

I would think one healthy young couple should be able to populate a planet parent/child or sibling/sibling couples look will see genetic mutations in about 25% of their offspring (you should never share 6.25% or more same genetic material if breeding).

 

So.. My answer is 2 people with good lifespans and circumstances.

 

Genetic mutations might be cruel however so I think the OP should revise the question to be how many would it take without suffering mutations.

 

I'm sure if you used the 6.25% rule you could do the math (any Math gurus here want to take a stab at it).

 

In small populations, the ability of Natural Selection to eliminate deleterious alleles is greatly weakened except in for those that are strongly deleterious. Genetic Drift will predominate for slightly and even intermediately deleterious alleles and can easily drive such alleles to fixation despite being harmful. The accumulation of such deleterious alleles within the population lowers the overall fitness of the population and without some counterbalancing inflow of beneficial alleles, leads to what is called a "mutational meltdown" and eventual extinction of the population. Its actually quite common amongst endangered species today and a major concern in conservation biology. It is really not feasible to start off with a population of only 2 individuals. For one, even though the chance of a male/female is supposed to be 50/50, at such a small population, it is very common to have skewed sex ratios that would also be a significant limitation.

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@

chadn737,

 

Good points. My main point was wondering if the OP was willing to have genetic defects in his populace, as the question could be with mutated population or without. Obviously if we wanted to colonize a planet we would likely not want abbe rations.

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My main point was wondering if the OP was willing to have genetic defects in his populace, as the question could be with mutated population or without.
It is all but impossible to find even a single couple entirely free of genetic defects - and the natural or background rate of mutation would introduce some within a few generations anyway.
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@

chadn737,

 

Good points. My main point was wondering if the OP was willing to have genetic defects in his populace, as the question could be with mutated population or without. Obviously if we wanted to colonize a planet we would likely not want abbe rations.

 

No such thing as a population without mutations. We all carry mutations.

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@

chadn737,

 

Good points. My main point was wondering if the OP was willing to have genetic defects in his populace, as the question could be with mutated population or without. Obviously if we wanted to colonize a planet we would likely not want abbe rations.

Let's say we don't mind them, even if it means that a large part (even as much as 10-15%) of the population is born with crippling genetic defects.

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In conservation biology, there is a concept called "effective population size". This is the minimum number of individuals that will represent the genetic variability within a population. Fifty is a minimum commonly used by animal breeders. This gives about a 2-3% loss of heterozygosity per generation. Some have suggested that 500 indivduals is enough so that the the new mutation rate balances the loss of heterozygosity due to small population sizes (inbreeding depression).
Source:
Franklin (1980): 50-500 reproductive individuals
Lande (1995): 5000 reproductive individuals
Essentials of Conservation Biology, Richard B. Primack

 

from
Since you have clarified the allowance of mutations a similar topic would be repopulating endangered species, and there is much reading material on the subject.
Some think a species can be reintroduced to society if we just clone a few using DNA. Will science be able to manipulate DNA to the point of preventing defects?
I still say under ideal circumstances (no disease or injuries), and with lots of babies one couple could do it, but the points made by ChadN737 in post 9 are great...
I will repost it here
In small populations, the ability of Natural Selection to eliminate deleterious alleles is greatly weakened except in for those that are strongly deleterious. Genetic Drift will predominate for slightly and even intermediately deleterious alleles and can easily drive such alleles to fixation despite being harmful. The accumulation of such deleterious alleles within the population lowers the overall fitness of the population and without some counterbalancing inflow of beneficial alleles, leads to what is called a "mutational meltdown" and eventual extinction of the population. Its actually quite common amongst endangered species today and a major concern in conservation biology. It is really not feasible to start off with a population of only 2 individuals. For one, even though the chance of a male/female is supposed to be 50/50, at such a small population, it is very common to have skewed sex ratios that would also be a significant limitation.

 

 

(added bolds for emphasis)

 

So for two people to work EXTREME LUCK would be required not only in DNA, but if they had all boys (Cain/Abel) then there would be no girls to be mothers.

 

I'd research "conservation biology" for more on this topic because repopulating a species from small numbers is similar to your question in the OP.

Edited by barfbag
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  • 2 weeks later...

Can someone (question directed mostly to forum geneticists - CharonY, Arete and chadn737) give me an example of an animal population that recovered it's genetic diversity after a severe bottleneck? Any answers will be much appreciated.

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