Smallest human population that can succesfully breed on?

[ArgonV]

I was having a discussion with a friend just now about surviving after an apocalypse. We were talking about how many people you would need to successfully carry on the human population. It's obvious that you'd need more women than men, but what's an optimal ratio? And how big should that group be for enough genetic variation? My friend claims 100 people should be enough, but that seems rather low.

Anyone here have any ideas?

[frezik]

I've heard various quotes of 33 up to 50,000. Some suggested that the surviving humans of Battlestar Galactica would not be enough to repopulate on their own.

I've yet to hear an answer from an authoritative source, though. Certainly, there have been geographically isolated tribes of humans surviving for centuries with less than a thousand people, hasn't there? I'm thinking of tribes on Pacific islands or perhaps aboriginal Australian tribes.

In my past research, I found that when a species' population gets low, mutation rates tend to go up. So it's hard to put an absolute lower bound on it.

Plus, incestuous problems may not actually show up for several generations. Queen Cleopatra's family tree was pretty narrow, but she still managed to seduce two of the most powerful men in the world at the time.

[Omegaton]

Wikipedia gives some estimates for species which I'm assuming are non-human (they don't specify) here: http://en.wikipedia.org/wiki/Minimum_viable_population

In my past research, I found that when a species' population gets low, mutation rates tend to go up. So it's hard to put an absolute lower bound on it.

You sure about that? You can get a higher chance of seeing recessive mutations due to inbreeding, but not because the mutation rate is actually higher.

[Interactive Civilian]

Wikipedia gives some estimates for species which I'm assuming are non-human (they don't specify) here: http://en.wikipedia.org/wiki/Minimum_viable_population

In my past research, I found that when a species' population gets low, mutation rates tend to go up. So it's hard to put an absolute lower bound on it.

You sure about that? You can get a higher chance of seeing recessive mutations due to inbreeding, but not because the mutation rate is actually higher.

It may not be that mutation rates are going up. It's that mutations that do appear have a better chance of getting fixed in the population when it is small. Unless a mutation has some sudden huge reproductive advantage, most will be lost in the noise of purifying selection on the "normal" gene in a large population. Or, put another way, a trait which was barely impacting a large population may suddenly become dominant in a small population after or during a bottleneck event.

This is probably part of why the Founder Effect and such happens, and probably why historically we see bursts of speciation after large extinction events. Weak purifying selection on smaller populations allows increased variation, allowing for more possibilities to fill the available niches. At least, this is my understanding, based on this paper and some of its various sources.

As for the question of smallest population which can successfully breed on? Well, it will depend on a lot, such as the amount of variation in the starting population, whether reproduction is "random" or highly controlled to maximize genetic variability in subsequent generations, etc. It seems to me that the more variation and the more control over who reproduces with who, the smaller your starting population can be and still survive without running into (too many) problems from undesirable recessive traits being expressed. As for an actual number, I have no idea, but I've often wondered about this and if the survivors at the end of Heinlein's "Orphans of the Sky" could possibly stand a chance, despite their extremely small number.

[psychosomaticism]

I think two variables (out of many, I'm sure) to consider are the problems with having offspring too close to one's immediate family, as well as the diversity of the population to withstand pressures like predation, disease, etc. We would definitely want to survive a genetic bottleneck, and as a K-selected species it's pretty important to have a large enough population. That being said, I don't really know the math behind it all. I figure there's something to be said about the Lotka-Volterra model and all that, but I was never that interested in population ecology.

And as for BSG, I always thought that that was a pretty small size for repopulation too, but was never able to back it up.

[nbonaparte]

Well, it's certainly not above 15,000. The Toba eruption brought the population down to 15,000 and we're still here. Now, we do have our fair share of genetic issues.

[poxic]

I'm trying to find an article I once read in SciAm about the population of Asia (I think) being traceable to 37 unique female ancestors. They took a lot of blood samples and examined the mitochondria, which are matrilineal.

Since I can't find the article, the number might have been different. In other words, vague recollections of science are not themselves scientific.

Edited to ruminate: this doesn't mean that a population with only 37 reproducing females will survive. Other lines of mitochondria might have died out when women had only male offspring, or when all their offspring died to save a few survivors (and other less-dramatic situations).

[kbltd]

Isn't it well established that all humans have a single female ancestor and a single male ancestor?

http://www.oxfordancestors.com/content/view/35/55/

... everyone alive on the planet today can trace their maternal ancestry back to just one woman. By all accounts, she lived in Africa about 150,000 – 200,000 years ago and is known as “Mitochondrial Eve” ...

http://www.oxfordancestors.com/content/view/36/56/

.... male ancestral lines ... converge on the common paternal ancestor of every man alive today. This man, know as “Y-Chromosome Adam”, lived in Africa 60,000 – 80,000 years ago...

So from the point of view of genetic diversity, very few individuals are needed. But it would certainly help them to have a larger community in place around them to share agriculture, child rearing and the like.

[Sharlos]

Yes but those two people had plenty of people to have children and share genetic information with.

[mmmcannibalism]

Assuming everyone started out unrelated; I just get the hunch it would be a few hundred at most.

Of course, my state of West Virginia proved that a pair of brother and sister would be enough

[sikyon]

It will depend on your tech level as well.

For example, a high tech group of people are much less vulnerable to natural disasters and can survive with a lower population.

In fact, if they had the capability for genetic selection/screening, and selective breeding (women become constant baby factories), I could see the viable population become very small indeed. It will also depend on the quality of the people you start with, ie. number of dangerous genetic defects. I am going to go out on a limb here and say that if you took the 6 people with the best genetic makeup in the world, 2 males 4 females, and gave them the best of the best technology available today they could be very much viable.

[lulzfish]

It will depend on your tech level as well.

For example, a high tech group of people are much less vulnerable to natural disasters and can survive with a lower population.

In fact, if they had the capability for genetic selection/screening, and selective breeding (women become constant baby factories), I could see the viable population become very small indeed. It will also depend on the quality of the people you start with, ie. number of dangerous genetic defects. I am going to go out on a limb here and say that if you took the 6 people with the best genetic makeup in the world, 2 males 4 females, and gave them the best of the best technology available today they could be very much viable.

Well, assuming that that technology can be manned staffed by 6 people who are also doing all the other work of surviving.

[sikyon]

It will depend on your tech level as well.

For example, a high tech group of people are much less vulnerable to natural disasters and can survive with a lower population.

In fact, if they had the capability for genetic selection/screening, and selective breeding (women become constant baby factories), I could see the viable population become very small indeed. It will also depend on the quality of the people you start with, ie. number of dangerous genetic defects. I am going to go out on a limb here and say that if you took the 6 people with the best genetic makeup in the world, 2 males 4 females, and gave them the best of the best technology available today they could be very much viable.

Well, assuming that that technology can be manned staffed by 6 people who are also doing all the other work of surviving.

Well yeah, but automation can get pretty cool.

[Omegaton]

Isn't it well established that all humans have a single female ancestor and a single male ancestor?

http://www.oxfordancestors.com/content/view/35/55/

... everyone alive on the planet today can trace their maternal ancestry back to just one woman. By all accounts, she lived in Africa about 150,000 – 200,000 years ago and is known as “Mitochondrial Eve” ...

http://www.oxfordancestors.com/content/view/36/56/

.... male ancestral lines ... converge on the common paternal ancestor of every man alive today. This man, know as “Y-Chromosome Adam”, lived in Africa 60,000 – 80,000 years ago...

So from the point of view of genetic diversity, very few individuals are needed. But it would certainly help them to have a larger community in place around them to share agriculture, child rearing and the like.

While it is true that we can trace ancestry back to a single female and a single male, this does not mean that there was ever a point in time where there were only 2 humans and that these single ancestors were the only members of the population. When both Y-chromosome Adam and mitochondrial Eve were around there were thousands of other people. Eventually, everyone else's mitochondria and Y-chromosomes eventually get lost due to either selection or random drift, but there were other Y-chromosomes and mitochondria around at the same time in the same species as both Adam and Eve! Mitochondria and Y-chromosomes are nice tools to use because they correspond to males and females, but if you take any of the other 22 chromosomes you could do the same thing, and find different most recent common ancestors. Therefore, it's a little misleading to name them Adam and Eve, as the preconception might be that they would be the only individuals at that time if everyone is descended from them, but that's not the case at all.

[eschaton37]

[url="http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1131883/"]It has been suggested[/url] that all Native Americans descend from 70-100 people, so that size seems to be considered viable.

That passes the 'sanity check', since generally second-cousins and more distant relations marrying don't have serious genetic consequences.

Realistically, less than that can probably *survive*, but genetic problems might turn up. And a specially selected group could probably be -very- small.

Remember, human genetic diversity is low *anyway* due to previous bottlenecks.

Some species have survived really bad bottlenecks - the Chatham Island Black Robin was down to 4 males & 1 female at one point! (There are now 250+ individuals.) There seem to be no problems from this, though it was only 20-some years ago and they might appear later; it's been speculated, though, that as a small-island species, it'd been through bottlenecks before and was able to survive them.

[Zamfir]

I am going to go out on a limb here and say that if you took the 6 people with the best genetic makeup in the world, 2 males 4 females, and gave them the best of the best technology available today they could be very much viable.

Dr. Strangelove: Well, that would not be necessary, Mr. President. It could easily be accomplished with a computer. And a computer could be set and programmed to accept factors from youth, health, sexual fertility, intelligence, and a cross-section of necessary skills. Of course, it would be absolutely vital that our top government and military men be included to foster and impart the required principles of leadership and tradition. Naturally, they would breed prodigiously, eh? There would be much time, and little to do. Ha, ha. But ah, with the proper breeding techniques and a ratio of say, ten females to each male, I would guess that they could then work their way back to the present Gross National Product within say, twenty years.

[...]

General "Buck" Turgidson: Doctor, you mentioned the ratio of ten women to each man. Now, wouldn't that necessitate the abandonment of the so-called monogamous sexual relationship, I mean, as far as men were concerned?

Dr. Strangelove: Regrettably, yes. But it is, you know, a sacrifice required for the future of the human race. I hasten to add that since each man will be required to do prodigious... service along these lines, the women will have to be selected for their sexual characteristics which will have to be of a highly stimulating nature.

Ambassador de Sadesky: I must confess, you have an astonishingly good idea there, Doctor.

[qetzal]

Mitochondria and Y-chromosomes are nice tools to use because they correspond to males and females, but if you take any of the other 22 chromosomes you could do the same thing, and find different most recent common ancestors.

In fact, we can go further. Autosomes recombine during meiosis, so even individual genes (or portions of genes) could theoretically trace to different ancestors. So yeah, we may all be descendants of mito-Eve & Y-Adam, but we are also descendants of lots of other humans that lived at the same time as them. Just like you and your first cousins by your mom's sister are all descendents of your mom's parents, but you are also a descendent of your father's parents, where your cousins are not.

[You, sir, name?]

Assuming we perfect human cloning, one.

Spoiler:

*badum-ching*

[Eebster the Great]

I think one male and one female gives a reasonable chance of success as a species for humans (unlike for most animals). Most humans survive long past breeding age and have the ability to reproduce several times. Inbreeding depression and mutations are serious risks, but we would only need a portion of each generation to survive and be fertile. If they could be selected (as is likely after, e.g., an apocalypse), I put their chances around 50%.

To get a 95% chance of success, the population would obviously have to be far larger, but it would also have to be far more spread out, as otherwise the entire population could be wiped out by one event. A post-apocalyptic world may be a very dangerous one, so you might need hundreds of individuals to ensure success. And yes, adding more females is much more beneficial than adding more males to the population for these purposes.

[Username4242]

Didn't the guy who traced the single mother/father in time for our species also suggest that at one point in time we were thinned down to only 2,000 people globally?

If so, I'm gonna have to say at least that.

[PhoenixEnigma]

The California Condor was down to 22 individuals, and the breeding program for them has been, by all accounts, successful. While clearly there are differences between a condor and a human, California Condors have reasonably late sexual maturity, long lifespan, and low reproduction rate (approx 1 offspring per year, best-case), so some parallels can be drawn.

Assuming the sole goal was the propogation of the human species (ie, don't worry about food production, selective nreeding, etc), I suspect we wouldn't need more then 100 people at the most. Probably even fewer if we were able to prescreen people to avoid problematic genetic traits.

If we aren't free from those contraints, it becomes much more an issue of how many people are needed to form a functioning society capable of producing it's own food, power, and other supplies. It would depend a lot on how much technology you want to maintain - a semiconductor foundry is going to need a lot of resources and people to keep running, for example.

[Meteorswarm]

I don't see why you couldn't make due with two, assuming that the population is minimally stressed. Sure, you'll get inbreeding depression, and sure, they'll be less fit than they'd otherwise be, but barring some sort of horrible unfortunate disease like hemophilia or Tay-Sachs, it's unlikely that they'd be so horrifically unfit as to not be able to reproduce. We inbreed plants and other animals all the time - look at lab rats - and they seem to live fine.

Now, add in disease and environmental stresses and you get a different story. A population with horrifically low genetic diversity would be quite vulnerable to disease.

[Eebster the Great]

I don't see why you couldn't make due with two, assuming that the population is minimally stressed. Sure, you'll get inbreeding depression, and sure, they'll be less fit than they'd otherwise be, but barring some sort of horrible unfortunate disease like hemophilia or Tay-Sachs, it's unlikely that they'd be so horrifically unfit as to not be able to reproduce. We inbreed plants and other animals all the time - look at lab rats - and they seem to live fine.

Now, add in disease and environmental stresses and you get a different story. A population with horrifically low genetic diversity would be quite vulnerable to disease

I think the vulnerability to disease is just one of many things that could go wrong. Before we even begin to worry about genetic diversity, any population that small is going to have challenges surviving in terms of getting enough food, building shelters, not becoming incredibly depressed, etc. My suspicion is that most pairs of people would learn to overcome these and repopulate successfully, but I am not certain about this, and I certainly doubt the success rate would be too far about 50%, especially when you consider the enormous variety of problems that could crop up. But with a few hundred people, you have a reasonably sized society, and would probably be fine for a long time, especially considering there have been isolated societies in history with just a few hundred people, and I don't think there is any evidence of these disappearing (with the notable and strange exception of the inhabitants of Easter Island), but rather they usually dissipate into surrounding tribes.

[Adacore]

I think, as others have touched on, there's no definite answer - every answer, even if it's 100% verifiable (which is likely impossible) will be probabilistic. It's certainly possible (imho) that a population of 2 (one female, one male) would be viable to repopulate the species, but the chances are almost certainly far lower than for a larger population. If the question is 'at what point does it become near-certain' or 'at what point is the chance of continuing survival greater than 50%' then that can possibly be answered more definitively.

[Meteorswarm]

I think, as others have touched on, there's no definite answer - every answer, even if it's 100% verifiable (which is likely impossible) will be probabilistic. It's certainly possible (imho) that a population of 2 (one female, one male) would be viable to repopulate the species, but the chances are almost certainly far lower than for a larger population. If the question is 'at what point does it become near-certain' or 'at what point is the chance of continuing survival greater than 50%' then that can possibly be answered more definitively.

I agree. What metric should we decide on? 95% confidence?

[mr-mitch]

Wouldn't this completely depend upon the DNA of the individiuals? If they're all riddled with 'flaws' then it would require a very large population, but if they weren't (all had 0) then it wouldn't matter?

I am under the impression the only problem is inbreeding increases the chances of two recessive genes which cause something 'bad'? Is this wrong? Or are there also other factors?

[Meteorswarm]

Wouldn't this completely depend upon the DNA of the individiuals? If they're all riddled with 'flaws' then it would require a very large population, but if they weren't (all had 0) then it wouldn't matter?

I am under the impression the only problem is inbreeding increases the chances of two recessive genes which cause something 'bad'? Is this wrong? Or are there also other factors?

Define "flaw."

And, yes, the most important impact of inbreeding is pairing of rare, recessive, harmful alleles that normally never find a matching pair in a population. However, the low diversity is also of importance in disease resistance, and ability to adapt quickly - with a low allele pool, it's unlikely that selection will have many phenotypes to work with, so if there is some sudden new selective pressure, the population may react badly.

[mr-mitch]

I can't define flaw, I don't think any one can, that's why I put inverted commas next to it, it was a bad word to use, but I had no other, yet it gets the point across. The impression may be about eugenics but I do not mean to cause that impression.

[Hiato]

I wonder if now is an appropriate time to quote The Architect: 7 females and 5 males. If the above of 1:1 holds, and twenty is good, why not? I'd say lower bound here.

Besides, if the apocalypse were to happen now, I would assume those left to be at least as knowledgeable as anywhere from now to thirty years back, allowing for forms of medical attention to be given if needed, friggen inbreeders

But who knows, maybe it's a doctor-apocalypse..

[mmmcannibalism]

I wonder if now is an appropriate time to quote The Architect: 7 females and 5 males. If the above of 1:1 holds, and twenty is good, why not? I'd say lower bound here.

Besides, if the apocalypse were to happen now, I would assume those left to be at least as knowledgeable as anywhere from now to thirty years back, allowing for forms of medical attention to be given if needed, friggen inbreeders

But who knows, maybe it's a doctor-apocalypse..

The type of apocolypse would probably be important to the amount needed. If we are talking nuclear war then a bigger population is probably needed because almost everyone is going to have some radiation problems; if we are talking virus then its possible you could just find twenty healthy people.

[Eebster the Great]

The type of apocolypse would probably be important to the amount needed. If we are talking nuclear war then a bigger population is probably needed because almost everyone is going to have some radiation problems; if we are talking virus then its possible you could just find twenty healthy people.

I thought we were all assuming it was Catnarok.

By the way, why don't the image resize BB tags work? (e.g. [img=320x240]http://example.com/example.jpg[/img])

[Meteorswarm]

On the other hand, with nuclear war, we'd have our genetic diversity problem partially solved...