Minimum population question

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notyouravgjoel
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Minimum population question

Postby notyouravgjoel » Tue Sep 18, 2007 7:12 pm UTC

I recently watched a Stargate episode and it got me thinking... if Earth was destroyed, would a population of a thousand or so humans have enough genetic diversity to continue the race?

What is the minimum sustainable species population? Is this minimum about constant across all species? If it varies, which qualities in a species affect this?

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Re: Minimum population question

Postby zenten » Tue Sep 18, 2007 7:21 pm UTC

notyouravgjoel wrote:I recently watched a Stargate episode and it got me thinking... if Earth was destroyed, would a population of a thousand or so humans have enough genetic diversity to continue the race?

What is the minimum sustainable species population? Is this minimum about constant across all species? If it varies, which qualities in a species affect this?


There's no hard number. From my understanding though once you get around a thousand or lower you often end up with more and more genetic defects. But that depends on the species, and I'm not sure if there is much hard data on this for humans.

It is actually possible to come back from just one pregnant woman, but humans are going to be fairly messed up for some time after that, just look at cheetahs.

This article is relevant, although I have no idea if it's accurate: http://en.wikipedia.org/wiki/Effective_population_size

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Postby notyouravgjoel » Tue Sep 18, 2007 7:26 pm UTC

I guess I'm assuming (maybe a bad assumption) that the defects will eventually get worse until humans start to die.

I've read that the Amish community is having a fair amount of problems with birth defects. As time goes along, people tend to leave, which makes the problem even worse; basically, everyone is closely related to everyone else.

Cheetahs =(

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Postby zenten » Tue Sep 18, 2007 7:29 pm UTC

notyouravgjoel wrote:I guess I'm assuming (maybe a bad assumption) that the defects will eventually get worse until humans start to die.

I've read that the Amish community is having a fair amount of problems with birth defects. As time goes along, people tend to leave, which makes the problem even worse; basically, everyone is closely related to everyone else.

Cheetahs =(


It's not as clear cut as that. People for instance tend to have medical care, so it could actually get very small before it starts making the survival of the human race unlikely. However, the quality of life of people would start to degrade well before then.

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Postby bigglesworth » Tue Sep 18, 2007 8:58 pm UTC

I've heard 300 as a lower limit for a colony on another planet, although these were of mostly unrelated people.

If the birthrate was high enough to allow for some wastage, probably.
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Postby SpitValve » Tue Sep 18, 2007 9:11 pm UTC

bigglesworth wrote:I've heard 300 as a lower limit for a colony on another planet, although these were of mostly unrelated people.


300 is not a number that bodes well for survival. But at least you get to yell a lot.

The New Zealand Black Robin got down to a single female and four males at one stage. There's now ~250, with large amounts of human intervention.

wikipedia wrote:Interestingly, this seems to have caused no inbreeding problems, leading to speculation that the species has passed through several such population reductions in its evolutionary past and thus losing any alleles that could cause deleterious inbreeding effects. It was generally assumed that the minimum viable population protecting from inbreeding depression was around 50 individuals, but this is now known to be an inexact average, with the actual numbers being below 10 in rapidly-reproducing small-island species such as the Black Robin, to several hundred in long-lived continental species with a wide distribution (such as elephants or tigers).

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Postby Pesto » Tue Sep 18, 2007 9:11 pm UTC

Are we assuming all the other things that go along with a destroyed Earth, like loss of technology and such?

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Postby Token » Tue Sep 18, 2007 9:35 pm UTC

Pesto wrote:Are we assuming all the other things that go along with a destroyed Earth, like loss of technology and such?

Well, it's no fun if you can just clone your way back to genetic diversity.

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Postby gmalivuk » Tue Sep 18, 2007 9:39 pm UTC

Token wrote:
Pesto wrote:Are we assuming all the other things that go along with a destroyed Earth, like loss of technology and such?

Well, it's no fun if you can just clone your way back to genetic diversity.


Um... how would cloning create diversity?
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Postby Pesto » Tue Sep 18, 2007 9:46 pm UTC

There would be lots of different yous?

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Postby Solt » Wed Sep 19, 2007 4:48 am UTC

gmalivuk wrote:
Token wrote:
Pesto wrote:Are we assuming all the other things that go along with a destroyed Earth, like loss of technology and such?

Well, it's no fun if you can just clone your way back to genetic diversity.


Um... how would cloning create diversity?


Well if you do it in a semi natural way and using two people, you could get a lot of diversity from just those two people.

I think humans evolved from a group of 10,000.
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Postby po2141 » Wed Sep 19, 2007 8:24 am UTC

Solt wrote:I think humans evolved from a group of 10,000.


Well, it was probably a little more complicated than that...

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Postby Sc4Freak » Wed Sep 19, 2007 11:45 am UTC

Solt wrote:
gmalivuk wrote:
Token wrote:
Pesto wrote:Are we assuming all the other things that go along with a destroyed Earth, like loss of technology and such?

Well, it's no fun if you can just clone your way back to genetic diversity.


Um... how would cloning create diversity?


Well if you do it in a semi natural way and using two people, you could get a lot of diversity from just those two people.

I think humans evolved from a group of 10,000.

You mean a group of 2.

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Postby zenten » Wed Sep 19, 2007 12:29 pm UTC

Pesto wrote:There would be lots of different yous?


No, in terms of genetic diversity there would be essentially one you, that can have lots of babies.

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Postby Hiro » Thu Sep 20, 2007 5:23 am UTC

Well if we have cloning, just obtain the genetic material of a million people and store it somewhere self. Now you clone them.
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Postby miles01110 » Thu Sep 20, 2007 1:11 pm UTC

Battlestar Galactica gets by with about 45,000. Of course....they're in space. And I haven't finished season 1.

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Postby evilbeanfiend » Thu Sep 20, 2007 1:15 pm UTC

surely you have to account for the rate of mutation somehow. perhaps BSG needs fewer as increased ration exposure from space travel is increasing genetic diversity
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Postby zenten » Thu Sep 20, 2007 2:47 pm UTC

By the way, the number changes depending on who you're having kids with. Monogamous relationships for instance will have a lower number than people breeding with almost anyone of the opposite sex, where the only people they won't sleep with are too closely related.

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Postby iop » Fri Sep 21, 2007 10:34 pm UTC

gmalivuk wrote:
Token wrote:
Pesto wrote:Are we assuming all the other things that go along with a destroyed Earth, like loss of technology and such?

Well, it's no fun if you can just clone your way back to genetic diversity.


Um... how would cloning create diversity?


Cloning would not directly create diversity, but you could just expose the cloned cells to mutagens (same thing you do for "classical" breeding). Plenty of embryos will die, but the rest will have all kinds of diversity.

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Postby Token » Sat Sep 22, 2007 2:42 am UTC

gmalivuk wrote:
Token wrote:
Pesto wrote:Are we assuming all the other things that go along with a destroyed Earth, like loss of technology and such?

Well, it's no fun if you can just clone your way back to genetic diversity.


Um... how would cloning create diversity?

It wouldn't really, if you only used living humans.

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Postby Solt » Sat Sep 22, 2007 2:48 am UTC

po2141 wrote:
Solt wrote:I think humans evolved from a group of 10,000.


Well, it was probably a little more complicated than that...


http://en.wikipedia.org/wiki/Toba_catastrophe_theory

according to the Wikipedia article on human evolution, humans are more genetically homogeneous than most species on earth.
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Postby ToLazyToThink » Sat Sep 22, 2007 3:29 pm UTC

evilbeanfiend wrote:surely you have to account for the rate of mutation somehow. perhaps BSG needs fewer as increased ration exposure from space travel is increasing genetic diversity


I don't think that would help. Wouldn't you just increase the number of bad genes that could end up getting concentrated by inbreeding?

I guess you'd also increase the number of good genes, but that doesn't really do you any good if the bad genes kill off the population anyway (especially since there would be more bad genes than the average in a stable population since they haven't had time to be selected against yet).

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Postby Copperhead » Mon Sep 24, 2007 10:43 pm UTC

IIRC the numbers i have seen tend to be a breeding population of about 500 for short term (A few generations) survival without selective breeding, and 2000 - 5000 for indefinite population survival. this seems to sync fairly well with this: http://www-personal.umich.edu/~dallan/nre220/outline12.htm
this number changes when you have an initial population that is very fit or unfit (genetically), as the different density of undesirable traits will make the effects of inbreeding less noticable (that is, there are fewer undesirable traits to reinforce.), or if you have an initial population that is already heavily inter-related. Ultimately it comes down to "the larger and more genetically diverse your population is, the better"
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Postby dumbclown » Tue Sep 25, 2007 4:40 pm UTC

Solt wrote:
po2141 wrote:
Solt wrote:I think humans evolved from a group of 10,000.


Well, it was probably a little more complicated than that...


http://en.wikipedia.org/wiki/Toba_catastrophe_theory

according to the Wikipedia article on human evolution, humans are more genetically homogeneous than most species on earth.


I don't think this is just wikipedia. I remember hearing about a human fossil they found which they could put 90% of human genetics back to.

If the earth was damaged badly there would be a few impacts from this. They could result in mutations.

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Postby Yakk » Tue Sep 25, 2007 5:37 pm UTC

I was under the impression that some recent work has found similar lack of diversity in other species: that might mean that the model we are using to detect the lack of diversity is wrong...

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Postby Daniel » Tue Sep 25, 2007 7:44 pm UTC

Cloning might not create diversity, but you could make out with yourself...

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Postby 4=5 » Wed Sep 26, 2007 1:01 am UTC

notyouravgjoel wrote:I've read that the Amish community is having a fair amount of problems with birth defects. As time goes along, people tend to leave, which makes the problem even worse; basically, everyone is closely related to everyone else.

the solution is for them to start adopting kids,

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Postby blob » Wed Sep 26, 2007 10:00 am UTC

It may depend on whether the society is monogamous or not. I heard there was a small tribe somewhere that organised marriage so that men married sets of sisters rather than single women, and more than one person could marry the same set of sisters. Apparently it helped increase genetic diversity relative to population size?
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Re: Minimum population question

Postby Teshi » Fri Sep 28, 2007 11:12 am UTC

Blob said what I was going to say. If you have 100 unrelated females and 100 unrelated males, and every woman has four children (since two would only be replacement and not very good) you can get four times as many genetic combinations if women are inseminated by four different men. I tried to work out the numbers but failed. You can also cross-generationally inseminate women with men from the previous generation with whom they are not related at all, increasing the pool.

You don't have to sacrifice marital or sexual monogamy to do this. Artificial insemination is very easy with a complete lack of technology.

If you have technology, you can easily import hundreds of extra sperm samples saving you tons of trouble.

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Re: Minimum population question

Postby McHell » Mon Oct 01, 2007 4:08 pm UTC

A simple observation is: if every man + woman get a daughter + son, so the population is stable (move to 2.1 children for actual replacement, compensating whatever fluctuations)... then theoretically every gene of the parents could end up in the children but on average not.

Take a locus A, where the male has allele A1 and A2, and female A3 and A4.
Say daughter gets (WOLOG) A1 and A3. Then the chance that son gets (A2,A4) is only 25%... 50% chance he's (A1,A4) or (A2,A3), and even 25% chance he's same as his sister (A1,A3). So per locus per generation 75% chance that 1 or 2 genes are lost from a genetically diverse locus.
Of course, on most loci humans are pretty uniform, with all having the same allele or slight variations on it; so if partners are (A1,A1) and (A1,A2) then A2 has a 75% chance of making it to the next generation, which includes 25% chance of being twice used. But this is drift, on average the number is the same. Branching processes theory will tell you that "in the long run, we are all dead": if the process is critical (neither growth nor decline ON AVERAGE), extinction is inevitable. This goes for any population with an upper limit and a stochastic element.

In nature minimal sizes for healthy survival differ on type of species: those that are always rare have healthier genes -- because recessive diseases will have been weeded out much more. Also they may have different adaptations to survive `bottlenecks' of extra-low numbers. Humans have evolved from ridiculously small sets of ancestors, leaving us with less diversity that e.g. the highland gorillas (a few hundred of them, where we're 6billion strong). [No you cannot show a single mother for all humans; you are thinking of mitochondrial dna, which is like family names --- your mother DID contribute to your genetic makeup even if you have your father's surname. Obviously. Similarly, your paternal grandmother gave you genes even though you have your maternal grandmother's `eve' marker. Thus this `eve' thing is irrelevant in this discussion.]

Your type of question is "the twofold cost of sex": basically, why are there males? If a female can raise two offspring, then a parthenogenic (=sex-less, at some level) organism can double every generation in size where the sexual requires one of the offspring (on balance) to be male which leaves the population at exactly the same size. [No, don't tell me a rare male can be having a harem of many females, leaving the doubling ratio close to that of the asexual situation; I think RA Fisher showed in the thirties already this doesn't work.] The observation that a lot of sex occurs (in higher animals) requires therefore a lot of proofs/calculations/research into why exactly? The obvious benefit of sex is recombination and thus weeding out badness, plus combining advances that occurred in different ancestors get combined (where each advantageous mutation would have to happen again in each line of decendants of an asexual creature).
[Of course, bacteria have ways around this, stealing each others dna without sex.]

Hope it helps.

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Re: Minimum population question

Postby McHell » Mon Oct 08, 2007 3:51 am UTC

Oh by the way -- even if your population survives short-term the population dynamical chance of extinction: if you then grow back to `normal' numbers, these are genetically so lacking in diversity that evolution can kill them, as it eventually finds a suitable pathogen.

Example: the tasmanian devils are very rapidly dying out even with large numbers present: they all succumb to the same disease. Similar cases are our bananas --- all we eat is Cavendish, from an infertile plant (no seeds in your bananas, please!). This variant has gotten popular when the older favourite died out from a disease --- and now there's fungi slowly spreading that will unavoidably kill the Cavendish, as the fungus grows in any climate that the banana can.

Edit: can't seem to get the url right? what am I failing at?
Edit2: ah, thanks yakk!
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Re: Minimum population question

Postby LoopQuantumGravity » Mon Oct 08, 2007 4:08 am UTC

McHell wrote:Oh by the way -- even if your population survives short-term the population dynamical chance of extinction: if you then grow back to `normal' numbers, these are genetically so lacking in diversity that evolution can kill them, as it eventually finds a suitable pathogen.

Example: the tasmanian devils are [url href=http://www.theregister.co.uk/2007/10/07/tasmanian_devil_cancer_inbreeding/]very rapidly dying out[/url] even with large numbers present: they all succumb to the same disease. Similar cases are our bananas --- all we eat is Cavendish, from an infertile plant (no seeds in your bananas, please!). This variant has gotten popular when the older favourite died out from a disease --- and now there's fungi slowly spreading that will unavoidably kill the Cavendis, as the fungus grows in any climate that the banana can.


This is why we need to keep making genetically engineered plants, damn it. But people have seen too many bad sci-fi movies where a genetically engineered plant/animal almost destroys the world due to some scientifically impossible series of events.

Also, IIRC, there have been a few points in human history where we've been reduced to <1000 people.
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Re: Minimum population question

Postby McHell » Mon Oct 08, 2007 10:24 am UTC

Well... 1000>>1 like the vegetable clones. [I think the latest estimates are 5000--10 000 for humans which is a bit safer.]

What genetical engineering has to do with it I don't see? Do you want to make immune devils? That incorporates making a whole MHC out of nothing... the space of possibilities is many orders of magnitude larger than that of viable/superior possibilities so a hit-and-miss approach is a bit problematic (also a wholesale transplant from e.g. a kangaroo species to the other marsupial is probably doomed for the same reason).

Also, evil Monsanto has now started a parallel program of non-genetically-engineered seed production. They basically let it randomly mutate while checking their markers whether they have mutations in the desired genes, instead of directly manipulating these genes. This is basically an enormous waste of time and resources, but the only way to crack the GM-resistent markets.
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Re: Minimum population question

Postby Yakk » Mon Oct 08, 2007 5:00 pm UTC

McHell wrote:Oh by the way -- even if your population survives short-term the population dynamical chance of extinction: if you then grow back to `normal' numbers, these are genetically so lacking in diversity that evolution can kill them, as it eventually finds a suitable pathogen.


Then humanity is screwed, because we are all nearly clones of each other. :)

The way I heard it put was "there is more genetic diversity in a single band of chimpanzees than the entire human species".

This variant has gotten popular when the older favourite died out from a disease --- and now there's fungi slowly spreading that will unavoidably kill the Cavendish, as the fungus grows in any climate that the banana can.


Apparently the pre-Cavendish banana is really tasty.

Note that there are many people working on post-Cavendish bananas.

Edit: can't seem to get the url right? what am I failing at?


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Re: Minimum population question

Postby Victorkm » Mon Oct 08, 2007 5:37 pm UTC

You could probably work out the minimum number given the ratio of male:female births and how many generations would need to pass before the offspring could breed with each other again. IE if you have Pair ZY and Pair XW, and each had so many children, say 9 children per pair ZYA-ZYJ and XWA-XWJ, the next generation you would end up with 81 children who all have ZYXWAA-ZYXWJJ and none would be able to breed without mixing genetics. Using this kind of model I figure for every 2 pairs you start out with, you have another generation. This is if you assume no one dies or goes infertile during the 81 months they bear children. How long does it take to dilute the pool so much that 2 people with ZYXW starting their strings no longer pass on those genetics?

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Re: Minimum population question

Postby McHell » Mon Oct 08, 2007 7:53 pm UTC

Not really. It's a stochastic process, so never exact numbers always with a *chosen* confidence interval; and any level of inbreeding is bad. Double the size of population is always better. Logically, any breeding within a population is inbreeding at some level (`because we all descent from adam&eve, we're all distant cousins).

The way to do it what you propose is to choose a risk you are willing to run, like one extinction per 1000 or 100 years: how large should my population be then? Or if I can have a network (`metapopulation') of separate groups that can seep together (with say 1% migrating from one `mating pool' to a random other one, per generation), is it riskier to have 10x100 individuals or 1x1000?

Notice that this doesn't even begin to address the question of: do I have an historically low-density species of which the (recessive) bad genes have been purged mostly through inbreeding, or a high-density species (more diverse, but may carry more rare crippling alleles that will only express themselves after inbreeding)?

There are many and more relevant issues, and together it means that you cannot hope to get an analytical result that deals with all. You can take one factor and conclude things (bigger pop = better), you can also get a feel for how bad any halving of a population is for example, but that's about it.

Actual ecology/species management is like water management: you make calculations how high the dykes and levees must be to withstand a once-in-100-years storm (meaning: expected great calamity within 3 generations!), or government can demand once-in-1500-years etc... And then you notice these (from the 70s) now start to fail because reality has changed in the meantime (global warming and all that). Thus species survival beyond 150years is probably meaningless to calculate.

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Re: Minimum population question

Postby mehmattski » Thu Oct 11, 2007 2:05 am UTC

For those interested in the original science (and have access to scientific papers), check out:

Lynch, M., J. Conery, et al. (1995). "Mutational Meltdowns in Sexual Populations." Evolution 49(6): 1067-1080.

They showed through modeling that sexual, outcrossing populations of effective size greater than 1000 will be free from the threat of extinction due to accumulation of deleterious mutations. Effective population size is the key, and represents the genetic diversity of the population, rather than raw numbers.

And the evidence for the effective population size of humanity being about 10,000 comes from this paper:

Harpending, Harry et al (1998). "Genetic traces of ancient demography." Proc Nat Acad Sci 95(4): 1961-1967

This one is based on mitochondrial DNA and has since been confirmed with other genes. But other population bottlenecks during the ice ages may have further restricted our genetic diversity, especially since eight of the ten groups of mitochondrial DNA never left Africa.

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Re: Minimum population question

Postby jwwells » Thu Oct 11, 2007 8:30 am UTC

Highly sequestered populations have done pretty well under some circumstances - see, for example, the [a href="http://en.wikipedia.org/wiki/Hutterite"]Hutterites[/a]. Part of it depends on what your selective pressures are, and what constitutes a deleterious mutation. If you're never faced by the one silver bullet pathogen that can kill all of you, you're fine. If you are, it may be that no amount of genetic diversity can help you.

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Re: Minimum population question

Postby McHell » Thu Oct 11, 2007 10:15 am UTC

mehmattski wrote:They showed through modeling that sexual, outcrossing populations of effective size greater than 1000 will be free from the threat of extinction due to accumulation of deleterious mutations. Effective population size is the key, and represents the genetic diversity of the population, rather than raw numbers.

I'd be really surprised if what you write is strictly true. If you assure me it is, I'll have to download it and read it; I think I have only online access beyond 2000 here.

I say this because the pop=1000 estimate is far older, from the early 60s at least, I'd say Kimura [who else? :shock: ]. It's a negligible risk of extinction in finite time in most models with >1000individuals, but as I pointed out what is negligible depends on personal preference and on timescale you watch. Even with 30individuals your time to extinction (or risk of extinction within N generations) can be acceptable, as stated before.

So >1000 we are happy from mutational extinction, good [unattainable number in many cases, but fine.]

As my Taz devil example points out, not fine if you're protected from that risk but die out from a pathogen you are actively spreading yourself (by fighting). Clearly this disease will spread slowly if there's very few devils left, but then it's not clear whether they'll die out from some Allee effect or on the other hand this saves them: can they be rare enough to let the disease go extinct but still find mates to repopulate the forests? There is not much of a sensible way to modelling this in general --- depends strongly on the ethology and on the disease in question [a fast flu-like thing or a slow aids-like thing? The fast thing could disappear easily if it's in a species that's very solitary outside of mating season, by bringing population densities low enough to not spread but allowing them to find mates (with some effort) later on.

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Re: Minimum population question

Postby mehmattski » Fri Oct 12, 2007 2:50 am UTC

I'm not entirely sure what you are questioning, but I agree that the idea of the 1000 number comes from a number of sources, like Kimura and then Kondrashov in the 1980s. What Lynch showed is that asexual, or inbreeding, populations will require higher numbers (10,000 or 100,000) to reduce the threat of extinction. In the sexual, outcrossing model, it is almost impossible to get extinction with effective population size greater than 1000, even after many generations. Asexual and inbreeding populations, meanwhile, have a higher threat of extinction as time goes on. If you do have access to the paper, I do suggest it, the figures helped me understand.

It is true that pathogens can increase the threat of extinction, provided the pathogen is epidemic and resistance response is slow. On the other hand, one of the best ways to get genetic diversity in a population is through host-pathogen coevolution. The human Major Histocompatibility Complex is one example. If the pathogen is not super virulent and resistance evolves relatively quickly, the population's standing genetic diversity will actually go up with time. I know that's not entirely related to the question at hand, but I find it interesting.

More on topic, I would be comfortable sending a colony ship with 300 carefully selected individuals, paying close attention to genetic diversity. Otherwise we may end up with a colony of furry, webbed-feet, small-brained humans. (Galapagos is one of my favorite books...)


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