Genetic Bottlenecks Question

[Kanastrous]

What is a plausible number for the minimum population of breeding pairs of human beings, necessary to avoid genetic damage as a consequence of inbreeding, over many generations?

[Junghalli]

I've heard it was 150-200 individuals (which would work out to 75-100 breeding pairs).

[Dominus Atheos]

I've heard it was 150-200 individuals (which would work out to 75-100 breeding pairs).

If you want to maximize genetic diversity, monogamy might not be the best practice to follow. Just let people breed with whoever they want to, and keep track of parentage to prevent incest.

[Duckie]

A certain species of seal fell to 30 individuals and still is alive today with thousands of members. Cheetahs were reduced to something absurd like 10 individuals or 30 individuals thousands of years ago. The Laysan Duck apparantly has an MVP of just 7 as estimated by biologists, although it only declined to 500.

So it's apparantly lower than that for certain species. However, MVP and No Genetic Damage are different- depends on how high of an inbreeding coefficient tolerance you have, for one.

[Bedlam]

Humans suffer more from inbreeding than other species as we're a fairly new species and most of the population outside of Africa are decended from a reletively small group so we're not very diverse.

[Kanastrous]

MVP?

I'm working on a story wherein spaceborne colonists get cut off from Earth, and die back to bare-survival levels before their population begins to grow, again. The idea is that they maintain breeding records and practice arranged pairings (in vitro/in vivo, whatever's practical) on top of the necessary palliatives to counter genetic damage from living in a high-radiation environment. But for story purposes the bottleneck needs to be severe enough that they have to be careful.

[andrewgpaul]

Minimum Viable Population?

[Broomstick]

MVP?

I'm working on a story wherein spaceborne colonists get cut off from Earth, and die back to bare-survival levels before their population begins to grow, again. The idea is that they maintain breeding records and practice arranged pairings (in vitro/in vivo, whatever's practical) on top of the necessary palliatives to counter genetic damage from living in a high-radiation environment. But for story purposes the bottleneck needs to be severe enough that they have to be careful.

Well, it also depends on reproductive technology and a willingness to eliminated defective fetuses. If a pre-implanted embryo can be analyzed for defects and discarded if not up to standard, and there is prenatal screening with similar effectiveness and willingness to abort, the minimum number of breeding individuals could be much lower. If you're relying on naturally evolved mechanisms you'll need a higher breeding population and also more births as you will continually lose some of each generation to genetic problems.

The diversity of the initial group might also be a factor - a group of 50 couples that were all somewhat related is less diverse that 50 couples with members from all over the world, chosen for maximum genetic diversity.

[Alyrium Denryle]

I dont know the math involved very well to actually calculate the minimum long term viable human population. However we have already been on the receiving end of at least two bottlenecks, one of which brought out population down into the single digit thousands world wide. So that is a good "conservative" estimate. 5000 or so. In reality though, provided there is not an infinite population size there will be some inbreeding. The smaller the population the more common and without long periods of time and a big population for mutation and drift to occur, a population in the low thousands even will start to get a little Arkansanian...

[Kanastrous]

Well, it also depends on reproductive technology and a willingness to eliminated defective fetuses.

There's +/- 250-years-into-the-future-grade genetics and biology knowledge and resources, to draw upon, although they're likewise constrained by the collapse of support from Earth. So the available technology is very advanced. Given that these guys have become a subsistence society a la Inuits or Koories, eliminating defective fetuses is countenanced.

If a pre-implanted embryo can be analyzed for defects and discarded if not up to standard, and there is prenatal screening with similar effectiveness and willingness to abort, the minimum number of breeding individuals could be much lower. If you're relying on naturally evolved mechanisms you'll need a higher breeding population and also more births as you will continually lose some of each generation to genetic problems.

The technical ability exists to correct a lot of genetic defects and damage, into adulthood (a necessity since it's a high-radiation environment). Although I'm positing a reluctance to modify natural genomes any more than absolutely necessary, as a cultural identity hangup.

The diversity of the initial group might also be a factor - a group of 50 couples that were all somewhat related is less diverse that 50 couples with members from all over the world, chosen for maximum genetic diversity.

The initial diversity pre-bottleneck was pretty decent; mostly drawn from Central and Southern Africa and China, with a leavening of Europeans and Latin Americans; a starting population pre-Crash of around 250,000.

[Mayabird]

A certain species of seal fell to 30 individuals and still is alive today with thousands of members. Cheetahs were reduced to something absurd like 10 individuals or 30 individuals thousands of years ago. The Laysan Duck apparantly has an MVP of just 7 as estimated by biologists, although it only declined to 500.

So it's apparantly lower than that for certain species. However, MVP and No Genetic Damage are different- depends on how high of an inbreeding coefficient tolerance you have, for one.

Cheetahs actually are having a pretty rough time because of that inbreeding. They have a lot of problems.

The initial diversity pre-bottleneck was pretty decent; mostly drawn from Central and Southern Africa and China, with a leavening of Europeans and Latin Americans; a starting population pre-Crash of around 250,000.

Genetically speaking, that's not diverse at all, except maybe for the Africans. Most of humanity's genetic diversity is in sub-Saharan Africa. IIRC, if you really wanted to separate our species into races based on genetic differences, it would be Pygmies (sorry, but I can't remember what the proper name is), Khoisan (aka South African Bushmen), and Everybody Else.

[Kanastrous]

For a while I was toying with the idea that these people were breeding themselves down to pygmy-size, for the obvious attendant advantages: lower food and oxygen requirements, small habitats (or more effective use of larger ones), etc.

[Broomstick]

There is some evidence that the reason "pygmies" are short is that they don't have the adolescent growth spurt most humans do, they just maintain a steady growth through childhood and adolescence and stop at adulthood. This may have evolved more than once in humans. If it's a fairly simple change they might be able to engineer it in one or two generations if they're open to in vitro fertilization.

[Junghalli]

There's +/- 250-years-into-the-future-grade genetics and biology knowledge and resources, to draw upon, although they're likewise constrained by the collapse of support from Earth. So the available technology is very advanced. Given that these guys have become a subsistence society a la Inuits or Koories, eliminating defective fetuses is countenanced.

With advanced genetic engineering, you could simply engineer all deletrious genes out of the population (at least from the second generation onward). That will make the MVP much lower than it is for unmodified humans, maybe even as low as one breeding pair. Asexually reproducing species manage with very little genetic diversity after all; it's the deletrious recessives you really have to worry about IIRC, and with mastery of biotech it shouldn't be hard to simply engineer those out. I expect by the time we start sending expeditions to other star systems we may very likely have done that anyway as a hygene measure, like mass vaccination today.

If they've got the tech and inclination to do stuff like engineer themselves into Pygmies they'd be stupid not to do this.

[Akkleptos]

I'm working on a story wherein spaceborne colonists get cut off from Earth, and die back to bare-survival levels before their population begins to grow, again. The idea is that they maintain breeding records and practice arranged pairings (in vitro/in vivo, whatever's practical) on top of the necessary palliatives to counter genetic damage from living in a high-radiation environment. But for story purposes the bottleneck needs to be severe enough that they have to be careful.

I found the following as an answer to this very similar question:

If the population can go flush (i.e., grow catastrophically), then the gene pool of two would be sufficient. If there was enough room and food for a rapidly growing population, the recessive genes can be selected out of the population without destroying it. What would happen would be in a few generation health problems would become VERY apparent. At some point, most of the population would die. However, since the population is growing rapidly anyway, the newborns would soon take up the slack.

Mutations would eventually occur in the population, and most of these would also be selected out. However, a few mutations would be beneficial. The population may end up diversifying. However, the real question that you are probably asking is what is the minimum gene pool necessary for the survival of a zero growth population. If the population was both zero growth and narrow in gene variety, the population could die. In a zero growth population which
is also highly inbred, a small statistical fluctuation could wipe everyone with a good gene out. Then the population would die. A small change in environment will kill off a zero growth population without genetic diversity and without mutations.

I conjecture that the minimum size gene pool necessary for the survival of a zero growth population depends on both the size of the zero growth population, and the rate of small mutations. The larger the zero growth population, the smaller the initial gene pool has to be. The larger the mutation rate, the smaller the initial gene pool has to be. The mutations will provide the necessary gene diversity. The downside is that most mutations die each generation.

A large population with a large rate of mutation can survive no matter how inbred the original population was. No matter how many get wiped out, a small number of mutants will survive. The zero growth population then becomes a catastrophically growing population and
comes back (see first paragraph).

At some point of size, mutations would help the population survive. A large zero point population would always have a small number of mutants who can adjust to the change in conditions. Most mutants would die each generation, but a change in environment will
make some of them viable.

So when you ask the question again, please distinguish between a
zero growth population and a growing population.

I still don't know what to make of it. Relying on mutations sounds a tad flaky to me, though. Personally I'd go for the 200+ estimate with unrestricted mating (barring incest) arrangement mentioned above.

[Andrew_Fireborn]

I've heard as few as a dozen once...

Provided everyone was willing to have at least one child of each sex with each other. Of course, that would confer a huge strain on the females... (Minimum 12 kids... ) That this would produce a stable gene pool in as few as three generations... (And a huge population boom.)


But I can't for the life of me remember where I'd heard it... And it's rather impractical for biological and sociological reasons.



Though, for a 0-growth population... I'd have to agree with the 200 figure. Though it would require either careful genetic supervision, or good old fashioned animal husbandry applied to people.