minimum viable population

[Hans de Vries]

What is the minimum population of humans that is necessary to maintain a stable population without significantly increased rate of genetic defects?

 

I do know that the Amish are descended from a group of 200-something individuals and Ashkenazi Jews descend from a group of some 350 individuals. 

[naitche]

Sorry, I can't verify the accuracy but I recall 2000 as a population minimum for longer term viability.

[Hans de Vries]

This seems way too high. AShkenazi Jews descend from a group of approx 350 individuals and although they have slightly increased risk of some diseases, they aren't unhealthy as a group overall. The Amish more so but their founding population was lower.

[iNow]

13 hours ago, Hans de Vries said:

What is the minimum population of humans that is necessary to maintain a stable population without significantly increased rate of genetic defects?

Define what you mean by “significantly increased rate” then do the math based on likelihood of genetic mutations 

[naitche]

On 12/24/2020 at 11:39 AM, Hans de Vries said:

This seems way too high. 

Doesn't seem so to me. 

Those populations though close to it, haven't been completely  'closed'. 

Even if they were, I would think effects to be cumulative, and still progressing

[swansont]

On 12/23/2020 at 7:39 PM, Hans de Vries said:

This seems way too high. AShkenazi Jews descend from a group of approx 350 individuals and although they have slightly increased risk of some diseases, they aren't unhealthy as a group overall. The Amish more so but their founding population was lower.

Are you sure this isn’t a manifestation of survivor bias? Just because one population didn’t suffer significant damage doesn’t mean they weren’t at risk.

[zapatos]

On 12/23/2020 at 6:39 PM, Hans de Vries said:

This seems way too high. AShkenazi Jews descend from a group of approx 350 individuals and although they have slightly increased risk of some diseases, they aren't unhealthy as a group overall. The Amish more so but their founding population was lower.

But it's not 350 any longer, and is way higher than 2000. If you kept the population at 350 it would not be viable. 

[CharonY]

From OP I assume that the question is about the minimum number of individuals to avoid inbreeding. There are multiple approaches to these calculations, and it also depend on factors such as population control selective mating and whether the stability is supposed to be indefinite.

Moore has developed a tool to do such calculations, with space travel in mind. I think he calculated something in the range of ~200 people. Others have came up with lower or (much) higher numbers, which are all based on different assumption. For example, accounting for random deaths, which could shrink the gene pool.

[TheAlchemist69]

at one point there were only 2000 people during an ice age they think

[swansont]

6 hours ago, TheAlchemist69 said:

at one point there were only 2000 people during an ice age they think

citation needed.

[John Cuthber]

I have seen an argument for a drop to 10,000 or so.
https://en.wikipedia.org/wiki/Population_bottleneck#Humans

 but even that is controversial.

 

 

However, at some stage in history, there were zero humans, there are now many. This implies that at some time in between, there must have been exactly one human.

And that "species" survived so the minimum viable population is 1 (together with a phenomenal amount of luck).

[zapatos]

10 hours ago, John Cuthber said:

And that "species" survived so the minimum viable population is 1 (together with a phenomenal amount of luck).

And we had two after the first virgin birth?

[Ken Fabian]

10 hours ago, John Cuthber said:

However, at some stage in history, there were zero humans, there are now many. This implies that at some time in between, there must have been exactly one human.

And that "species" survived so the minimum viable population is 1 (together with a phenomenal amount of luck).

I'm not sure it works like that. It isn't a new species until there is a population. An individual with the characteristics of a new species isn't a new species it is a variant within the existing species. That may seem a technicality but I don't think so; he/she needed to successfully breed within that existing population and the subsequent new species will be descendants of a lot of other individuals besides that one even if the whole new species counts that individual as a progenitor. Whether the traits are recessive or dominant will matter too; an individual with traits of the yet to come new species could need the combination of recessive mutations already part of the population, originating from an individual who did not have those traits; the population has the genes.

I'm inclined to think two healthy humans could go on to found a viable population if they were lucky and began free of deleterious mutations. Incest taboos would have to be set aside. Australia's problem rabbit population began with the release of 13 individual rabbits; they went on to populate and overpopulate a continent. I seem to recall an island overrun with moose or elk or something that began with 5 individuals.

These may not do well in competition with other related populations - they may well be outcompeted and displaced by healthier, better adapted populations - but may do well in isolation. But perhaps founder effect will see important traits emerge that ultimately let them outcompete the species they came from - maybe low probability, but possible.

[Ferrummageticium]

plants become polymorphic all the time . showing odd types between species that dont really place them in either but are obviously related.  I forget when I visited the museum of natural history in D.C. there is a panel with at least 25 species of Homo, maybe more. its fathomable to me that many of these species bred with each other. 

On 9/7/2021 at 6:42 PM, Ken Fabian said:

I'm inclined to think two healthy humans could go on to found a viable population if they were lucky and began free of deleterious mutations. Incest taboos would have to be set aside. Australia's problem rabbit population began with the release of 13 individual rabbits; they went on to populate and overpopulate a continent. I seem to recall an island overrun with moose or elk or something that began with 5 individuals.

I think this is plausible. deleterious mutations may not have been lethal and still allowed beneficial ones to aspire.  and if they did perhaps new genetics were introduced. like why a certain percentage of homo neanderthalensis genetics have been found higher in certain groups currently.  also time, there is a lot of time for a benefiting mutation to occur in 50k plus years. 

 

having a large set of beneficial genes then setting a small system for them to mingle together only for only a couple thousand years may not be very harmful at first

[mistermack]

The question is a bit like "how long is a piece of string?", there are so many possible anwers.

A lot depends on how mobile the population is. If you had two similar populations of similar animals, and one was more mobile than the other, then you have more chance of mixing the genes. On the other hand, if you have a much bigger population, but of far less mobile individuals, then you are more likely to get inbreeding even with the greater numbers. 

To illustrate that, imagine a big population, that lived in small numbers on scattered islands. They are more likely to inbreed than a smaller population all living in one herd. And you can get the island effect without islands, with barriers like mountain ranges or rivers or deserts.