the minimal viable population (the minimum number of individuals necessary to maintain the level of genetic diversity to repopulate Earth)
I think this is a narrower usage of minimum viable population (which I think is the term you mean, rather than minimal viable population) than is standard. (Though I’m not an expert here.) Wikipedia says:
Minimum viable population (MVP) is a lower bound on the population of a species, such that it can survive in the wild. [...] MVP refers to the smallest possible size at which a biological population can exist without facing extinction from natural disasters or demographic, environmental, or genetic stochasticity.
That indicates the MVP is determined not just by whether there’s sufficient genetic diversity, but also whether the population can survive natural disasters and other things. (See also this comment “from the perspective of a person with a population biology/evolution background”, who argues that factors other than genetic variation would be more important in determining humanity’s MVP following a catastrophe/collapse.)
(This is just a comment on terminology. You do cover whether the groups would be small enough to be wiped out by natural shocks. And I think it makes sense to consider the genetic diversity and natural shock questions separately. I just think it’d be best to not use the term “MVP” when one means something like “MVP from the perspective of genetic diversity specifically”.)
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Later, you write:
The minimum viable population (100–1,000 people, assuming a sufficient balance of the sexes) is really, really small compared to the current population: Just 0.00001% of the people alive today would need to survive for repopulation to be possible.
Do you have a citation for the 100-1000 figure? I ask partly because I think the size of the MVP is plausibly an important question (and I’d thus be interested to know if there’s a relatively definitive source on it), and partly because I wonder if that figure is based on considering only genetic variation or also other factors.
Also, I think the last bit of that passage is somewhat misleading: We don’t just need 0.00001% of the people alive today to survive, but for there to end up being a single group of that size. As you note elsewhere, people are likely to be initially quote scattered, and will likely remain at least somewhat scattered. (But I think you address this point adequately elsewhere, and I don’t think it’d really change our bottom-line conclusions, so this is just a minor thing.)
It seems that groups of about seventy people colonized both Polynesia and the New World (Murray-McIntosh, Scrimshaw, Hatfield, & Penny, 1998; Hey, 2005). So let us assume, as a reference point for analysis, that the survival of humanity requires that one hundred humans remain, relatively close to one another, after a disruption and its resulting social collapse. With a healthy enough environment, one hundred connected humans might successfully adopt a hunter-gatherer lifestyle. If they were in close enough contact, and had enough resources to help them through a transition period, they might maintain a sufficiently diverse gene pool, and slowly increase their capabilities until they could support farming. Once they could communicate to share innovations and grow at the rate that our farming ancestors grew, humanity should return to our population and productivity level within twenty thousand years.
(Murray-McIntosh, Scrimshaw, Hatfield, & Penny, 1998; Hey, 2005)
This is an upper bound for the number required to prevent extinction. Smaller groups would suffer more seriously from inbreeding, especially since our society’s genome has a lot more deleterious mutations in it than a hunter gatherer one. But quite badly inbred humans could still survive, breed up to more survivable numbers and gradually fix their genome by natural selection. The real number is probably a good deal lower.
Minor comment regarding minimum viable population
I think this is a narrower usage of minimum viable population (which I think is the term you mean, rather than minimal viable population) than is standard. (Though I’m not an expert here.) Wikipedia says:
That indicates the MVP is determined not just by whether there’s sufficient genetic diversity, but also whether the population can survive natural disasters and other things. (See also this comment “from the perspective of a person with a population biology/evolution background”, who argues that factors other than genetic variation would be more important in determining humanity’s MVP following a catastrophe/collapse.)
(This is just a comment on terminology. You do cover whether the groups would be small enough to be wiped out by natural shocks. And I think it makes sense to consider the genetic diversity and natural shock questions separately. I just think it’d be best to not use the term “MVP” when one means something like “MVP from the perspective of genetic diversity specifically”.)
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Later, you write:
Do you have a citation for the 100-1000 figure? I ask partly because I think the size of the MVP is plausibly an important question (and I’d thus be interested to know if there’s a relatively definitive source on it), and partly because I wonder if that figure is based on considering only genetic variation or also other factors.
Also, I think the last bit of that passage is somewhat misleading: We don’t just need 0.00001% of the people alive today to survive, but for there to end up being a single group of that size. As you note elsewhere, people are likely to be initially quote scattered, and will likely remain at least somewhat scattered. (But I think you address this point adequately elsewhere, and I don’t think it’d really change our bottom-line conclusions, so this is just a minor thing.)
Do you have a citation for the 100-1000 figure?
Comes from here https://mason.gmu.edu/~rhanson/collapse.pdf and the papers it cites:
This is an upper bound for the number required to prevent extinction. Smaller groups would suffer more seriously from inbreeding, especially since our society’s genome has a lot more deleterious mutations in it than a hunter gatherer one. But quite badly inbred humans could still survive, breed up to more survivable numbers and gradually fix their genome by natural selection. The real number is probably a good deal lower.