Thanks for writing this post! I enjoyed looking over these, many of which I have also been puzzling about.
What’s the minimum viable human population (from the perspective of genetic diversity)?
After seeing this question picked up here I thought I would share some quick thoughts from the perspective of a person with a population biology/evolution background. I think this is a reasonable question to ask, but I suspect is not as important as the other factors that go into the broader question of what is the minimum population size from which humanity is likely to recover, period. Genetics are just one factor and probably not the most important when we consider the probability of recovery after a severe drop in global population.
Suppose that after some catastrophic event the population of humanity has suddenly dropped to a much smaller and more fragmented global population, e.g. 10000 individuals scattered in ~100 groups of 100 each across the globe. While the population size is small, it will be particularly susceptible to going extinct due to random fluctuations in population size. The population size could remain stationary or gradually decline, until eventually a random event causes extinction. Or it could start increasing, until eventually it is large enough to be robust to extinction from a random event.
The idea of a minimum viable population size (MVP) from a purely genetic perspective is that, since small populations are predicted to have lower average genetic fitness due to an increase in the expression of recessive deleterious mutations (“inbreeding depression”), an increased fixation of deleterious mutations in the population, or a lack of genetic variation that would allow adaptation to environment, there is in theory a population size small enough where a population would decline and go extinct due to low genetic fitness.
But in reality, the population seems more likely to go extinct because of poor environmental conditions, random environmental fluctuations, loss of cultural knowledge (which, like genetic variation, goes down in small populations), or lack of physical goods and technology, none of which have much to do with genetic variation.
Another way in which the concept of a MVP is too simplistic is that it is defined with respect to a genetic “equilibrium”—it assumes that conditions have been stable enough that there is a constant level of genetic variation in the population. However, after a sudden population decline, we would be far from equilibrium—we would still have lots of genetic variation from the time the population was large. This variation would start to decay, but as different local populations become fixed for different variants, much of this variation would be maintained at the global level and could be converted back into local variation by small amounts of migration. Such considerations are not usually included in MVP considerations. (Some collaborators and I have written about this last point at it relates to conserving endangered species here)
Perhaps we should keep the term “minimum viable population size” but use a broader definition based on likelihood to survive, period. I see that Wikipedia uses a broad definition that includes extinction due to demographic and environmental stochasticity, but often MVP is used as in the OP to refer just to extinction due to genetic reasons, so it is important to clarify terms.
in reality, the population seems more likely to go extinct because of poor environmental conditions, random environmental fluctuations, loss of cultural knowledge (which, like genetic variation, goes down in small populations), or lack of physical goods and technology, none of which have much to do with genetic variation.
This matches what I had tentatively believed before seeing your comment—i.e., I had suspected that genetic diversity wasn’t among the very most important considerations when modelling odds of recovery from collapse. So I’ve now updated to more confidence in that view.
I raised MVP (from a genetic perspective) just as one of many considerations, and primarily because I’d seen it mentioned in The Precipice. (Well, Ord doesn’t make it 100% clear that he’s just talking about MVP from a genetic perspective, but the surrounding text suggests he is. Hanson also devotes two paragraphs to the topic, again alongside other considerations.)
Perhaps we should keep the term “minimum viable population size” but use a broader definition based on likelihood to survive, period. I see that Wikipedia uses a broad definition that includes extinction due to demographic and environmental stochasticity, but often MVP is used as in the OP to refer just to extinction due to genetic reasons, so it is important to clarify terms.
I’d agree that clarifying what one means is important. This is why I explicitly noted that here I was using MVP in a sense focused only on genetic diversity. To touch on the other “aspects” of MVP, I also have “What population size is required for economic specialisation, technological development, etc.?”
It seems fine to me for people to also use MVP in a sense referring to all-things-considered ability to survive, or in a sense focused only on e.g. economic specialisation, as long as they make it clear that that’s what they’re doing. Indeed, I do the latter myself here: I write there that a seemingly important parameter for modelling odds of recovery is “Minimum viable population for sufficient specialisation to maintain industrialised societies, scientific progress, etc.”
Another way in which the concept of a MVP is too simplistic...
I wasn’t aware of these points; thanks for sharing them :)
Thanks for your response and the link to your newer post and the Ord and Hanson refs. I’ll just add a thought I had while reading
This is why I explicitly noted that here I was using MVP in a sense focused only on genetic diversity. To touch on the other “aspects” of MVP, I also have “What population size is required for economic specialisation, technological development, etc.?”
It seems fine to me for people to also use MVP in a sense referring to all-things-considered ability to survive, or in a sense focused only on e.g. economic specialisation...
This all makes sense, but sounds to me like to be at risk of leaving out the population/conservation biology perspective (beyond genetic considerations). A large part of what motivated me to write my original post is that I do think it is indeed valuable to use frameworks from population and conservation biology to study human extinction risk - but it is important to include all factors identified in those fields as being important; namely, environmental and demographic stochasticity, as well as habitat fragmentation and degradation, which could pose much greater risks than inbreeding and genetic drift.
Yeah, that sounds right. Those factors were left out just because I didn’t think of including them (because I don’t know very much about these frameworks from population and conservation biology), rather than because I explicitly decided to include them, and I’d guess you’re right that attending to those factors and using those frameworks would be useful. So thanks for highlighting this :)
There are probably also various other “crucial questions” people could highlight, as well as questions that would fit under these questions and get more into the fine-grained details, and I’d encourage people to comment here, comment in the google doc, or create their own documents to highlight those things. (I say this partly because this post has a very broad scope, so a vast array of fields will have relevant knowledge, and I of course have very limited knowledge of most of those fields.)
Thanks for writing this post! I enjoyed looking over these, many of which I have also been puzzling about.
After seeing this question picked up here I thought I would share some quick thoughts from the perspective of a person with a population biology/evolution background. I think this is a reasonable question to ask, but I suspect is not as important as the other factors that go into the broader question of what is the minimum population size from which humanity is likely to recover, period. Genetics are just one factor and probably not the most important when we consider the probability of recovery after a severe drop in global population.
Suppose that after some catastrophic event the population of humanity has suddenly dropped to a much smaller and more fragmented global population, e.g. 10000 individuals scattered in ~100 groups of 100 each across the globe. While the population size is small, it will be particularly susceptible to going extinct due to random fluctuations in population size. The population size could remain stationary or gradually decline, until eventually a random event causes extinction. Or it could start increasing, until eventually it is large enough to be robust to extinction from a random event.
The idea of a minimum viable population size (MVP) from a purely genetic perspective is that, since small populations are predicted to have lower average genetic fitness due to an increase in the expression of recessive deleterious mutations (“inbreeding depression”), an increased fixation of deleterious mutations in the population, or a lack of genetic variation that would allow adaptation to environment, there is in theory a population size small enough where a population would decline and go extinct due to low genetic fitness.
But in reality, the population seems more likely to go extinct because of poor environmental conditions, random environmental fluctuations, loss of cultural knowledge (which, like genetic variation, goes down in small populations), or lack of physical goods and technology, none of which have much to do with genetic variation.
Another way in which the concept of a MVP is too simplistic is that it is defined with respect to a genetic “equilibrium”—it assumes that conditions have been stable enough that there is a constant level of genetic variation in the population. However, after a sudden population decline, we would be far from equilibrium—we would still have lots of genetic variation from the time the population was large. This variation would start to decay, but as different local populations become fixed for different variants, much of this variation would be maintained at the global level and could be converted back into local variation by small amounts of migration. Such considerations are not usually included in MVP considerations. (Some collaborators and I have written about this last point at it relates to conserving endangered species here)
Perhaps we should keep the term “minimum viable population size” but use a broader definition based on likelihood to survive, period. I see that Wikipedia uses a broad definition that includes extinction due to demographic and environmental stochasticity, but often MVP is used as in the OP to refer just to extinction due to genetic reasons, so it is important to clarify terms.
Very interesting, thanks! Strong upvoted.
This matches what I had tentatively believed before seeing your comment—i.e., I had suspected that genetic diversity wasn’t among the very most important considerations when modelling odds of recovery from collapse. So I’ve now updated to more confidence in that view.
I raised MVP (from a genetic perspective) just as one of many considerations, and primarily because I’d seen it mentioned in The Precipice. (Well, Ord doesn’t make it 100% clear that he’s just talking about MVP from a genetic perspective, but the surrounding text suggests he is. Hanson also devotes two paragraphs to the topic, again alongside other considerations.)
I’d agree that clarifying what one means is important. This is why I explicitly noted that here I was using MVP in a sense focused only on genetic diversity. To touch on the other “aspects” of MVP, I also have “What population size is required for economic specialisation, technological development, etc.?”
It seems fine to me for people to also use MVP in a sense referring to all-things-considered ability to survive, or in a sense focused only on e.g. economic specialisation, as long as they make it clear that that’s what they’re doing. Indeed, I do the latter myself here: I write there that a seemingly important parameter for modelling odds of recovery is “Minimum viable population for sufficient specialisation to maintain industrialised societies, scientific progress, etc.”
I wasn’t aware of these points; thanks for sharing them :)
Thanks for your response and the link to your newer post and the Ord and Hanson refs. I’ll just add a thought I had while reading
This all makes sense, but sounds to me like to be at risk of leaving out the population/conservation biology perspective (beyond genetic considerations). A large part of what motivated me to write my original post is that I do think it is indeed valuable to use frameworks from population and conservation biology to study human extinction risk - but it is important to include all factors identified in those fields as being important; namely, environmental and demographic stochasticity, as well as habitat fragmentation and degradation, which could pose much greater risks than inbreeding and genetic drift.
Yeah, that sounds right. Those factors were left out just because I didn’t think of including them (because I don’t know very much about these frameworks from population and conservation biology), rather than because I explicitly decided to include them, and I’d guess you’re right that attending to those factors and using those frameworks would be useful. So thanks for highlighting this :)
There are probably also various other “crucial questions” people could highlight, as well as questions that would fit under these questions and get more into the fine-grained details, and I’d encourage people to comment here, comment in the google doc, or create their own documents to highlight those things. (I say this partly because this post has a very broad scope, so a vast array of fields will have relevant knowledge, and I of course have very limited knowledge of most of those fields.)