Sibylla Bostoniensis

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[Special content warning: discussion of existential threats to humanity. Highly depressogenic content. This might not be the best thing to read if you're given to despair or if despair is unsafe for you. Maybe give this post a pass if you are not in a place, emotionally, to handle it.]


Previous: Part 0: Introduction



2.

There are three ways that what we can call population development has consequences for the risk of contagious illness in a species: population size, population density, and population connectedness.

There's a very obvious naive idea of how population density impacts transmission of disease: the more closely packed a species is, the more opportunity there is for disease to transmit from one organism to another. This is often translated to a concept of population density that is reckoned in terms of organisms per area unit (such as per square meter). This isn't a completely terrible model, but it has some problems.

Humans are not, say, tigers padding about the country-side on their bare feet, free-range and lonesome. Tigers' evolutionary adaptation for acquiring calories is to spread out – to minimize their population density – to reduce competition for game. Our evolutionary adaptation for acquiring calories is grocery stores, so it doesn't matter the over-all population density of our habitation: we're all, left to our own devices, going to try to cram into the same Giant Eagle on Sunday afternoon. And breathe on one another.

When talking about modeling contagious illness, population density is actually a proxy measure: it's a proxy for how likely one organism of this species is to encounter conspecific (fellow species-member) who is carrying the contagion. But in humans, simple area-based population density is of limited value as a proxy for that. Sure, it will tell you that there's increased chance of encountering a random infectious conspecific in your vicinity, which will, indeed, vary, with the number of fellow humans you are likely to casually encounter going about your life, which, yes, will probably be a lot more in Manhattan than a small town in the back woods of Maine.

But the thing about us humans is, we clump. We may reside far away from most of our conspecifics, but we'll deliberately get in our automobiles and travel many, many miles to the gathering places of our species, because that is what our species has evolved to do. We do it for the pleasure of others' company and we do it because that's how our species handles survival: by specialization of production and economies of scale. Everything we want and need, instead of personally wresting it from nature, we get from one another – and, traditionally, by showing up in person to get it, whether it's a good, a service, or employment.

You may recall, early on in the Covid-19 pandemic, while it burned through certain urban metropolises there was speculation that the more rural parts of the US might be protected from the pandemic by their low population density. But that's not how it works and not how it worked. It doesn't matter, for modeling the spread of an airborne illness, if there is only one human per sq km for a two thousand sq km area, if all two thousand of those humans all go to worship in the same church, or half of them all work in the same slaughter house.

What this reveals is that while pure geographic population density is not without some impact on the transmission of contagion, the temporary population densities we humans (and other species) attain in our migratory patterns are far more significant. That's what we're talking about when we talked about superspreader events: temporary localized peaks in population density that facilitated the spread of Covid.

But how big human gatherings get is, to some extent, a function of how many humans there are to gather. The more people there are to gather, the bigger a gathering can be. Which brings us back to a concept of geographical population density. The annual Fourth of July fireworks in Boston typically are attended by over a million people; in 1776, there weren't even half a million people in the entire state. In turn that's also a function of the total number of humans.

But it's not just how many humans there are to gather that influences the size of our gatherings, it's how many want to gather. And that, in turn, is partially a function of to what extent we have to.

This brings us to population connectedness. Like I said above, our species' way of handling sourcing nourishment is basically grocery stores. Very few humans meet their nutritional needs by prising calories directly from nature by direct food production. Most find some way to do other labor in exchange for money, which they then use to buy food. In other words, those of us who do not create all our ingredients from nature need to have some sort of connection with some entity that will take our money in exchange for giving us food, or we will starve. It doesn't have to be a deep personal connection, we just need to know a guy, or a store (that's an organization of people who are engaged in the enterprise of exchanging food for money).

Now multiply that times every single thing we do for our continued survival: what we do for earning money, what we do to provide care for our kids, what we do to get medical care, and on and on. Plus everything else we do for our well-being and pleasure, like socialize, and entertain, and worship, and celebrate, and on and on. Everything we want to do – until quite recently – was mediated by human contact. Want to earn a wage? Go to work and get breathed on. Want to check out a library book? Go to the library and get breathed on. Want to go bowling with friends? Go to the bowling alley and get breathed on. Want to get married? Go to city hall to get the license and get breathed on then hold a party for your friends and have them all breathe on you.

We don't really think about this – or we didn't – because we just completely take it for granted. That's how our species rolls. And it's got wonderful advantages. I'm not knocking it. But the very interconnectedness our way of living and survival entails induces us to clump more, and increase our temporary local population density, in ways that expose us to more risk of contracting disease.

This is what Covid starkly limned for us in the early days of the pandemic: all the ways we wind up in one another's company in the course of our daily activities, regardless of whether we lived in a city or the countryside.

Obviously, we responded with alacrity to Covid by doing what we could to minimize, as much as we possibly could, the amount of human contact that was needed to do a whole bunch of survival activities. For the first time, there was an obvious compelling reason to find mechanisms to engage in survival and enrichment activities in ways that didn't involve actually colocation, so we mitigated some of that risk. But, of course, not all of it. We found there were some pretty stark limits to the virtualization of contact – for instance, there are jobs that cannot be done remotely.

What we suddenly had shown to us is how much our species depends on these social connections, which have traditionally entailed in-person social contact, for basic survival and functioning.

This illustrates that the more a population is socially connected in its functioning – meaning dependent for basic survival on interactions with conspecifics – the more it will wind up tending to clump, and the more vulnerable it will be to being a super-spreader event, and to contagious disease transmission in general.

Finally, the absolute size of the population of a species matters to the risk of infectious illness spreading through a population, too: every additional human is yet another "attack surface", to borrow a term from computer security, for a contagious illness to get into the human population to begin with.

You've heard of the idea of "six degrees of separation". That may or may not be true (it's complicated) but it captures our awareness that the human social world is vast, but that we are very connected to it. If there are but six degrees of separation between any one of us and any other human, then there are only six transmission events between any one of us and any possible patient zero in the entire human species. Thus our individual risk of catching a contagious illness is a function of how many other humans there are to catch it from, and our species' collective risk of one of us curious hairless apes getting exposed to a novel contagious illness will be a function of how many of us humans there are poking around in places there are novel contagious illnesses to catch, which is itself a function of how many humans there are. The more of us there are, the more of us there are to catch something and spread it to the rest of us.

So there's a lot of face validity to the idea that increased population – absolute size and geographic density and social/material interconnectedness – leads to an increase of infectious disease burden in that population.

But there's also some really disturbing science.




3.

Back in the late 60s or 70s (I have not been able to nail down the actual cite!) a very mathematically-minded evolutionary biologist named Richard Lewontin noticed something interesting.

To recap a bit of basic evolutionary biology: a mutation is a typo in DNA. When DNA is copied – such as when one cell makes another – sometimes there's a glitch. One letter is substituted for another, and the protein it is a blue-print for is made a bit differently.

It is on these errors that evolution proceeds. Is this new way of building protein better than the old way? Often depends on circumstances. If the new protein is better for surviving the circumstances the organism finds itself in, then that organism has a better chance of surviving to sexual maturity and passing its genetic code – those blue-prints, mutation and all – on to to its progeny. If it should prove less helpful, well, nature is not gentle, and it will struggle more to pass on its genes. The mutation that hinders the survival of the organism it occurs in does not get propagated, or less so; the mutation that helps the survival of the organism it occurs in is passed on. This is what is meant by "survival of the fittest". The word "fitness" has come to mean muscular development or health, but orginally Darwin meant "fit" in the sense of fit for a purpose, i.e. suitable. It is survival of the most suitable for a specific place and time and situation.

What Lewontin noticed was this. If the causes of mutations are random – and why shouldn't they be? – then the more organisms there are in a species to have mutations, the more mutations you will have. Imagine an extremely oversimplified model, wherein we could say there is, say, a certain species of microbe which has a 5% chance than any given microbe in that species will reproduce with a mutation. In that case, if you have 20 microbes, you have one mutation, but if you have 2000 microbes, you will have 100 mutations. And if you have 2,000,000 microbes, you'll have 100,000 mutations.

So it stands to reason that the number of mutations in a species should rise with the population size. It should be a linear relationship. It stands to reason the more members a species has, the more mutations – the more genetic diversity – it should have among its members, in pretty direct relation.

But nope. That's not what we find in nature.

This is called Lewontin’s paradox, and it's been vexing evolutionary biologists for half a century or so. Why isn't it the case that genetic diversity varies with the population size of a species?

There's been a number of hypotheses about that. There's one in particular I want to tell you about.

In 2017, there was a paper published in Science, about passenger pigeons. Yes, passenger pigeons.

If you know only one thing about passenger pigeons, you know that we hunted them to extinction. But, yes, we still have preserved dead passenger pigeons, so we can still study at least their bodies – including their DNA.

If you know only one other thing about them, it should be their incredible abundance. Native to North America, they used to be the single most numerous bird species on this continent. Wikipedia:

The pigeon migrated in enormous flocks, constantly searching for food, shelter, and breeding grounds, and was once the most abundant bird in North America, numbering around 3 billion, and possibly up to 5 billion. [...] It practiced communal roosting and communal breeding, and its extreme gregariousness may be linked with searching for food and predator satiation.

They were described as having flocks that blackened the sky.

This paper, "Natural selection shaped the rise and fall of passenger pigeon genomic diversity" was an investigation into Lewontin's paradox:

The extinct passenger pigeon was once the most abundant bird in North America, and possibly the world. Although theory predicts that large populations will be more genetically diverse, passenger pigeon genetic diversity was surprisingly low.

They explain how they did this:

To investigate this disconnect, we analyzed 41 mitochondrial and 4 nuclear genomes from passenger pigeons and 2 genomes from band-tailed pigeons, which are passenger pigeons’ closest living relatives.

Band-tailed pigeons are nowhere near as numerous, and don't live in big groups. They serve as a comparison for their extinct close genetic relatives, the previously profoundly gregarious, once awesomely abundant passenger pigeon. What the researchers concluded was this:

Passenger pigeons’ large population size appears to have allowed for faster adaptive evolution and removal of harmful mutations, driving a huge loss in their neutral genetic diversity. These results demonstrate the effect that selection can have on a vertebrate genome [...]

Notice this sentence: "Passenger pigeons’ large population size appears to have allowed for faster adaptive evolution and removal of harmful mutations, driving a huge loss in their neutral genetic diversity".

There's some trickiness in that language. "Allowed" is a word with positive valence, and "removal of harmful mutations" sure sounds like a good thing. But let's be very clear what is being described here: the way natural selection "adapts" a species and "removes" any gene from a species is by preventing the individuals that carry that gene from passing any of their genes on to offspring. And, overwhelmingly, the way it does that is by killing off the organism before it manages to reach sexual maturity and reproduce.

What they are saying is that, apparently, there was something about having a large population size that increased the evolutionary pressure on the individuals in that species, by tending to kill them off. A doom that didn't stalk their less numerous, less gregarious cousins.

This finding is very interesting and weird, because, presumably, living in huge groups conveyed enormous evolutionary advantage to passenger pigeon, or it wouldn't have evolved to do so. If living in groups was a net bad for passenger pigeon survival, they wouldn't have still been blackening the skies in the 19th century. One of the authors of the study was quoted in a press release as saying, "Passenger pigeons did really well for tens of thousands of years". The species was doing great until we screwed it up for them. So living in huge vast flocks clearly worked for them as an evolutionary adaptation.

And yet, here the researchers were finding evidence that, actually, living in huge groups also had some sort of cost for them. That their otherwise very successful adaption came with a price they paid. Something other than us stalked them, and put them under huge evolutionary pressure not to have genetic diversity.

Or rather not to have specific kinds of genetic diversity. Because what the scientists found didn't quite square with Lewontin's paradox, either.

What they found was that some parts of the passenger pigeons' DNA showed all the diversity you could want. Lots and lots of mutations. But other parts of the passenger pigeons' DNA was extraordinarily homogeneous from bird to bird. That is where the apparent lack of genetic diversity comes from.

The implication is that in the passenger pigeon, no changes or variations in those particular stretches of DNA were survivable. The implication is that any random change to that part of the blue-print for a passenger pigeon resulted in a dead passenger pigeon – in a way that was not true for their closest genetic relatives, that didn't live in huge groups.

So what did these rigidly homogeneous stretches of DNA code for? Three things.

One had to do what what they could eat. Apparently, they had a valuable adaptation in "ability to eat lots of certain foods", to quote the press release. Makes sense: if you and the several hundred thousand others just like you are all trying to forage in the same field, you might need some specific adaptations to not starve to death.

Another had to do with better tolerating stress – presumably social stress. That also makes sense: if living in groups causes social stress (hell, as Sartre observed, is other pigeons) then being able to reap the benefits of living in groups requires a superior ability to survive social stress.

The third?

Oh, their immune systems.

It would seem that one of the things you need to have to survive living in the giant flocks the passenger pigeons did is a really optimized immune system. Indeed, it would seem from how conserved their immune system DNA is, that to survive being a passenger pigeon living among millions of other passenger pigeons, you needed to have an immune system so optimized, so finely tuned, that the least deviation from the optimal plan was fatal.

Their findings suggest three strongest evolutionary pressures on the passenger pigeon were starvation, intraspecies conflict, and infectious disease.

So maybe this is the answer to Lewontin's paradox: there's something about being numerous, itself, that puts evolutionary pressure on a species – and that selective pressure gets stronger the more numerous it is. Maybe the more conspecifics (fellow species members) you have, the more you are at risk of starvation because of competition for food. Maybe the more conspecifics you have, the more you are at risk of violent conflict over constrained resources. Maybe the more conspecifics you have, the more you are potentially exposed to contagious illness. And maybe, these things being true, a species has to evolve adaptions to survive these sources of fatality which only get worse and worse the more and more biological success your species has.

Maybe the reason that big numerous, gregarious species – not just the passenger pigeon, but in general – don't show the anticipated genetic diversity is because they can't afford it. Maybe for all big, numerous species, as for the passenger pigeon, it becomes too risky. They're already up against the ropes of survival already. There's no more give in the system. Like fine race cars, their DNA, some of it at least, is optimized to run within very narrow tolerances.

What this suggests is that population growth is essentially and intrinsically lethal, and therefore population growth itself is something that a species has to evolve some sort of solution to, or they will not survive their own reproductive success.



4.

We are not used to thinking of evolution this way. When we imagine an organism being fit for its environment, we imagine its environment being all the other sorts of organism it interacts with – its predators and its prey, its competitors and its symbionts – and inorganic forces around it like the weather. That's what we think of as what puts evolutionary pressure on species.

But for a sufficiently social species, its own kind is its primary environment. That is the environment to which it must be fit, and the environment which puts the most evolutionary pressure onto the species. The one environment a social organism absolutely must be able to survive is its own society.

We're not used to thinking of society as a source of evolutionary pressure, because properly conceived of, as I explained above, "evolutionary pressure" means organisms getting killed off and we aren't given to thinking of society, in the abstract, whether ours or other species', as a source of threat to the individual. Especially not in evolutionary terms. This is probably an anthropic bias: we're a social species, so we're given to like "society" as an abstract concept and think only good things of it. Society, to our social ape souls, is security!, and economies of scale!, and abundance of resources! and cognitive specialization! How could society put evolutionary pressure on a species, if it's only a benign force?

Well, it would seem that society, as wonderful as it is, is not only beneficial. That's what these findings in the DNA of passenger pigeons suggests: that as wonderful and advantageous for a species as gregarity is as an evolutionary strategy, it has a serious shadow side. There's several things intrinsic to being a highly numerous social species which threaten the lives of individual members of the species, and that as a species – especially a gregarious one, especially a social one – becomes more numerous, those risks increase, and put even more evolutionary pressure on that species.

So maybe the reason that genetic variation doesn't increase linearly with population size is because in any species there's these countervailing forces that also grow with population size: starvation, strife, and infectious disease.



5.

This research into passenger pigeons was to investigate Lewontin's paradox, which is not specific to passenger pigeons. Lewontin's paradox is not about passenger pigeons, but about all animals.

Which is kind of unfortunate for us, because we're animals. And when researchers went looking at passenger pigeon DNA to figure out what might be causing Lewontin's paradox, what they found – that the evidence suggests big population causes evolutionary pressure through increasing starvation, strife, and the proliferation of infectious disease – might apply to us too.

There were some 3 to 5 billion passenger pigeons, but there are 8 billion humans.

We, too, are an enormously gregarious species that lives in huge contact networks.

It would seem we, too, are subject to the same evolutionary pressures as shaped the DNA of passenger pigeons.

You might not have ever thought of it this way, but we've been fighting these three threats to our species, all along, as we have proliferated down through the ages.

The whole of history of agriculture is the history of fighting famine and providing food security for our species. It would be hard to overstate the advances in food productivity of 20th century agrarian science. I remember hearing on TV as a child that at the start of the 19th century something like 80% of the US population worked in agriculture, and that "today" (in the 1970s) less than 2% of the US population "was needed to work in agriculture to feed the whole country". There are things wrong with that sentence, but the underlying idea is sound: so long as industrialized society endures (which admittedly may not be long) we need not fear running out of food, because we have become masters of food cultivation.

We've been less successful in our war against war. Arguably the history of political and legal philosophy has been the history of reducing interpersonal and international conflict among larger and larger populations of people. We live in one of the most peaceful times in history, which is not the comfort we would like, given how unpeaceful it is, but it is also not nothing. And there's an argument to be made that that is what religion has been up to, not because religion is nice or pacifist (it is neither), but because above all, a religion survives and propagates that convinces its members to make common cause with their coreligionists.

Obviously we've been fighting disease with all the might of 20th and 21st century medical science... okay, with all the might of 20th and 21 century medical science that we were willing to fund, which was regrettably less than, in retrospect, we perhaps ought to have. But the fight didn't start with the discovery of the microbe. Arguably the history of civilization – literally "city-fication" – is the history of sewage management, that being the historical limiting factor on how many people can live in close proximity before they start dying of, e.g., dysentery. We've been industriously bent to the project of fixing the way our own waste tends to kill us when we congregate for as long as we've congregated.

These things we, as a species, have been doing deliberately, intentionally, with our big, big neocortices. That's been our chief adaptation against starvation, strife, and infectious disease.

There are others, of course: our teeth are a testament to our evolution to wrest calories out of just about anything organic we can fit in our mouths, and our opposable thumbs evidence of our evolution of the ability to rend large things smaller so they fit in our mouths; our evolved capacity for language, both larynx and brain, exists for no other reason than to allow humans to make common cause instead of killing one another; and our hairlessness has been argued a sanitary adaptation, like the bald heads of vultures. We have a bunch of things going for us, evolutionarily speaking, for living in groups without group living killing us.

But mostly, we've left those physical adaptations behind, instead fending off the evolutionary pressures of population on our bodies with our minds. We have gotten into an arms race with starvation, strife, and infectious disease whereby we are trying, wholly and solely, to win by out-smarting them.

That's how we've gotten away with our population hockey-sticking as it has. So far.

Unfortunately, as our population grows and with it grows the natural selection pressures of having a large population, we're not getting any smarter.

We may have gone down an evolutionary dead-end, like (as has been theorized) the peacock growing larger and larger and more and more unmanageable tails. Our species evolving intellects to fight off the negative consequences of numerical success (and congregate survival strategies) might not be an ultimately successful adaptation, because our intellectual capacity, as a species, doesn't seem to be scaling adequately with our population size.

Which brings me back to the passenger pigeons. The researchers on that genetic diversity paper theorized that that genetic optimization for living in large groups may have been what actually doomed them:

The passenger pigeon is famous for the enormous size of its historical population in North America (estimated at 3 to 5 billion) and for its rapid extinction in the face of mass slaughter by humans. Yet it remains a mystery why the species wasn't able to survive in at least a few small, isolated populations.

One theory, which is consistent with the findings of [this] study [...], suggests that passenger pigeons were well adapted to living in huge flocks, but poorly adapted to living in smaller groups, and the change in population size happened so fast they were unable to adapt.

It would seem that the passenger pigeons, as a species, pursued an evolutionary "strategy" that required them to bet big on innate resistance to the three dooms that that strategy courts.

We, as a species, have also put all our evolution eggs in the basket of living in huge groups, and have bet on general intelligence to protect us from those three dooms, by being smart enough to figure out, intellectually, how to protect ourselves from starvation, strife, and infectious disease.

I have to say, nothing about the last two and a half years have left me feeling good about the human species' evolutionary trajectory relying on species-wide intelligence.

As a side note, an essential lethality of population growth may not just explain Lewontin's paradox. It may explain Fermi's.

As survival strategies go, generalized intelligence may prove merely enough rope for a species to hang itself. Intelligence may not be able to scale up fast enough to keep up with the rising of selection pressures that ensue from the rapidly rising population, chief among them the increased incidence of contagious illness.

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The Great Age of Plagues
Table of Contents

• 0. Intro
• 1. Population – You are here
• 2. COVID-19
• 3. Climate Change
• 4. Climate Change, II
• 5. Conclusion

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