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Cultural innovation leads to small populations being successful; small populations just lead to extinction

In a pretty informative piece in Gizmodo, Scientists Say New Research Tracing the Origin of Modern Humans to Botswana Is Deeply Flawed, there is an interesting quote that I would like to follow-up on:

That said, Curtis Marean, a professor of archaeology at Arizona State University who wasn’t involved with the new research, is not a fan of the idea that multiple origin locations exist for modern humans. In an email to Gizmodo, Marean said this idea doesn’t “make sense at all,” and that evolution “happens the quickest in small isolated populations—we know this—so that will always remain the favored hypothesis, until proven otherwise.”

The intuition that evolution happens fast in a small population is a common one. And, on some level it has justification. As per the neutral theory, the time until the fixation of a new allele is proportional to the effective population size. Basically, in small populations frequencies can change faster due to drift.

The idea is that you can “random-walk” your way into innovation. A lot of this thinking draws consciously or unconsciously from the shifting balance theories of Sewall Wright, where small populations subject to higher drift can stochastically explore more of the evolutionary parameter space than a large well-mixed population. The problem as outlined in Will Provine’s Sewall Wright and Evolutionary Biology is that the shifting balance theory operated as more of a metaphor than a formal model that could be tested. In The Structure of Evolutionary Theory Stephen Jay Gould argued that Wright deemphasized drift and played up gene-gene interactions in his later years because of the expectations of the Modern Synthesis. But this was possible probably because as Provine documents Wright was not always clear himself how the shifting balance worked. When drift became less fashionable he tweaked the shifting balance accordingly.

Gould was probably the most influential in promoting the idea that small populations are where the “action” in evolution is to the general public. Many evolutionary biologists demurred on this. The reasoning is that small populations may evolve fast due to drift, but drift can lead to fixation of deleterious alleles, and selection is more effective in large well-mixed populations. Additionally, in structured meta-populations, you may have scenarios where drift is periodically powerful, but a great deal of adaptation occurs during periods of mixing, so that drift may not be consequential.

Probably the dominant position in regards to speciation and differentiation of populations in an evolutionary genetic framework is that allopatry matters. Separation. Isolated populations are necessarily allopatric in relation to parent groups, but you can have two large populations that are separated as well. Rather than a small population leading to some sort of biological innovation, I suspect it will more often lead to extinction.

Which brings us back to humans, and the intuition that modern humans derive from a small distinct population. We know from ancient DNA that many human populations were often quite isolated and inbred. This holds for many Neanderthals, Denisovans, and Mesolithic European hunter-gatherers. These groups were on the edge of the range of humans, so it’s not surprising. And, it looks like there was a lot of local extinction of these populations. The archeology and genetics imply that it was Africa, with larger numbers of hominins, which was the source for new populations several times (the ancestors of Neanderthals and Denisovans replaced earlier hominin groups, and arrived from Africa ~750,000 years ago).

There are human groups that went through bottlenecks or were isolated. The ancestors of all non-Africans, for example. The ancestors of Amerindians. The ancestors of Oceanians. You can see this is aspects of the allele frequency; these populations have undergone more drift due to small populations. There is no evidence that I know that isolation was evolutionarily beneficial in a biological sense (in The Dawn of Human Culture Richard Klein leveraged Gould’s ideas to propose that a small ancestral human population was subject to a macromutation which allowed for recursive language).

Rather, let me suggest that cultural changes or geographical contingencies lead to evidence of bottlenecks being associated with successful populations. ~15,000 years ago a group of humans managed to make it to the Americas. This was a small ancestral group which by chance was afforded the opportunity to expand their range very fast. Such a demographic expansion from a small founding population has left its impact on the genetics of Native Americans. The genetics of the small isolated group did not lead to demographic expansion. Similarly, ~4,000 years ago Austronesians began to expand their range through long-distance voyages. The bottlenecks left an imprint genetically. But again, the source of innovation was not genetic isolation, genetic isolation emerged due to cultural innovation allowing for rapid fission.

Humans, at least before the Holocene, were not numerous. Meta-population dynamics and periods of isolation between demes seems to have been normal. Rather than isolation resulting in laboratories for experimentation, I suspect most of the time it just led to extinction for many groups. But not all groups. And because small isolated populations tended to be on the margins of the species’ range, you have scenarios such as where the ancestors of non-Africans expanded very fast and were quite successful (the expansion of Amerindians and Oceanians was similar). Additionally, while small populations may not be beneficial for biological evolution, the conditions for cultural revolution are different. Peter Turchin has observed, along with many others, that some of the most innovative and energetic populations tend to be on the fringe or boundary of a broader cultural zone (e.g., the rise of Rome and Qin from the fringe of the Classical and Chinese worlds).

4 thoughts on “Cultural innovation leads to small populations being successful; small populations just lead to extinction

  1. I completely agree with your notion of large scale human (meta-) populations and ecosystems creating a uniform, large scale habitat being more interesting for a beneficial evolution, for two reasons:
    1) There are simply more people and therefore genetic variants in a large, densely populated region
    2) If a people are isolated, they only compete with themselves individually and the environment! That’s the main reason for a lot of isolated Hominids evolving in a rather one sided, less competitive direction and they went extinct as soon as the isolation was ended.

    Its the same with a lot of animal species too. They adapt to, e.g. an island. They evolve slow and develop a one sided set of traits solely adapted to the very small niche they are living in.

    The same couldn’t be done by humans in larger habitats of Africa and even less so in Eurasia once Homo sapiens was out. Also because of human group selection and war. If a group was not competitive, it was pushed away or even going extinct.

    So the best chances for a generally adaptive, classic human package of traits coming up is in large populations with many subpopulations which compete with each other.

    Obviously it is not on small islands or dense rainforests, usually not in deserts and dry Savannah as well. Not for the culture and not for the genetic package. Because they lack in their isolation a large enough genpool, oftentimes the energy and food resources too, but also the group competition for the best genetic and cultural assets if talking about an expansion.

    There might have been extremely conservative groups of early Homo sapiens in the Near East, which didn’t change and isolated themselves from others. But contrary to some islands and remote places in general, sooner or later, they were faced with human competitors. And if they were not adaptive enough, what they usually wouldn’t have been, they might have left and obscure archaeological culture and remains behind, but nothing else. The longer a fairly small human population was completely isolated, the less competitive they became and the higher the chance they were going extinct once their isolation was no more. Human genes and culture are always competing.

    Competition is key for the human evolution. Adapting to a “natural environment” was not, because that kind of success and ability to survive in diverse environments achieved early Hominids already.

    But of course, there could have been extreme situations in isolation, leading to generally favourable traits in exceptional cases (!). So its still possible, but its the exception, not the rule. And then people have to prove why this environment and subpopulation was superiour in developing classic Hominid traits better than elsewhere.

  2. Isn’t there often some introgression advantage in the successful small populations, perhaps most frequently in the HLA area of chr 6, but also elsewhere … all the talk about the adaptive Neanderthal / Denisovan introgression, the outsize role of African HLA haplotypes in Native Americans, the not-yet-well-characterized North-East Asian admixture in the Ashkenazim, the strange HLA spectrum of the Quebecois? And perhaps in other species as well?

    Perhaps being admixed for higher diversity after, or even before, the bottleneck event is a helpful factor for success?

  3. Perhaps being admixed for higher diversity after, or even before, the bottleneck event is a helpful factor for success?

    yes i think basically one model you could posit are cycles of structure/separation and admixture. the admixture phases could then be subject to strong selection due to new variants.

    if you had a fully mixed population though selection could happen all the time. i guess the issue is how much gene flow diminishes the dynamic in the face of localized selection coefficient.

  4. @Dx: Obviously it depends. The question is: What kind of variation is useful in the given situation short term and for adaptive qualities long term. Oftentimes these two factors can contradict each other, because evolution has never a long term plan, but just “knows” short term adaptation. Whether this will end in a dead end or not, the next step in the same direction will lead you to the abyss, is of no importance to your current situation for which you adapting.
    That’s why from our human perspective, something like “negative selection” is thinkable and real. Because we can anticipate future developments – at least its in our (future?) potential.

    If you have the constant pressure from other groups of your genus and species, you are simply less likely to fall into the trap of an extreme niche adaptation, which is more likely to bring you to extinction on the long term.

    Actually, if a trait is really successful, it will spread in a small and large population alike. Examples being pigmentation, lactase persistence, intelligence and immune factors. If selection favours the advantageous trait in a region, a given context, you don’t need an extraordinarily isolated population for its spread. Even on the contrary, some of these genetic advantages will spread even with minimal admixture events from one population to another without subsequent isolation. On the other hand, if selection doesn’t favour the traits in question, for whatever reason, they will never spread, with or without isolation.
    So the decisive factor is always selection.

    If the admixture event occurred before a bottleneck, this is just a more ideal scenario which might result in the negative aspects of the admixture being purified quicker and more efficiently. But then again, it all depends on the selection. Because if the selection doesn’t purify and simple drift events change the frequencies, you might end up with a negative recombination instead!

    That’s something all too often neglected in the current debates, whether any sort of admixture event is positive or not depends in the majority of cases solely on selection. Because most populations will have advantageous traits another one doesn’t have, even if their overall profile will be less positive. But the outcome is not predetermined, its the selection on the mixed breed which will make it an improvement or a degradation.

    Because after a large scale mixture event, nothing of either side will be lost. This means even if one side has 90 percent more favourable traits, the other just 10, it doesn’t matter, if selection produces exactly that kind of end result or something close to it, like let’s say 85 : 15. Might still be better than 100 percent of both originals phenotype. But if its 50 : 50, no selection purifies, it will be a degradation for the given context.

    The Neandertal and Denisovan examples are perfect, because a large portion of these human species/subspecies genetic variation was rather detrimental for human hybrids and general development. But still they seem to have had “something to offer”. Neandertals at least, Denisovan advantages might have been rather low, considering how little survived in the large metapopulations. It was about the purifying selection to make the difference in any case.

    By the way, I would call them species, because we don’t know the hybrids fertility and whether the sexual contact was natural from either side, or rather a violent, one sided accident for one, because their behaviour and sexual preferences were already too different for a regular intercourse.

    So isolation is no major factor, but it makes it easier to keep innovations and regional adaptations alive and frequent, especially if genflow from a less adaptive population took place already and “the best parts were taken” already. Any additional admixture event would just result in a reset, resulting in lower quality and the need for stronger selection and more suffering to reach the status quo ante.

    In this sense the best thing for most populations would have been regular, small scale admixture events which introduce innovations from other populations. Then these new variants could be “digested” and reshaped to the needs of the given environment. Constant admixture will destabilise, complete isolation makes you less adaptive.

    So the ideal situation is, like I said above, a large meta-population with significant structure in a favourable environment for human development. Between its subpopulations LIMITED REGULAR genflow takes place. So that they constantly update each other, without destroying the regional advantages and larger variation of the metapopulation.
    The only exception would be that one of these subpopulations reaches a level of adaptive quality, beyond its regional adaptation, that its time for a large scale replacement. Then this new group/subspecies/species replaces most of the other subpopulations (like Homo sapiens did) while still taking up, from the remains of the other subpopulations, what’s useful.
    They settle down, regional variants and subpopulations come up and the same game starts anew with limited isolation and limited genflow. That’s, quite obviously, the best for a species like humans occupying most of the world.

    If a population being not part of a large metapopulation, it will, most of the time, fall behind. Because they miss the upgrades circulating in the centers of development and spreading there because of human competition. The most important upgrades are those which are adaptive beyond the regional climatic, ecological character of the habitat.

    Neandertals were adapting to the West Eurasian climate for hundreds of thousands of years, it didn’t help because they missed most upgrades for human competition and development happening in the then all important metapopulation in Northern, North Eastern Africa or the Near East. They could contribute something of value to the new Eurasian metapopulation, but they were no match for it overall.

    Apparently only after Homo sapiens reached a certain level of development. Because as we know now, they couldn’t outcompete Neandertals before. This means to me, that some kind of upgrade took place in between the earlier and later contacts of Homo sapiens and neanderthalensis. This upgrade was still important enough to give the new and fully developed sapiens the edge over not just the other species, but also older layers of an earlier sapiens colonisation outside of Africa/Near East. If the current narrative is true in all details.

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