Book Club: Darwin’s Unfinished Symphony, 6, gene-culture co-evolution

lactose tolerance

map of lactose tolerance

Kevin Laland’s book, Darwin’s Unfinished Symphony, which I have been discussing for several posts now, is basically one long argument in favor of the thesis that human evolution has been shaped by a feedback process involving a cultural drive mechanism initiated by natural selection, a mechanism that favored the acquisition of accurate and efficient copying. Chapter 9, to be examined here, is devoted to the classic approach of gene-culture co-evolution, the fundamental notion that cultural changes affect genetic evolution, and indeed that the more time passes the more human evolution is increasingly driven by culture and less so by biology (though biology always remains a fundamental constraint to be reckoned with):

“Genetic propensities, expressed throughout development, influence the cultural traits that are learned, while cultural knowledge, expressed in behavior and artifacts, spreads through populations and modifies how natural selection affects human populations in repeated, richly interwoven interactions.” (p. 217)

While the chapter begins with an interesting treatment of the phenomenon of right-handedness, the standard example of gene-culture co-evolution is, of course, lactose tolerance. In most humans, the ability to metabolize milk disappears in adulthood, as it was not pre-historically needed. But some populations have large numbers of adult individuals that retain a functional version of the gene coding for lactase activity, resulting in the phenotype of lactose tolerance. We now know that lactose tolerance evolved independently at least six times, and that this happened after the switch to agriculture following the last glaciation, making it a strong candidate for culture-driven genetic change in humans. Interestingly, mathematical models show that the rapidity of spread of the genetic trait depends on the fidelity of transmission of the cultural one: the more likely children of milk drinkers are to become milk drinkers themselves, the stronger the selection coefficient favoring the continued expression of the lactase gene into adulthood.

Several other traits have been shown to have evolved in a similar fashion in recent human history, including genes involved in skin pigmentation, salt retention, and heat stress, all obviously related to the sorts of climate changes experienced by human populations during their migrations. Unfortunately for us today, some of these strongly selected genes facilitate a highly efficient usage of food sources, as well as storage of energy into fats. Hence the trouble that many moderns are experiencing with obesity, leading to diabetes and heart problems, among other negative effects. Another fascinating example is the sarcomeric myosin gene MYH16, expressed mostly in the jawbone. A sizable chunk of the gene has been deleted, leading hominins to lose a lot of jaw muscles. This genetic event occurred at about the time we invented cooking, which made strong jaw muscles unnecessary (and likely metabolically expensive). And of course, many genes involved with brain development, particularly the neocortex, are now known to have undergone very strong positive selection in recent time.

As Kevin is careful to point out, none of this means that natural selection stopped working in humans. So long as there will be differential survival and reproduction, selection will be active on our genomes. But its mode and tempo have been dramatically altered by the onset of cultural evolution, which has become a drive, rather than an outcome, of natural selection in our species. As Laland puts it:

“Theoretical models consistently find that gene-culture dynamics are typically faster, stronger, and operate over a broader range of conditions than conventional evolutionary dynamics. … This picture of the evolution of the human mind is radically different from the portrayal advanced by evolutionary psychologists and many popular science writers.” (p. 239)

I think Kevin is a bit too mild when he discusses the limitations of evolutionary psychology (whose initial central hypothesis, a massive modularity of the human mind, has now been definitively rejected empirically). He states that current research in gene-culture co-evolution shows that the degree of mismatch between our genetic endowment and our culturally created environment is “far more limited” than evopsych authors envisioned. I’d say that’s a dramatic understatement, but certainly still an observation that should lead serious evolutionary psychologists to revise a great deal of what they are doing, abandoning the increasingly silly idea that the Pleistocene was a crucial “environment of evolutionary adaptedness” (EEA), as if the genetic evolution of Homo sapiens had suddenly stopped at that point in time.

“Far from being trapped in the past by an outdated biological legacy, humans are characterized by a remarkable plasticity. Our adaptiveness is reinforced by both cultural and biological evolution.” (p. 240)

If people who write about evopsych were to take this conclusion a bit more seriously, especially when they write for a general public, there would be a lot less garbage floating around the pop science literature. But I ain’t holding my breath…

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112 thoughts on “Book Club: Darwin’s Unfinished Symphony, 6, gene-culture co-evolution

  1. Robin Herbert

    If I put it this way :

    X drove changes in Y
    X happens in many diverse ways
    The changes that X drove in Y were almost identical.

    That appears to need at least some unpacking.

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  2. Liam Uber

    The great value of Laland’s book for me is that it brings together, in an easily accessible format, a vast amount of scientific information for the non-specialist. But we must not forget that new information will likely lead to major revisions for many decades to come.

    I just read in the newspaper that multiple Neanderthal sites have been found in Spain, predating the arrival of H. sapiens, and that exhibit artistic expression as in cave drawings. This may be very significant, perhaps indicating that Neanderthals had greater cognitive abilities than we had thought. The possibility of an as yet unidentified human ancestor is suggested. The only hominids and primates that flourished were the ones that were not in direct competition for our niche, or that of our direct ancestors?

    Liked by 1 person

  3. Robin Herbert

    Bunsen Burner

    After all, we will always have a certain percentage of people that lack certain cognitive faculties due to the statistical nature of genetics.

    That is what I would have thought would be the case too, but apparently gene flow means that this is not necessarily the case.

    That is the puzzle I am trying to get at. I would have expected there to be some diversity of cognitive function. Why isn’t there any, as far as anyone can tell? Why isn’t there even a diversity of sense of humour?

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  4. Robin Herbert

    Massimo,

    To clear up my confusion, are you or aren’t you denying that we would expect to see some diversity of cognitive function between populations that have evolved in different geographical areas around the world?

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  5. Massimo Post author

    Robin,

    “You are saying that there can be gene transmission to a population that no one knows exists.”

    How on earth can you make that claim? Especially when there is genetic evidence of admixture pretty much everywhere among human populations?

    Liked by 1 person

  6. Massimo Post author

    Robin,

    “why, if culture drove genetic change and culture is diverse, why wasn’t the genetic change that it drove also diverse?”

    Because there is no simple, linear relationship between the two. As Kevin says in the chapter, genetic evolution is much slower, and human brains are incredibly behaviorally plastic.

    Liked by 1 person

  7. Massimo Post author

    Robin,

    “That is the puzzle I am trying to get at. I would have expected there to be some diversity of cognitive function. Why isn’t there any, as far as anyone can tell? Why isn’t there even a diversity of sense of humour?”

    The answer to the first question is, again, plasticity. The observation about humor is simply incorrect. There is obviously cultural variation in humor, since what’s funny depends on cultural context. I’ve lived in the US 27 years, and I still dont get some American humor.

    Liked by 2 people

  8. Robin Herbert

    Massimo,

    I am not clear about what you are disagreeing with me about. Lets clear this up. Bunsen Burner said:

    After all, we will always have a certain percentage of people that lack certain cognitive faculties due to the statistical nature of genetics.

    Do you agree that this is an expected consequence of there having been significant evolution in cognitive function in recent history?

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  9. Massimo Post author

    Robin,

    “To clear up my confusion, are you or aren’t you denying that we would expect to see some diversity of cognitive function between populations that have evolved in different geographical areas around the world?”

    You mean genetic based diversity? No. Again: humans are very genetically homogeneous, because of gene flow. Also, high behavioral plasticity, with the consequence that cultural evolution is far mpre rapid than genetic evolution.

    Liked by 1 person

  10. SocraticGadfly

    Oh, speaking of genetic convergence plus gene-culture coevolution (the culture being migrating into particular places to live) — many Southeast Asians have something somewhat similar to sickle-cell. It offers somewhat less protection, and is based on a different genetic variation.

    Liked by 2 people

  11. SocraticGadfly

    Robin, per human nature, Tribe 1 had a war with Tribe 2. Some man from Tribe 2 captured a woman from Tribe 1, had a baby, etc. That baby’s son later was part of a Tribe 2 war with Tribe 3; he captured a Tribe 3 woman, etc.

    Wash. Rinse. Repeat.

    I don’t know what else people can say to you on this issue.

    (Note; That’s not the only way genetic homogeneity is maintained, but it’s one obvious one.)

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  12. synred

    You are saying that there can be gene transmission to a population that no one knows exists

    Adjacent group to adjacent group, genes diffuse. People at one end likely don’t know about those at the other. Ten thousand years is pretty a long .. a lot of spread can and did happen…

    And there’s plenty of actual moving too. The pacific Island’s where only colonized in the last few 1000 years.

    Liked by 2 people

  13. synred

    and human brains are incredibly behaviorally plastic.

    which is exactly the general purpose adaption evolving…

    If we have been evolving to have increasing plastic behavior, you’d expect us to be plastic in our behavior…and as result vary from place-to-place…

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  14. Robin Herbert

    Massimo

    How on earth can you make that claim? Especially when there is genetic evidence of admixture pretty much everywhere among human populations?

    It is just what I read in the history of the area. In 1935 the colonial authorities which had control of the region for around 50 years had no idea that there were even people there.

    As I understand it genetic studies show the highland people of New Guinea show no evidence of non Sahul ancestry, while the lowlanders show evidence of a small genetic inflow from the rest of south east Asia.

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  15. Robin Herbert

    Massimo

    The answer to the first question is, again, plasticity. The observation about humor is simply incorrect. There is obviously cultural variation in humor, since what’s funny depends on cultural context. I’ve lived in the US 27 years, and I still dont get some American humor.

    Yes, it depends on cultural context and not at all on genetics. That is just what I am saying.

    A lot of Americans don’t get some American humour. I don’t get a lot of British humour.

    But yet I and a New Guinea Highlander can share a joke and both find it funny.

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  16. Bunsen Burner

    Given that these recent traits occurred during recent history, wouldn’t we expect to see a time in the archaeological or even historical record where a population had these genes and no other population did? Has anyone looked into this? Can we match up any significant cultural transformations with the development of these recent traits?

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  17. Bunsen Burner

    There may be some crosstalk here with Robin’s inquiry. I believe he is asking a question about variability across populations. I , however, am interested in variability within a population. I always though that every heritable trait had a non zero variance in a population. However, it seems to me at least, that you are arguing that in Human populations its more normal for this variance to be zero? Is there an a priori reason for this? Can geneticists give a theoretical account of which heritable traits will have zero variance, and which, like lactose tolerance, will have non zero variance, even in a single isolated population?

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  18. Massimo Post author

    Robin,

    “After all, we will always have a certain percentage of people that lack certain cognitive faculties due to the statistical nature of genetics.

    Do you agree that this is an expected consequence of there having been significant evolution in cognitive function in recent history?”

    The claim is too vague for me to evaluate. I’m sure there are some differences among people, but they are not systematic by population. We know of major mutations that impair cognitive functioning, but again those are individual-level phenomena, not evolutionarily significant.

    Also recall that humans went through a fairly recent bottleneck, where the global population was dramatically reduced. That’s the sort of thing that in part accounts for the little genetic variation we see. And gene flow, again.

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  19. Massimo Post author

    Robin,

    “It is just what I read in the history of the area. In 1935 the colonial authorities which had control of the region for around 50 years had no idea that there were even people there.”

    For the last time: that’s entirely irrelevant to the issues being discussed. No bearing whatsoever. Nothing.

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  20. Massimo Post author

    Robin,

    “But yet I and a New Guinea Highlander can share a joke and both find it funny.”

    Good for you. But, again, irrelevant to any story about gene-culture coevolution.

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  21. Massimo Post author

    bunsen,

    there is a well developed branch of human population genetics that tracks historical gene flow and compares it with archeological findings. The map accompanying this post (on lactose tolerance) is one example.

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  22. Massimo Post author

    As for the degree of genetic variation in humans, there of course is some, and it’s definitely not zero. But it is much lowered that comparable species of primates, because of the panmixia of humans (high level of gene flow). This also means that variation across populations is a fraction of variation within populations (i.e., it’s hard, though not impossible, to tell, genetically, where a human being comes from, because we are all so similar).

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