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

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…