A piece in yesterday’s science section of The New York Times drew my attention to a new article by Laland et al. in Nature Reviews Genetics summarizing the intriguing reciprocal interactions between human culture and human genes. It’s an interesting article, well worth reading if you’d like to see the recently-acquired evidence for natural selection in our species, and how that selection might have been affected by our culture (and vice versa).
In principle, any species that has some kind of culture (which the authors define as “information that is capable of affecting individuals’ behaviour, which they acquire from other individuals through teaching, imitation, and other forms of social learning”) can show these mutual interactions between DNA and culture: the so-called “gene-culture co-evolution.” And any species that can change the environment through its own activities, like moles digging burrows, can be viewed through the lens of “niche construction,” in which an organism actually affects the ways that natural selection impinges on it by their own behavior, evolved or otherwise. Once moles “decided” to go underground (and certainly the initial stages of that “decision” may have been based partly on genes), they then subjected themselves to all kinds of novel selection. One is selection to lose or reduce their eyes, which aren’t needed in a dark tunnel and, indeed, can be a source of infection, or use metabolic energy that could be more profitably diverted to other traits.
These phenomena may be especially important in humans since we have a rich culture, much of which does not rest on genetic adaptations, and that culture can affect our susceptibility to natural selection. We have “invented” doctors and antibiotics, for example, so infections that would have felled our ancestors (and selected for resistance to bacteria) is no longer so strong. Laland et al. discuss several examples like this.
The article gives examples of each phenomenon. Perhaps the best example of gene-culture coevolution is one that I discuss in WEIT: the evolution of lactose tolerance in those “pastoral” human populations that keep cows, goats, and sheep for milk. This was a cultural change, but one that seems to have profoundly influenced the evolution of at least one gene.
Whether or not you can digest lactose (milk sugar) depends on whether you have one of two forms of the gene producing the enzyme lactase. Ancestral human populations were lactose intolerant as adults: while babies obviously needed to drink milk, adults did not, and so the gene producing lactase (the enzyme breaking down lactose) became inactivated during development, as it still does in many humans. This is the form of the gene that produces lactose-intolerance in its carriers.
(Analysis of DNA from ancient human bones showed that, as predicted, they had the “intolerant” form of the gene: a cool result.)
But, starting about 9,000 years ago, some human populations became pastoral, and those populations now show a high frequency of lactase persistence: the gene breaking down lactose is not turned off, so that adults can digest and get nutrients from milk sugar. So they have the “new” derived form of the lactase gene, the “tolerance” allele. Groups that historically were not pastoral, like many Asian and African populations, retain the ancestral intolerance allele—the one that gets switched off in adults. The difference between the “on” and “off” versions of the lactase gene is based on a simple DNA change in its regulatory region.
What is nice is that we can, through population-genetic analysis, get an idea of when the new “switched-on” form of the gene arose by mutation. It was between 3,000 and 8,000 years ago, making its appearance and rise coincident with the rise of pastoralism—and the huge energy advantage of drinking milk. One can draw a pretty strong conclusion that individuals able to use this rich new source of food—a source deriving from the cultural adoption of pastoral behavior—had a selective advantage over non-tolerant individuals. You can even calculate this advantage from population-genetic analysis of how fast the “tolerant” gene increased in frequency. Apparently, tolerant individuals in pastoral cultures would have left 4-10% more offspring than non-tolerant individuals, probably because they were better fed. This is pretty strong selection, and would have promoted rapid genetic change.
The evolution of lactose tolerance is a splendid example of gene-culture coevolution. The authors give others. One that I didn’t know about was the correlation between yam cultivation in Africa and the frequency of the sickle-cell hemoglobin allele, which in heterozygous form confers some protection against deadly malaria. Apparently those human populations that cultivate yams, which involves cutting down forest, have a higher frequency of this allele than those who don’t grow yams. This genetic difference is ascribed to one of the byproducts of yam culture: the creation of pools of water that serve as breeding sites for mosquitoes.
The authors also suggest a hypothesis that I’ve long found appealing (see WEIT): many of the superficial physical differences between human populations—those differences affecting hair texture, facial features like eye and nose shape, and the like—could reflect sexual selection, but sexual selection based on cultural preferences. I like this idea because, first, differences in appearance between human ethnic groups must have arisen very quickly, since they presumably weren’t present when our ancestors migrated out of Africa less than 100,000 years ago; and sexual selection motivated by cultural preference can be incredibly strong. Second, genetic differences between human groups are much larger for genes affecting external physical traits than for genes in general, which are pretty homogeneous among groups. To me, this again suggests sexual selection.
The authors give a big table of genes (Table 2) in our species that may have experienced natural (or sexual) selection deriving from human culture. There are lots of them, and of course the evidence is stronger for some than for others. Population-genetic analysis that can pinpoint “the footprints of selection” on DNA sequences is not perfect, and there can be false positives. I, for one, am dubious about the MYH16 gene. The authors note that “this gene is expressed primarily in the hominid mandible, and its loss is thought to result in a massive reduction in jaw muscle, with a timing that may coincide with the appearance of cooking.” This may be true, but showing that its loss really did have that phenotypic effect may be hard (it’s impossible to do those experiments in humans!), and showing that its evolution may reflect relaxed natural selection for chewing ability even harder. Such cases may always remain speculative.
Nevertheless, the idea of gene-culture coevolution is a good one, and certainly must explain some recent cases of selection in our species. Likewise, the idea of “niche construction” bears thinking about, though I think its ubiquity may have been overstated by some of its proponents. Yes, some species can affect their own evolution through their behavior, but in other cases species must certainly be passive responders to the environment. Does the hoof of the chamois really have an effect on the tensile properties of Swiss granite? Do those Lithops plants that resemble stones have any effect on the appearance of the stones themselves? Does the polar bear’s white coat have any effect on the reflective properties of ice and snow? (I suppose one could counter that the bear subjected itself to coat-color selection by “choosing” to live in cold climates—if it did!).
At any rate, if you’re not up on the latest evidence for selection in our species, and how that selection may have been driven by our culture, the Laland et al. paper is a good place to start.
Laland, K. N., J. Odling-Smee, and S. Myles. 2010. How culture shaped the human genome: bringing genetics and the human sciences together. Nature Reviews Genetics 11, 137-148 (doi:10.1038/nrg2734).