Culture and the human genome: a synthesis of genetics and the human sciences

Humans are immersed in culture from birth. It is so fundamental to our experience, and what it means to be human itself, yet we often overlook the consideration that “cultural practices might have transformed the selection pressures acting on humans” (Laland, Odling-Smee & Myles, 2010, pg. 137).

For those of you with some sort of investment in human evolution, it’ll be quite clear that gaps between culture and biology are being broached by a variety of researchers. Anthropologists are highlighting how cultural practices rapidly modify environmental conditions and subsequently impact upon allele frequencies (Holden & Mace, 1997). Meanwhile, geneticists delving into the human genome are uncovering a whole host of genes showing signs of recent positive selection (Tang et al., 2007), whilst theoretical biologists are taking advantage of mathematical modelling to provide a quantitative approach to the cultural processes affecting human evolution (Cavalli-Sforza & Feldman, 1981; Boyd & Richerson, 1985). In the current review by Laland, Odling-Smee & Myles, they show how the data emerging from these disciplines is consistent with two branches of mathematical evolutionary theory analysis: “gene-culture co-evolutionary theory, which explores how genetic and cultural processes interact over evolutionary time, and niche-construction theory, which investigates the evolutionary impact of the modification of environments by organisms” (ibid, pg. 137, my emphasis).

Like the evolution of biological organisms, culture too can be viewed as following three Darwinian principles: variation, differential fitness, and inheritance. It is an evolutionary system subject to the selective and non-selective forces Darwin wrote about in The Origin of Species: that there is a vast amount of variation within a species, which leads to selection for particular traits, and that these traits are then inherited by successive generations. When looking at the spread of culture, we see evidence for all three of these processes. For instance, of the 6-8000 languages spoken throughout the world (Evans & Levinson, 2009) there is a considerable amount of variation. Meanwhile, cultural selection is taking place at many levels and is the result of many factors, including cognitive constraints on memory, attention and expression (Mesoudi, Whiten & Laland, 2006). Lastly, Mesoudi, Whiten & Laland (ibid) identify the inheritance “of successful cultural traits has been demonstrated in numerous studies of transmission of skills and beliefs in traditional societies… and in studies of social learning in children.” (pg. 331-2).

The central idea emerging from this insight is that genes and culture are two interacting forms of inheritance, “with offspring acquiring both a genetic and a cultural legacy from their ancestors” (Laland, Odling-Smee & Myles, pg. 138). Gene-culture co-evolution then, is a dynamic system where genetic and developmental processes influence cultural learning, with culturally transmitted information, such as certain types of behaviour and artefacts, spreading throughout populations. In turn, this modifies the selection pressures that act on the populations in which they were propagated. Interestingly, such dynamics show culture is not uniform in affecting the rate of change of allele frequencies: some instances show a slowing down, or buffering, of selection, whilst in some cases cultural is in fact accelerating adaptive evolution. One often cited incident of extraordinarily strong selection involves the spread of the lactose-tolerance allele, which has seen it go from “low to high frequencies in less than 9,000 years since the inception of farming, with an estimated selection coefficient of 0.09–0.19 for a Scandinavian population” (ibid). Yet such strong selection sweeps may be the exception rather than the rule, with Coop et al. (2009) claiming “that neutral processes – especially population history, migration, and drift – exert powerful influences over the fate and geographic distribution of selected alleles” (pg. 9).

In understanding how genes and culture interact over evolutionary time, Laland et al (2010) offer cultural niche construction as an explanation for the geographical variation found in human genes. Over the past 50,000 years, humans have modified selection pressures through a myriad of scenarios: from the exploitation of agriculture and domestication of plant and animal species to the invasion of new environments. For instance, counteractive niche construction provides a means of buffering selection, effectively nullifying the effects of environmental change on selection for certain sets of genes. Laland et al use the example of climate to demonstrate how cultural products, such as the ability to put on or take off clothes, can counteract the need for specific selection pressures in response to a particular niche. As such, humans are adept at operating in  hot and arid or cold and moist environments. Furthermore, this buffering leads to situations where:

One prediction from this cultural mitigation of selection is that we now expect more (of what would otherwise be) deleterious alleles in the human gene pool than we would in the absence of cultural activities.

As we know, the evidence for gene-culture co-evolution is mounting, but it is also very far from completion. Just looking at the situation from a genetics perspective, we can see signs of recent positive selection not just confined to single-base-pair substitutions or protein-coding regions: genomic rearrangements, copy-number variants (CNVs) and regulatory gene networks all show signs of having been shaped by natural selection. Simply put: the puzzle is composed of many, highly complicated, but solvable, pieces. As the authors note, one major task facing researchers is discerning “the connections from genotype to phenotype to selection pressure for the long list of selected genes generated from genome-wide scans” (pg. 142). Given the clarity of the argument put forward for gene-culture co-evolution, it is clear these interactions are happening. However, the next major question is how prevalent has gene-culture co-evolution been in shaping the human genome? To answer this, we are first going to need more causal links between gene variants and their adaptive phenotypes. Secondly, we are then going to have to rule out other extraneous factors before claiming culture is the central selection pressure.

References

Laland KN, Odling-Smee J, & Myles S (2010). How culture shaped the human genome: bringing genetics and the human sciences together. Nature reviews. Genetics, 11 (2), 137-48 PMID: 20084086