The bulk of human evolution is now through culture (let's keep it that way)

Humans stopped genetically evolving some time ago, and now evolve almost exclusively through cultural change. The simple reason is that cultural change allows human populations to generate solutions to the problems they face at least as fast as the problems emerge and much faster than their own generation time (that is, well within the time between the births of successive generations).

In a previous post, I introduced two time scales, T and \(\tau \). The first time scale is T, the generation time, or the time between the births of successive generations. It's also the time scale of natural selection, or the smallest approximate period of time over which genetic changes can sweep through a population. The second time scale I introduced is \(\tau \), or the time scale of culturally effective change. This is the time it takes for an cultural change to sweep through a population and effectively solve a cultural problem. In general, \(\tau \) has the potential to be much smaller than T, but it can also be much larger if there is a significant risk of loss associated with the cultural change. In general, \(\tau \) is smaller than T in those time when adopting a cultural change will confer an immediate advantage with comparably few and minor near-term disadvantages, or when failing to adopt the change would be catastrophic (i.e., deadly), while \(\tau \) is likely to be larger than T in those times when adopting a cultural change would lead to large near-term risks of loss.

Now I would like to introduce a third time scale, E, which is the time it takes for an environmental change (of any kind, including those that you might otherwise attribute to internal conflict) to adversely if not catastrophically affect a population.

Let's say that the environmental change we're talking about is a multi-decadal cold snap, meaning that the climate of a region changes over 5 years (E = 5 yrs) such that temperature, on average, is 5deg C colder than before for decades at a time, with dangerously cold winters. Let's also assume that, overall, the population is not prepared to survive this change (i.e., their culture does not have a solution to the multi-decadal cold snap problem). Let's further assume that some individuals within the population are genetically predisposed to carrying around a lot of extra "blubber". We'll call these individuals sub-population A, while the rest of the population (the skinny ones) we'll call sub-population B. The extra fat carried around by the A's confers a distinct advantage during the dangerously cold winters. Not only are A's more likely to survive the winters, but they are more likely to be able to do excess work, beyond simply staying alive, enabling them to build up a surplus in resources that allows them to successfully raise more offspring (who, one would assume, are genetically more likely than not to be A's themselves).

B's, on the other hand, have it tough. They either migrate away to warmer climates (assuming they know how) or they live out a meager existence, either dying from the cold, or being so beset by frigid temperatures that they never manage to successfully raise any offspring. The end result of the differential in reproductive success between the A's and B's leads to the dominance of the A's and the change in the nature of the population from one that looks like a mix between A's and B's, to one that mostly just looks like the A's. The time scale of this change via natural selection is the generation length, or T.

An cross-sectional example of an Inuit shelter.

If the percentage of the population comprised of A's is small, or if the advantage conferred during the winter months by being an A is small, then the implications of the cold snap could be very, very grim, indeed. Most if not all of the population would likely die off, leaving other human populations elsewhere to carry the torch for the species. But the B's (and the A's) have a third option; they don't simply have to leave or die. They can also find and adopt cultural solutions to the cold snap. Rob Boyd and his colleagues Peter Richerson and Joseph Henrich (2011, PNAS, 108:10918-10925, open-access article) outline the intensely complex cultural solutions adopted by the Central Inuit in polar North America in response to deep cold, including very well-tailored clothing, cunning shelters, ample sources of heat, inventoried food, and a wide-variety of seasonally appropriate hunting tools, among others.

None of these solutions sprang up overnight, but you can imagine the ancestral populations of the Inuit incrementally adopting these kinds of cultural solutions as the climate cooled, or they migrated pole-ward. Therefore, in the moment, \(\tau \) would always be less than or equal to E, and always less than T, keeping the population from having to undergo a bloody round of strong natural selection. As they were adopted, these cultural solutions would confer advantages to both A's and B's, while the A's would have the additional advantage of their "spare tires", and it would no longer be a foregone conclusion that the B's would entirely die out or move away. In fact, keeping the B's around could be useful during the summer months. Bottom line: cultural solutions allow a population to persist, without the need for deep change in its underlying genetics.

So, if \(\tau \) is smaller than T and and no greater than E, then populations will evolve by cultural means, not genetic means (the "cultural selection regime"). If, on the other hand, \(\tau \) is incapable of being less than T, and is larger than E, then populations will evolve by genetic means (the "natural selection regime"), subject to Tennyson's "nature, red in tooth and claw." Cultural evolution helps to avoid or soften these very hard times by introducing population-level innovations that are not reflected in the gene pool of the population, which can take effect much faster than the time it takes for favorable A-related genes to sweep through a population.

This has real meaning for us today. We are facing grave cultural problems: nuclear proliferation, global climate change, mass migrations, water shortages, lack of clean water, broken politics, massive income inequality, and many others. Genetic evolution can't help us here. In fact, all of our broader cultural efforts (humanitarian aide, relief efforts, nuclear non-proliferation treaties, agricultural reform, environmental protection, etc.) are in service of avoiding natural selection, and keeping us in the "cultural selection regime." But it is our inability to quickly find cultural solutions to these problems that should worry us. If we can't find them, and \(\tau \) creeps higher and higher, while E gets smaller and smaller, then expect humans to enter a very dark time in our history, where population change occurs by genetic rather than cultural means.