Uchiyama et al. have made an important contribution to dual inheritance theory. To date, dual inheritance theory has focused mainly on the coevolution of a pair of genetic and cultural traits (e.g., Gerbault et al., Reference Gerbault, Liebert, Itan, Powell, Currat, Burger and Thomas2011) or on the genetic evolution of cultural evolution itself (e.g., Henrich & McElreath, Reference Henrich and McElreath2003). The question of how genetic evolution might itself evolve culturally has received less rigorous inquiry. Uchiyama et al. broach this issue, showing how cultural adaptation (especially group level cultural adaptation) can change genetic heritability by intervening between genes and their effects on survival and reproductive outcomes. In a recent article, two of us explore this same dynamic from the perspective of long-term gene–culture coevolution (Waring & Wood, Reference Waring and Wood2021). Here, we consider the implications of the insights of Uchiyama et al. on long-term gene–culture coevolution.

The authors argue that cultural contributions to phenotypes can modify genetic heritability, an important and often overlooked point. Phenotypic variation in humans is the result of genetic, environmental, and cultural factors, and their interactions. Genetic heritability can be given as:

$$\displaystyle{{V_{\rm G}} \over {V_{\rm G} + V_{\rm c} + V_{\rm e}}}$$

where V _G, V _c, and V _e denote phenotypic variation with a genetic, cultural, and environmental basis, respectively. Our capitalization is consistent with Uchiyama et al., but our formulation differs in order to highlight the idea that cultural effects on phenotype are not limited to acting through environmental variation.

The influence of culture on genetic heritability is complex, and the result of indirect feedbacks by which one V component affects another. Culture may generate phenotypic variation directly (increasing V _c), affect environmental (or ecological) variation indirectly (changing V _e), and mask or unmask genetic variation (decrease or increase V _G). For example, medicine can reduce the effect of diseases (an environmental variable) on health, reducing the role of immunity genes in determining phenotypic outcomes, but making health contingent on the health system one is part of. In this example, the novel cultural adaptation decreases V _e and V _G and increases V _c. These changes would decrease genetic heritability if changes in V _G and V _c outweigh those in V _e. Therefore, the overall impact depends on the relative phenotypic contribution of each type of variation.

Uchiyama et al. appear to assume that cultural evolution tends to decrease cultural and environmental variation within groups, citing mechanisms such as conformist learning (Henrich & Boyd, Reference Henrich and Boyd1998), prestige-biased learning (Henrich & Gil-White, Reference Henrich and Gil-White2001), and success-biased learning (Baldini, Reference Baldini2012). However, structured group-level cultural traits complicate the argument (Smaldino, Reference Smaldino2014). Because of specialization and divisions of labor, social learning mechanisms often generate adaptive cultural complexity within a society, rather than merely homogenize it. For traits that mask genetic effects, increases in cultural variation, V _c, cause decreases in genetic heritability. Thus, there may be more scenarios which decrease genetic heritability than previously thought.

Cultural influences on genetic heritability have major consequences for human evolution well beyond those discussed by Uchiyama et al. A key point they omit is how these cultural influences would alter genetic adaptation and evolution. Increased heritability strengthens the evolutionary response of functional genes to selection, while decreased heritability weakens this response (Lush, Reference Lush1943). Laland (Reference Laland1992) has shown how the transmission of an adaptive behavior via social learning can preempt adaptation by natural selection on genes. This effect should be the expected result. Therefore, students of human evolution should ask whether there is any average long-term trend in culturally mediated changes to heritability.

We hypothesize a general directionality to the role of culture in determining phenotypic variation in the long term: Culturally determined phenotypic variation is increasing (V _c is growing), and cultural evolution is simultaneously decreasing genetically determined phenotypic variation by breaking the link between genotype and phenotype (V _G is shrinking). For example, educational attainment depends on both genetic and cultural factors, and generally comes at a reproductive cost. But, as Hong (Reference Hong2020) shows, educational attainment is likely to continue to increase even while the genetic component declines.

We think that V _c has increased, not for all traits at all times, but as part of a long-term average trend across human societies over the course of evolution. Evidence for this comes from the broad and striking increase in the emergence, diversification, and refinement of cultural systems and technology that improve human fitness outcomes in food production, collective defense, health, and so on. Such complex group-level cultural adaptations increase cultural variation and, thus, V _c, in the human species. When V _c increases, then genetic heritability decreases. Thus, we suspect, along with Uchiyama et al. and others, that cultural adaption has already been replacing genetic adaptation in humans (e.g., Mathew & Perreault, Reference Mathew and Perreault2015). However, we also hypothesize that this trend, highlighted by decreasing genetic heritability, will continue in the long term.

Together, cultural preemption (or masking) and increased V _c are expected to reduce genetic heritability, and this has dramatic implications for the future of human evolution (see Waring & Wood, Reference Waring and Wood2021). As the response of any trait to selection depends in part on that trait's heritability, culturally mediated reductions in genetic heritability could weaken the role of genetic evolution in shaping human fitness and adaptation. At the same time, reductions in genetic heritability and genetic adaptation in humans could pave the way for a more predominant role of culture in human evolution, creating an accelerating positive feedback (Crespi, Reference Crespi2004). This trend is not just a curious possibility but represents a dominant and growing mode in human gene–culture coevolution over long time scales (Waring & Wood, Reference Waring and Wood2021).

We feel that Uchiyama et al. help geneticists and social scientists better understand how genes and cultures interact to shape human heritability. But, we believe the most important implication of their work is the emergence of culture as our primary system of evolutionary adaptation, a long-term pattern which uniquely defines the human species in the past, present, and future.