Genetic diversity, phenotypic diversity and the founder effect
In two recent posts I examined the causative mechanisms underlying Ashraf and Galor’s hypothesis linking genetic diversity to economic growth (innovation and conflict). In those posts, I avoided examining whether genetic diversity could be considered a proxy of phenotypic diversity unrelated to that genetic diversity (such as language).
Part of the reason for this is that Ashraf and Galor do not indicate in their paper or web appendix that they intended to use genetic diversity as a proxy in this way. As I have posted about before, the language of the paper is focused on genetic diversity and the phenotypic expression of that genetic diversity. In dissecting the paper, I wanted to focus on what the paper states.
However, given that Ashraf and Galor have now made an argument [Update: the response is no longer online] that genetic diversity is a proxy in their response to a critique of their paper, the argument is worth assessing. Ashraf and Galor write:
The key is that the measure of intra-population genetic diversity that we employ should be interpreted as a proxy (i.e., a correlated summary measure) for diversity amongst individuals in a myriad of observable and unobservable personal traits that may be physiological, behavioral, socially-constructed, or otherwise. …
The fact that the measure of genetic diversity we use is based on variation across individuals in non-protein- coding regions of the genome (and, thus, in genomic characteristics that are not necessarily phenotypically expressed so as to be subject to the forces of natural selection) is clear reason why our findings should be interpreted through the lens of our measure serving as a proxy for diversity more broadly defined.
The more relevant question to ask therefore is to what extent the measure we use can reasonably be considered a proxy for diversity in unobserved phenotypic or socially-constructed characteristics. There is indeed an emerging body of scientific evidence that establishes remarkable correlations in this regard.
Ashraf and Galor refer to two articles on this point - one on diversity in head shape, which I noted in my post on innovation, and a second diversity in language. On the second, they cite a Science paper from 2011 in which Atkinson reports a finding that diversity in phonemes - perceptually distinct units of sound that differentiate words - declines with distance from Africa. This pattern reflects that found for genetic diversity, and Atkinson suggests that similar forces were acting on each. There are still significant hurdles to show a causative link between diversity in phonemes and innovation and conflict, but the persistence of the phenotypic diversity leaves opens this possibility. The task is to identify what forms of phenotypic diversity might be relevant.
Ashraf and Galor build the case further in a new paper that will be published in the American Economic Review Proceedings and Papers in May. They propose that:
Building on the role of deeply-rooted biogeographical forces in comparative development, this research empirically demonstrates that genetic diversity, predominantly determined during the prehistoric “out of Africa” migration of humans, is an underlying cause of various existing manifestations of ethnolinguistic heterogeneity.
Rather than proposing that genetic diversity is a proxy for other diversity shaped by the Out of Africa event, they propose that genetic diversity is a cause of ethnolinguistic heterogeneity. Depending on whether they use a modern global or old world sample, they find that genetic diversity is responsible for between 7 and 11 per cent of ethnolinguistic heterogeneity.
An alternative approach would have been to attribute genetic and ethnolinguistic diversity to a common cause in the founder effect. I asked Ashraf and Galor by email why they preferred an explanation of genetic diversity causing ethnolinguistic heterogeneity and they replied as follows:
[T]he ethnicities located at greater migratory distances from East Africa (especially those outside of the African continent) DO NOT represent a subset of the ethnicities in Africa. Had the serial founder model (as applicable in generating the global distribution of genetic diversity) been equally applicable in explaining global spatial variation in ethnic diversity, we should observe that ethnic groups extant outside of Africa are also present within Africa. This is clearly NOT the case.
Thus, our hypothesis is that of quasi-random migrant selection from the origin in each step of the “out of Africa” demic diffusion process, with the migrants engaging in endogenous group selection (or endogenous sorting) upon reaching their destination in that step of the diffusion. Moreover, this group selection process would take into account the trade-off associated with intragroup diversity (i.e., diversity across individuals WITHIN the new group), and possibly, also reflect the interaction of intragroup diversity with location-specific geographical factors.
They further expand on the mechanism in their paper:
Following the “out of Africa” migration, the initial level of genetic diversity in indigenous settlements presumably facilitated the formation of distinct ethnic groups through a process of endogenous group selection, based on the tradeoff between the costs and benefits associated with heterogeneity and scale. While heterogeneity raised the likelihood of disarray and mistrust, reducing cooperation and thus adversely affecting group-specific productivity, complementarities across diverse productive traits and preferences stimulated productivity. Since in a given environment, diminishing marginal returns to diversity and homogeneity entail an optimal size for each group, higher initial genetic diversity would have positively contributed to the number of groups, and thus to the degree of fractionalization. Further, to the extent that higher initial diversity did not lead to an excessively large number of groups, it would have positively contributed to the degree of polarization as well.
The causative argument relies in part on the conflict between genetically dissimilar individuals limiting group size, which would in turn affect ethnic fractionalisation, which would then affect inter-group conflict. I need to think about this argument more, but one possible implication of this group-sorting argument is that groups with less genetic diversity would become larger. Part of the benefit to lower diversity would be to enable a scale effect - that is, more people leading to more ideas. A larger group would be more innovative simply through having more innovators.
So, to answer the question of whether genetic diversity can be a proxy for phenotypic diversity beyond phenotypic expression of that genetic diversity, yes. And as a plausible causative link may exist between ethnolinguistic heterogeneity and conflict, this builds the case for the link between genetic diversity and conflict. But, as their new paper suggests, Ashraf and Galor propose a more direct relationship than through a common cause in the founder effect, at least as it relates to ethnolinguistic diversity. Rather than pursuing the proxy argument, their new paper builds the case that the relationship between genetic diversity and conflict is more direct.
My posts on Ashraf and Galor’s paper on genetic diversity and economic growth are as follows:
Is genetic diversity a proxy for phenotypic diversity? (this post)
Is population density a good measure of technological progress?
What are the policy implications of the effects of genetic diversity on economic development?
Earlier debate on this paper can also be found here, here and here.