Impact of Selection and Demography on the Diffusion of Lactase Persistence:

The lactase enzyme allows lactose digestion in fresh milk. Its activity strongly decreases after the weaning phase in most humans, but persists at a high frequency in Europe and some nomadic populations. Two hypotheses are usually proposed to explain the particular distribution of the lactase persistence phenotype. The gene-culture coevolution hypothesis supposes a nutritional advantage of lactose digestion in pastoral populations. The calcium assimilation hypothesis suggests that carriers of the lactase persistence allele(s) (LCT*P) are favoured in high-latitude regions, where sunshine is insufficient to allow accurate vitamin-D synthesis. In this work, we test the validity of these two hypotheses on a large worldwide dataset of lactase persistence frequencies by using several complementary approaches.

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Our results show that gene-culture coevolution is a likely hypothesis in Africa as high LCT*P frequencies are preferentially found in pastoral populations. In Europe, we show that population history played an important role in the diffusion of lactase persistence over the continent. Moreover, selection pressure on lactase persistence has been very high in the North-western part of the continent, by contrast to the South-eastern part where genetic drift alone can explain the observed frequencies. This selection pressure increasing with latitude is highly compatible with the calcium assimilation hypothesis while the gene-culture coevolution hypothesis cannot be ruled out if a positively selected lactase gene was carried at the front of the expansion wave during the Neolithic transition in Europe.

The "calcium hypothesis" idea is of course one of the explanations for light skin in Northern Europe as well. The locus responsible for 1/3 of the skin color difference between Africans and Europeans, SLC24A5, is a relative recent sweep, on the order of the last 10,000 years. The authors do caution to be careful about the assumptions of their model. Point taken to heart, as I don't think they have a good enough grasp on the fine-grained variation in the lactase persistence alleles and how they track ecology within Europe. The Greenland Norse did not raise cattle just because of lack of Vitamin D (which they ended up getting through a shift toward a marine diet in any case), rather, there were ecological constraints in terms of the maximum productivity of grain-based subsistence farming (particularly with wheat in cold damp climates). In the conclusion of the paper it is noted that Iberia is a good test case of the model, and more data needs to be gathered there. If it is gene-culture coevolution than many Iberian peoples should be lactase persistent, but if it is due to Vitamin D, they should not be.

Labels: Genetics