Simply put, the frequency of the Lactase Persistence (LP) allele in most of southern Europe can be explained by demography alone, as the result of genetic drift during the Neolithic diffusion from the Near East. In Europe itself, there is a very strong correlation of this allele with latitude, and it is in Northwestern populations where the allele finds its higher frequency. This is more compatible with the calcium assimilation hypothesis, since lactase persistence is beneficial in low sunshine (high latitude) regions than with the gene-cultural co-evolution hypothesis, according to which pastoralists become LP because of their need to rely on animal milk.
These results can of course be improved by sampling more populations and micro-sampling communities that have traditionally practiced pastoralism vs. agricultural ones, where available, e.g., in the Balkans. However, they do suggest that the hypothesis of LP spreading during the Neolithic, and being selected at higher latitudes (south-to-north) is more attractive than its spread by pastoralists who were genetically adapted to consume milk in adulthood, as is the case in Africa.
PLoS ONE doi:10.1371/journal.pone.0006369
Impact of Selection and Demography on the Diffusion of Lactase Persistence
Pascale Gerbault et al.
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.
We first analyse the distribution of lactase persistence in various continents in relation to geographic variation, pastoralism levels, and the genetic patterns observed for other independent polymorphisms. Then we use computer simulations and a large database of archaeological dates for the introduction of domestication to explore the evolution of these frequencies in Europe according to different demographic scenarios and selection intensities.
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.