July 24, 2009

Demography and selection in the spread of Lactase persistence in Europe (Gerbault et al. 2009)

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.

Abstract

Background

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.

Methodology

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.

Conclusions

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.

Link

2 comments:

Gioiello said...

This paper is important for at least 2 results:
1) The exclusion of the Linear Band Keramic population for the peopling of Europe, not being this hypothesis that that explains the diffusion of the Lactase persistence in Western Europe.
2) The cal (calcium assimilation) hypothesis is better than the gcc (gene-culture coevolution).
If we exclude the Vizachero fallacy (that was the Renfrew one) of an origin of R1b1b2 (and of Indo-European languages) from Middle East and the openings of this paper on two other hypotheses (“An alternative hypothesis would be that a strong genetic structure for LCT*P was already present during the Mesolithic Era and lead to differential milk consumption in European populations” (p. 5) and “as well as migration and possible admixture between hunter-gatherers and Neolithic farmers” (p. 6)), I can do my hypotheses.
The age of the mutation of the LP, that some scholars calculate between 7,450 and 12,300 YBP, must be dated at 12,300 years as only one man (or his woman) had first the mutation. In an epoch of low population’s expansion, 7,450 YBP is the date when this population took agriculture from the Middle Eastern J2’s and G2’s and began to expand. Of course this population were the R1b1b2 of the Italian refugium, who, from 12,300 years ago, as the mitochondrial U5b3, have had a low expansion to Middle East till Mesopotamia and Iran, but was who remained in Italy or nearby who peopled Europe.
Took this hypothesis and you will be that everything squares and mind, on the calculation, behind the latitude, also the rainfall.

Ponto said...

I am not a believer in the Paleolithic/Neolithic paradigm. Personally I just think it to be akin to Nazi racist propaganda and the likes of Madison Grant. Mediterranean people are not of Neolithic age, Near Eastern origin. That is just a fanciful theory to account for the appearance of agriculture, a Levantine cultural invention, in Europe,and the darker appearance of Mediterranean Europeans. Europe always was in contact with the Near East since long before the Neolithic. All those so called Neolithic haplogroups are way older than the Neolithic by more than 10 ky. It beggars belief that haplogroups of that age, diversity and spread around Africa, Europe and Asia and originating in the east Mediterranean zone just stayed put for many thousands of years when johnny come latelys aka Asian immigrants like R1b and R1b moved spectacularly fast from their Central Asian home in just a few thousand years.

Lactose persistence is part of the Pastoral way of life utilizing raw milk as food. Agriculturalists by and large use milk converted to cheese, yogurt and butter. Pastoral peoples like the Indo-European speaking peoples used raw milk as food and its presence in Europe can only date from the entry of IE languages into Europe, which was not coincidental with agriculture into Europe but much younger in age. R1b* and R1a* date from the appearance of IE languages into Europe which occurred from the east from Central Asia. The date of 12300 ya is unlikely. The entry of IE languages is more of the order of 5 to 6 kya well after the first appearance of agriculture in Europe, sites in Portugal date older than 7 kya, in Cyprus to around 10 kya.

The mtDNA U5b is likely much older than the appearance of IE languages in Europe and its bearers were most probably lactose intolerant like most Europeans were prior to the appearance of the Easterners with their cattle and their overpowering languages. In Italy look to the ancestors of the Italic language speakers for lactose persistence. The men were most likely of the R1b haplogroup.

Those people you think survived in refugia were probably overwhelmed and outbreed by those Easterners with their R1b haplogroup in most of Europe except the south. The women probably stood a better chance of passing their mtDNA on than the men.