The trait of lactase persistence (lactose tolerance) is probably one of the better schoolbook examples of natural selection in human populations. The reasons for this are probably two-fold. There is a very strong signature of selection within a specific gene known to associate with the trait in question in many populations. And, there is a very compelling historical narrative which explains rather neatly how this particular functional change could have undergone such strong selection within the past ~5,000 years across these populations. But the elucidation of the origin and spread of this genetic adaptation is also interesting because it looks as if it was not a singular event. Populations as disparate as Arabians, Danes, and Masai seem to carry different alleles around the locus of interest which confer the ability to digest milk. This illustrates the fact when selection pressures have a viable target, there is a rapid response on the genomic level. At some point during the maturation of a mammal the regulatory pathway which produces lactase enzyme shuts down. Yet within numerous human populations this gradual shutdown process has been short-circuited.
The variety of response in relation to this adaptation was brought home to me as I read Diversity of Lactase Persistence Alleles in Ethiopia – Signature of a Soft Selective Sweep, in the latest issue of The American Journal of Human Genetics:
Dienekes and Maju have both commented on a new paper which looked at the likelihood of lactase persistence in Neolithic remains from Spain, but I thought I would comment on it as well. The paper is: Low prevalence of lactase persistence in Neolithic South-West Europe. The location is on the fringes of the modern Basque country, while the time frame is ~3000 BC. Table 3 shows the major result:
Lactase persistence is a dominant trait. That means any individual with at least one copy of the T allele is persistent. As Maju noted a peculiarity here is that the genotypes are not in Hardy-Weinberg Equilibrium. Specifically, there are an excess of homozygotes. Using the SJAPL location as a potentially random mating scenario you should expect ~7 T/C genotypes, not 2. Interestingly the persistent individual in the Longar location also a homozygote.
Unlike in some Asian societies dairy products are relatively well known in South Asia. Apparently at some point my paternal grandmother’s family operated a milk production business. This is notable because Bengal is not quite the land of pastoralists. In much of North India milk and milk-products loom larger, in particular ghee. People don’t tend to consume what makes them ill, and even accounting for some processing in the form of butter, most researchers have assumed a substantial number of South Asians must be lactase persistent. That is, they can extract nutritive value out of the lactose sugar present in milk (in addition to fat and protein). Additionally, many South Asians have the well known -13910 C>T common in Western Eurasia. How do I know this? Because I share my genetic information with lots of South Asians, and some of them, especially Punjabis, come up as “lactose tolerant” on that allele.
A new paper in Molecular Biology and Evolution confirms this with a larger data set, over 2000 samples from South Asia. The geographical pattern is exactly what you’d expect:
My recent focus on the lack of genetic continuity between hunter-gatherer and farming populations genetically and culturally is primarily due to the fact that we’re not in theory-land; the extraction of ancient DNA samples is steady-as-it-goes and is sharpening and overturning our understanding of the past. The relationship between culture and genetics is of particular importance in this case, genes serve not only as markers which we can track population movements, but genes themselves are embedded in dynamics which need not be connected to population movements.
Consider lactase persistence, which confers the ability to digest milk as an adult. In the 20th century “lactose intolerance” was assumed to be a pathology, but it turns out that most human populations can not digest milk sugar as adults due to the lack of production of the lactase enzyme. This is the ancestral type. Rather, different mutations which result in the persistence of lactase production into adulthood seem to have arisen independently in several regions of western Eurasia and Africa. This suggests that the mutational target zone here is large, that is, given particular selection pressures (cattle culture) mutants will arise in the background and increase in frequency which produce the phenotype of lactase persistence.