Lactose tolerance has always been a favorite with researchers in human genetics because it is obviously a recent adaptation. Lots of populations still don’t have it, or have it only at very low incidences–the population my wife comes from, for example: in common with most East Asians, Mrs. D is lactose intolerant.
The striking thing here is the great survival advantage conferred by the adaptation:
“Genetic evidence shows that the mutations conferred an enormous selective advantage on their owners, enabling them to leave almost 10 times as many descendants as people without them. The mutations have created ‘one of the strongest genetic signatures of natural selection yet reported in humans,’ the researchers write.”