Lactose intolerance is fairly uncommon in much of Europe and parts of Asia and populations which are mostly Euro-Asian in origin. In these populations, where milk forms an integral part of an adult diet, the problems associated with not being able to digest milk are a problem needing special diet, etc. For that population, it is seen as a medical condition needing treatment.
However, in many populations, especially African ones, lactose intolerance is the norm, but this is not normally a problem because milk does not feature strongly in the diet of many African peoples. There is a very good reason for this. The source of most of our milk is the domestic animals and most domestic animals do not thrive in Africa because of the presence of the tsetse fly so it has proved difficult to establish herds of dairy cattle for example in most of tropical Africa. This in part explains why there are still vast herds of wildebeest and zebra in Africa - humans have not driven them off the grass land and taken it for grazing.
If we look at most mammals we find the norm is for babies to be able to digest milk but this ability is turned off at about the time they are weaned. The evolutionary advantage of this are obvious from the genes' point of view. Suckling young act as a natural contraceptive, inhibiting ovulation in the mother by stimulating production of an inhibitory hormone by the pituitary gland. Stopping the young from suckling by making milk seem unpalatable allows the mother to produce more babies, so she is more likely to pass on lactose intolerance to other children and produce more children than a female who suckles her babies for longer.
However, mutations have arisen which prevent this ability to digest milk from being turned off. You would expect this to be quickly eliminated from the gene pool if it meant the carriers produced fewer children by suckling them for longer. But, in societies where other milk is readily available, all the mother need do is to use this alternative milk for her older babies and she can ovulate and produce more young just as though they were intolerant of her breast milk.
And, with the milk of domestic cattle being able to sustain a much bigger and healthier population by providing both calcium and vitamin D (to help counter the lack of sunshine in northern climates) so lactose tolerance became a distinct advantage and not the disadvantage it was when we all lived in Africa. In Africa, however, it remained a disadvantage so the mutation never gained any traction in the gene pool. In fact, lactose tolerance there should be regarded as an abnormality as it keeps babies at the breast for longer, resulting in smaller family sizes.
All perfectly straight-forward and fully understandable in terms of Darwinian Evolution where different populations diversified due as always to differences in their allele frequency because of local environmental factors, in this case domesticated cattle, sunshine and tsetse flies, producing variations on a basic mammalian theme. This article about lactase persistence, as lactose tolerance should more properly be called, explains this in far greater detail.
The only slight problem is one of nomenclature: to be biologically precise, now we know the evolutionary origin of 'lactose intolerance', we should really call the ability to digest milk as an adult 'lactose tolerance' (which is implied in the name anyway) or 'lactase persistence' because the 'intolerance' was the norm in the founder population and in other members of our family tree and so it is fair to assume, in our common ancestors, hence lactose tolerance is regarded as the mutated allele.
Now see how Creationists represent these undeniable and fully supportable scientific facts in Can’t drink milk? You’re “normal”!:
Firstly the misleading headline. There is of course no value judgement of normality or abnormality in evolution. Whatever produces the most descendants becomes the predominant allele in that environment. Lactose tolerance is no more normal or abnormal in Euro-Asians than intolerance is in Africans or other lactose-intolerant populations which have not co-evolved with domesticated cattle.
Then there is the attempt to discredit the theory by pointing out that some African populations have also evolved lactose tolerance, but with slightly different mutations. Given that we are talking about something being turned off, it is not at all surprising that this can happen with several different mutations. The deactivation of lactase is likely to be an active process and failure in any part of it would result in lactase persistence. In fact, it is maybe more surprising that there are only three so far discovered but we are talking about a mutation which has only gained traction in some populations during the last 5-10,000 years. The African populations mentioned are in Sudan, Tanzania and Kenya, all of which border the Ethiopian highlands where cattle are farmed because the tsetse fly is absent from there. Milk also forms part of the diet of several Niloto-Saharan people including the Masai from Kenya.
Lastly, there is the quite disgraceful attempt to associate the Theory of Evolution by Natural Selection with white supremacist racism with a grotesque claim that the evolutionary explanation of differences in lactose tolerance between peoples implies that black people are less evolved, when it implies quite the opposite. Lactose intolerance in the absence of a substitute milk because of the presence of the tsetse fly, is no less evolved that is lactose tolerance in the presence of a substitute milk. Both populations are equally evolved in the context of their specific environments.
The hypocrisy here, in a political movement which invented 'social Darwinism' to justify its white supremacism, is quite breath-taking. It was only when the Civil Rights Campaign, which the conservative Christian right had opposed with violence, got American black people something approaching full emancipation that the white right realised they could no longer rely on violence and discrimination alone to keep black people at the bottom of the peck-order as a supply of cheap labour and domestic servants.
So we see a right-wing conservative, 'Christian' political movement overtly misrepresenting evolution and simultaneously playing on the understandable fear of racism in the black population; the same black population that this political movement seeks to keep poor, excluded and disenfranchised, and only just tolerated so long as they turn out to 'vote for Jesus' - and the white supremacist pseudo-Christian neo-Fascists who use their religion to keep them in their 'place'.