Unfortunately, not everyone can stomach a 3 scoop sundae covered in chocolate fudge and whipped cream - some get some rather discomforting digestive issues we don't want to describe in detail here. This is very often the consequence of lactose intolerance: the inability to break down lactose, a common sugar in milk and milk-derived products. Lactose intolerance in adults is often viewed as a disease, yet this condition is the norm not only among humans, but all mammals.
The lactase enzyme in intestinal cells allows young mammals to digest milk. After weaning, however, its expression decreases dramatically as lactose is no longer an essential part of a mammal's diet. Humans, on the other hand, are rather unique mammals: some of us like milk with our breakfast cereal and in our coffee even as adults. Most humans are lactose intolerant to some degree in adulthood (in fancy terms, this condition is known as "lactase nonpersistance" or "adult hypolactasia"). As we've already said, this is because the body naturally makes less and less lactase as you get older because milk no longer forms the major part of your diet (unlike when you were a baby) and its wasteful in terms of resources to keep on making the same amount as when you were dependent on milk for all of your nutrition. But in some people this happens at a very young age, sometimes as early as two or three years old; this type of early lactose intolerance is genetically inherited. It's even possible for newborn babies to be lactose intolerant due to other inherited gene mutations, meaning that there's either little or no lactase, or the lactase produced (in normal quantities) just doesn't work. And, as you can imagine, if you can get some forms that don't work at all, then you can also get some forms that work even better than average, so some people's lactase is just more effective than others, even if there's only a small amount of it being made. As ever, environment can have an effect, too: some ladies who are lactose intolerant regain their ability to happily process dairy products whilst they're pregnant! Some drug treatments, such as chemotherapy and antibiotics, can also cause lactose intolerance, as can some illnesses, like Crohn's disease.
In some Northern European, African and Middle Eastern populations and their descendants, lactase activity persists into adulthood thus making them lactose tolerant. This condition is genetically determined, with lactose tolerance being the dominant trait. The mutations that gave rise to this new phenotype evolved recently in evolutionary terms; research suggests it only appeared in Europe within the last 8000 years (Burger, et al., 2007). Also, lactose tolerance has evolved in humans more than once: in other words, the same phenotype arose in different populations independently - the mutations that allow Northern Europeans to digest lactose in adulthood are different than those that let East African or Saudi communities enjoy milk (Enattah, et al., 2008; Tishkoff, et al., 2007). Lactose tolerance in all these different groups most likely evolved through natural selection because of the many adaptive benefits of milk consumption. Thus losing the ability to produce lactase as you get older is considered the "wild phenotype," while persistent lactose tolerance, the rare and more recently evolved trait, is the "mutant" phenotype.