Lactose

For breast feeding mammals, only the infants and the young are able to breakdown lactose.  Once they reach an age of maturity, the gene which produces lactase, the enzyme which breaks down lactose, is turned off.  Since lactose cannot be broken down into simpler sugars, it reeks havoc on the small intestine.  Perhaps, this the gene turning off was to keep the breast milk only going to the infants of the population and not to the older individuals which have more food choices available to them.  Although not everyone can break down lactose, there are certain human populations which have this ability.  These populations are those who domesticated animals and use their milk as a key part of their diet.  Through the four forces of evolution, the ability to digest lactose in adulthood has persisted around the world.  It only takes a single point mutation to turn on the gene which produces lactase.  There is no way of telling if this mutation occurred once in history but it likely that convergent evolution was at play.  Next, gene flow or the interbreeding of humans continued to spread the gene.  Natural selection also increase the prevalence of this gene in population that domesticated animals.  Finally, the founder’s effect, a type of genetic drift, would have spread this ability to new colonizations with a high probability of the members having the mutation.