The mutation for milk-drinking evolved in different parts of the world over the last 10,000 years as a result of strong natural selection, but why was it so advantageous?
The data show that the mutations providing lactase digestion spread unusually rapidly. It had been thought that this was due to positive selection, perhaps due to Vitamin D needs. Those with the mutation simply were healthier, lived longer and had more offspring.
But the data do not show this as likely. So why did it spread so fast?
It may well be due to negative selection. Evidence indicatses that people were using fermented dairy products such as cheese and yogurt before the lactase mutation spread.
So we wre used to using dairy products. But drinking raw milk would result in massive diarrhea.
This would not be a bad thing during good times. But what happens during bad times, when water and other liquids are hard to find?
It is likely anyone drinking raw milk would die from the diarrhea. Only those with the mutation would survive at all.
It is much more likely for a useful mutation to spread throughout a population rapidly not only when it provides a positive trait for survival and lots of offspring but also when the normal gene leads to death and no offspring.