Comment by seanhunter

My understanding is that lactose tolerance is a particularly interesting case because lactose tolerance is in fact the mutation and lactose intolerance is the "default". It's just that for historical reasons lactose tolerance obviously conferred an advantage in Europe in particular which is why the mutation persisted. That's why around 40% of the global population are lactose tolerant and intolerance is the global norm.

https://en.wikipedia.org/wiki/Lactase_persistence