A story just came out today about drinking high fat vs. low fat milk, and the positive effect on fertility that the former can have. Remember the report that drinking milk increases twinning? Issues like this should be kept in mind when considering the spread of lactose tolerance, anything that increases fitness should spread. Why didn’t it? Well, it seems likely that cattle can’t be raised everywhere, so you have a situation where the selective benefit is geographically constrained. Also, modern lifestyles are characterized by no scarcity of calories so comparing this to pre-modern situations might be a false trail. The world wide spread of milk consumption should be changing things genetically though….