Here is an interesting post on John Hawks’s blog: Selection and lactase persistence.
After reading an article by John Hawks in Scientific American (No, Humans Have Not Stopped Evolving), a reader reached out to him, asking for clarification about a point that left him puzzled: why would a genetic mutation that allows lactase persistence after weaning age (meaning the lactase enzyme keeps being produced in adults so that they can still efficiently digest the lactose in milk) stick around in a population when it (apparently) does not confer a direct advantage in survival/reproduction?
In other words: it’s easy to conceive that a genetic mutation allowing individuals to be more resistant to malaria will help them survive and reproduce, therefore passing on the beneficial allele to their offspring and allowing that allele to expand in the population ; by contrast, being unable to digest lactose past infancy/childhood will certainly makes an individual uncomfortable when consuming milk, but as it is not life-threatening, how does that affect one’s survival and reproduction chances?
For a short and easy to understand answer, it’s here: Selection and lactase persistence.
Spoiler: it has something to do with nutrition, energy availability, and its effect on reproductive function.