Answer:
Natural selection
Explanation:
AMY1 produces amylase, a protein that breaks down starch from food into maltose, which is the first step in properly digesting starch.
A mutation in the AMY1 gene that causes it to produce more amylase, would mean that individuals carrying that mutation would have more amylase in their saliva, and would be better at breaking down the starch.
If you can more efficiently break down the starch in your diet, it will be a better source of energy for you. This would have been particularly important thousands of years ago, when food security was poorer. If you lived in a region or were part of a culture where starch was an important part of your diet, you might have been better nourished if you could properly break down these starches.
Over time, natural selection would be at work: the individuals carrying the AMY1 mutation would have improved fitness, because of their ability to digest starch and get all the nutrition and energy from it (i.e. maybe they were less likely to get sick, to starve etc.). Therefore, very slowly, this mutation would become more frequent in those regions where it is beneficial.
