The best-known example involves the gene that regulates a person's ability to make an enzyme required to digest lactose, the sugar in milk. Historically, the LCT gene shut down in early childhood as babies were weaned off breast milk. But after cows, sheep and goats were domesticated, people with a mutation that allowed them to drink milk as adults had a nutritional advantage that made it easier for them to propagate their genes.
DNA analyses have shown that the mutation cropped up in Europe about 8,000 years ago, and quickly spread all the way to India. Today, it is carried by more than 95 percent of people of northern European descent.