In Slate's Blogging the Human Genome series, Sam Kean says cases of sickle-cell anemia illustrate how genetics and environment can work together to affect a population's evolution. Thousands of years ago, people in West Africa began growing yams — but the fields had standing water that attracted mosquitoes, which carry malaria, and people who had certain mutations couldn't get malaria, Kean says. One mutation in particular on chromosome 11 changed the shape of some red blood cells — a change that prevents people from getting malaria — and that spread further into the population as people with it were better able to survive and have children. This allowed the yam farmers to spread further into the land as they were less affected by the mosquitoes and malaria. "Unfortunately, when the mutated, crescent-cell-producing gene became relatively common, people started having children with two copies of it," Kean says. "And while having one copy still provided resistance to malaria, having two copies proved deadly, since the crescent, or sickle-shaped, blood cells died off prematurely, and also jammed up inside small blood vessels. Today we call this condition sickle-cell anemia."
The cultural choice to chop down forests and raise yams changed the genetics of entire populations, he adds. And it's not just sickle-cell anemia — different kinds of diets favor differences in human teeth and jaws structures, domesticating cows led to lactose tolerance, and so on. "In fact, modern civilization has curbed the ancient threats to our survival — exposure, droughts, starvation, predators, parasites — so well that culture probably shapes our genes as much or more than anything else nowadays," Kean says.