America’s population comprises many races and ethnicities, and the concept of “race” itself is complex. While it often seems easy to tell at first glance whether a person is white, black, or Asian, scientists point out that race has no biological basis. As geneticist Craig Venter explains, “Race is a social concept, not a scientific one. We all evolved in the last 100,000 years from the same small number of tribes that migrated out of Africa and colonized the world.” External differences in skin color, he points out, reflect traits that developed over time due to environmental pressures: “Equatorial populations evolved dark skin . . . to protect against ultraviolet radiation, while people in northern latitudes evolved pale skin, the better to produce vitamin D from pale sunlight.”

The concept of race, then, arose as...