Jon Entine, author of Taboo: Why Black Athletes Dominate Sports and Why We’re Afraid to Talk About It,” takes stock of the DNA London Olympics–where, as usual, African-descended athletes swept the running events while whites and Asians dominated in the water sports, field competition and strength events. What’s going on here? Segregation was on display in London over the past two weeks–which, surprisingly, should spark no concerns and may even help educate us all about the wonders of human biodiversity. Let me explain. Led by 100-meter world record holder Usain Bolt, Jamaican men swept the sprinting events at the London...