The South Fore people of Papua New Guinea used to eat their dead relatives' brains as a sign of respect, passing on the deadly prion disease kuru–a relative of mad cow disease–in the process. But long before the Fore stopped the tradition on the advice of scientists in the 1950s, evolution was already at work. Less than 200 years ago, according to New Scientist, a member of the Fore was born with a gene mutation that protected against kuru. They passed it to their children.
Because having the mutation helped you live longer (and, thus, have more children), it quickly spread through the Fore population. Today, several Fore families descended from people who took part in the brain-eating rituals owe their existence to the reality of evolution.
(Via Mind Hacks.)