Just before the winter solstice brings autumn to an end, here’s a chance to blog about the great evolutionary biologist–and student of fall foliage–William Hamilton. Hamilton, who died in 2000, has never reached the household-name status of other evolutionary biologists such as E.O. Wilson or Richard Dawkins or Stephen Jay Gould. But he deserves a place of privilege, for all his profoundly influential ideas. He found an explanation for altruistic behavior in many insect species by expanding biology’s notion of fitness to include the genes an individual shares with its relatives. He offered one of the best-supported theories for the origin of sex–as a way for a species to keep ahead of its parasites in their evolutionary arms race.