Ground squirrels don't always dive for safety at the first sign of a predator, but instead sometimes they stand stiffly and give a shrill alarm call that alerts other squirrels to the predator. Evolutionary biologists refer to behaviors like alarm calls that increase the fitness of the recipients but lower the fitness of the actor as altruism. The evolution of altruistic behavior had a strong role in defining sociobiology, and it continues to be one of its core problems. This chapter shows why natural selection can favor altruism when relatives interact. It introduces several new tools for...

Ground squirrels don't always dive for safety at the first sign of a predator, but instead sometimes they stand stiffly and give a shrill alarm call that alerts other squirrels to the predator. Evolutionary biologists refer to behaviors like alarm calls that increase the fitness of the recipients but lower the fitness of the actor as altruism. The evolution of altruistic behavior had a strong role in defining sociobiology, and it continues to be one of its core problems. This chapter shows why natural selection can favor altruism when relatives interact. It introduces several new tools for building and solving analytical models, including George Price's covariance genetics framework. It also considers the prisoner's dilemma, positive assortment, common descent, and inclusive fitness. It reconstructs Hamilton's rule, using more-detailed genetic models of the evolution of altruism, and demonstrates why it can be used to understand the evolution of social behavior. It then derives Hamilton's rule using Price's method.