In the Origin of Species (1859), Charles Darwin introduced his theory of natural selection, the generally accepted mechanism for evolutionary change. More organisms are born than can survive and reproduce; there will consequently be a struggle for existence. Given naturally occurring variation, the struggle will bring on a process equivalent to a breeder's artificial selection: a differential reproduction leading to evolutionary change of a kind that centers on adaptation, producing contrivances like the hand and the eye. A matter of immediate interest was the level at which natural selection was supposed to operate. Does the struggle occur between individuals or between groups like species? If the latter, can adaptations benefit the group at the expense of the individual? Could one have "altruistic" adaptations where, instead of an organism selfishly serving its own ends, it sacrifices its well-being and...