Inclusive fitness theory from Darwin to Hamilton.

ONE of the central dogmas of modern behavioral ecology is that blood kinship plays a critical role in understanding the evolution of social behavior, particularly of costly social behavior such as altruism and cooperation. But it was not always so, and what I would like to do in this Perspectives is provide some historical context that led up to William Hamilton’s seminal work developing inclusive fitness theory. The story begins, not surprisingly, with Charles Darwin. The worker bees that sacrifice themselves to protect their hives—the ultimate example of animal altruism—were deeply troubling to Darwin. If increased reproduction is the currency of natural selection, then altruists should disappear—and fast. But they did not disappear, and Darwin was so puzzled by this that he spoke of altruism as a problem that he feared was ‘‘one special difficulty, which at first appeared to me to be insuperable, and actually fatal to the whole theory’’ (Darwin 1859, p. 236). Eventually, however, Darwin came up with an explanation. Since sterile worker bees were helping their blood relatives—especially the queen—Darwin hypothesized that natural selection might favor altruism at the level of blood kin. In a section of On the Origin of Species entitled ‘‘Objections to the theory of natural selection and instinct: neuter and sterile insects,’’ Darwin proposes that the problem of natural selection producing sterile castes that often risk their lives to protect others ‘‘disappears when it is remembered that selection may be applied to the family, as well as the individual and may thus gain the desired end’’ (Darwin 1859, p. 204). Blood kinship, and interactions among relatives, it turned out, was the key to solving Darwin’s problems with both social insect sterility and altruism. One hundred and four years later, biologist William Hamilton would formalize Darwin’s idea, but the path from Darwin to Hamilton was not smooth (Dugatkin 2006). That fact that it was not is not surprising. The nature of altruism makes it all too easy to drift from a scientific to a political, philosophical, and even a religious approach to this subject. Studying the structure of an atom is not personal, and neither is studying, for example, night vision in mammals. Studying altruism can be personal, however, because we all want to understand the origins of goodness. And it certainly was personal for the next two figures in the story of blood kinship and altruism—Thomas Henry Huxley and Peter Kropotkin (Woodcock and Avakumovic 1950; Desmond 1994).