 It is easy to see how the two-pronged definition of a unit of selection—as interactor and manifestor of adaptation—held sway for so long in the species selection debates. After all, it had dominated much of the group selection debates for so long. Some of the confusion and conflict over higher-level units of selection arose because of an historical contingency—the early group selectionist’s implicit definition of a unit of selection and the responses it provoked (Wynne Edwards 1962; Borrello 2010).