Ricklefs and Wikelski 2002)]. In this context, Pianka (1970) argued that, “…natural selection will usually act to maximize the amounts of matter and energy gathered per unit time.” Brown et al. (1993) likewise offered an energetic definition in which fitness is “reproductive power, or the rate of conversion of energy into offspring.” This reproductive power was taken to be a function of both the rate of assimilation of energy from the environment and the rate of conversion of energy to offspring (but see (Kozlowski 1996)).

To Gowdy and Krall, the ultra-social nature of human groups allowed for a shift in the primary level of selection from the individual level to the group level. Thus, “With the transition to agriculture the group as an adaptive unit comes to constitute a wholly different gestalt driven by the imperative to produce surplus