The evolution of aggression and the level of selection controversy

Abstract

This paper has two objectives. The first is to examine the “level of selection” controversy in an historical light, particularly as it relates to the evolution and function of aggressive behaviors. Individual and kin selection, on the one hand, and group selection, on the other, are viewed as alternative scientific paradigms for interpreting aggression and other behaviors. It is suggested that the crux of the conflict between these positions involves different ways of explaining altruism. Often, restraints in fighting are tacitly assumed to function first and foremost for the good of the group or species to the detriment of the actor – ie, to be altruistic and to have evolved by group selection. The frequent use of the group level “good for the species” explanations in various disciplines is noted. A second and overlapping objective of this paper is to present evidence against group selection level interpretations and to propose instead that individual level selection and inclusive fitness theory explain better and provide a heuristically more useful way of examining aggressive behavior. Toward these ends, ritualized aggressive behaviors, formerly assumed to exist for species preservation, are reinterpreted along individualistic lines. Next, the concept of inclusive fitness [Hamilton, 1964] is discussed as a theoretical breakthrough which adds support to an individual level approach. In short, the argument is set forth that animals, by and large, do not kill or seriously injure members of their own species because it usually is not in their genetic self-interest to do so; social patterns of interaction such as aggressive encounters should be examined in terms of their effects on the fitness of the individuals engaging in the behaviors before group adaptations are simply assumed. In illustration, data from several animal behavior studies are discussed. Consideration is also given to instances of intraspecific killing and wounding, which in fact occur much more frequently than has been suggested by members of a group selectionist school of thought. Infant killing reported in lions and various primate species is examined and shown to be explicable through individual selection and inclusive fitness theory. It is concluded that this approach represents a more viable theoretical framework for the evolutionary and functional study of both ritualized and other forms of limited animal aggression as well as more violent incidents of intraspecific wounding and killing.