Abstract
A model is presented for the evolution of several aspects of sociality based on reciprocal ties of social cooperation, modeling especially cooperative hunting behavior in carnivores. This model captures the possibility of a critical threshold in gene frequency, which, if reached, will lead to an explosion toward fixation of the "social" trait. This threshold phenomenon might be restated as follows: the precondition for evolution favorable to the specific form of social behavior considered is hard to satisfy, but-once this condition is satisfied-the tendency toward sociality is effectively irreversible. The simple model proposed appears to be highly robust, with most realistic changes additionally favoring the social gene.