THE EVOLUTION OF TEMPORAL PATTERNS OF SELFISHNESS, ALTRUISM, AND GROUP COHESION

In intrademic selection models, individuals interact in groups, and th is interaction phase is usually treated as a point in time. It is like ly, however, that interactions take place over some time period. If se lfishness is treated as a quantitative trait and this time period is e xplicitly considered, how does the evolutionarily stable strategy (ESS ) level of selfishness or altruism change through time? Our main resul t is that, under biologically reasonable conditions, the ESS level of selfishness is expected to increase. Tire of the assumptions behind th is result are that there is a finite time horizon on the life of the g roup and that reproduction occurs continuously throughout the time per iod in question. If there is no time horizon or if all reproductive ou tput is concentrated at the end of the time period, the ESS level of s elfishness is constant. Our main result suggests that care must be tak en when interpreting empirical data that is collected at different lim es and that altruism will often be most pronounced when groups first f orm. The model also demonstrates that, when individuals interact repea tedly over time, the evolution of altruism can be promoted through a m echanism other than reciprocity.