The evolution of temporal patterns of selfishness, altruism, and group cohesion

In intrademic selection models, individuals interact in groups, and this interaction phase is usually treated as a point in time. It is likely, however, that interactions take place over some time period. If selfishness is treated as a quantitative trait and this time period is explicitly considered, how does the evolutionarily stable strategy (ESS) level of selfishness or altruism change through time? Our main result is that, under biologically reasonable conditions, the ESS level of selfishness is expected to increase. Two of the assumptions behind this result are that there is a finite time horizon on the life of the group and that reproduction occurs continuously throughout the time period in question. If there is no time horizon or if all reproductive output is concentrated at the end of the time period, the ESS level of selfishness is constant. Our main result suggests that care must be taken when interpreting empirical data that is collected at different times and that altruism will often be most pronounced when groups first form. The model also demonstrates that, when individuals interact repeatedly over time, the evolution of altruism can be promoted through a mechanism other than reciprocity.