  Individual success in group-structured populations has two components. First,
an individual gains by outcompeting its neighbors for local resources. Second,
an individual's share of group success must be weighted by the total
productivity of the group. The essence of sociality arises from the tension
between selfish gains against neighbors and the associated loss that
selfishness imposes by degrading the efficiency of the group. Without some
force to modulate selfishness, the natural tendencies of self interest
typically degrade group performance to the detriment of all. This is the
tragedy of the commons. Kin selection provides the most widely discussed way in
which the tragedy is overcome in biology. Kin selection arises from behavioral
associations within groups caused either by genetical kinship or by other
processes that correlate the behaviors of group members. Here, I emphasize
demography as a second factor that may also modulate the tragedy of the commons
and favor cooperative integration of groups. Each act of selfishness or
cooperation in a group often influences group survival and fecundity over many
subsequent generations. For example, a cooperative act early in the growth
cycle of a colony may enhance the future size and survival of the colony. This
time-dependent benefit can greatly increase the degree of cooperation favored
by natural selection, providing another way in which to overcome the tragedy of
the commons and enhance the integration of group behavior. I conclude that
analyses of sociality must account for both the behavioral associations of kin
selection theory and the demographic consequences of life history theory.
