ABSTRACT
This chapter considers the possibility that intercolony selection may be an effective evolutionary force in the cooperative spiders. It discusses the evolution of their highly female-biased sex ratios within the context of a multilevel selection model. The chapter also discusses empirical and theoretical evidence that cooperation may be responsible for giving rise to sufficiently cohesive groups on which selection may act. It argues that the nonlinear effects of cooperation on individual fitness may have important consequences on the existence, persistence, and dynamics of social groups. The chapter shows that the nonlinear effects of cooperation on individual fitness may not only be responsible for the existence of social groups, but may also contribute to their dynamical instability. By causing individuals to become dependent on one another under harsh environmental conditions, cooperation may start to switch the primary focus of selection from the individual to the group.
