  Many organisms live in populations structured by space and by class, exhibit
plastic responses to their social partners, and are subject to non-additive
ecological and fitness effects. Social evolution theory has long recognized
that all of these factors can lead to different selection pressures but has
only recently attempted to synthesize how these factors interact. Using models
for both discrete and continuous phenotypes, we show that analyzing these
factors in a consistent framework reveals that they interact with one another
in ways previously overlooked. Specifically, behavioral responses
(reciprocity), genetic relatedness, and synergy interact in non-trivial ways
that cannot be easily captured by simple summary indices of assortment. We
demonstrate the importance of these interactions by showing how they have been
neglected in previous synthetic models of social behavior both within and
between species. These interactions also affect the level of behavioral
responses that can evolve in the long run; proximate biological mechanisms are
evolutionarily stable when they generate enough responsiveness relative to the
level of responsiveness that exactly balances the ecological costs and
benefits. Given the richness of social behavior across taxa, these interactions
should be a boon for empirical research as they are likely crucial for
describing the complex relationship linking ecology, demography, and social
behavior.
