The evolution of mutualism with modifiers

Mutualisms are widespread, yet their evolution has received less theoretical attention than within-species social behaviors. Here, we extend previous models of unconditional pairwise interspecies social behavior, to consider selection for donation but also for donation-suppressing modifiers. We present conditions under which modifiers that suppress costly donation receive either positive or negative selection; assortment only at the donation locus always leads to selection for donation suppression, as in within-species greenbeard traits. However, genomewide assortment with modifier loci can lead to intermediate levels of donation, and these can differ in the two species even when payoffs from donation are additive and symmetric. When costly donation between species can evolve without being suppressed, we argue that it is most appropriately explained by indirect fitness benefits within the donating species, using partner species as vectors for altruism. Our work has implications for identifying both the stability and the ultimate beneficiaries of social behavior between species.