Data from: Sanctions, partner recognition, and variation in mutualism

Mutualistic interactions can be stabilized against invasion by
noncooperative individuals by putting such “cheaters” at a selective
disadvantage. Selection against cheaters should eliminate genetic
variation in partner quality—yet such variation is often found in natural
populations. One explanation for this paradox is that mutualism outcomes
are determined not only by responses to partner performance but also by
partner signals. Here, we build a model of coevolution in a symbiotic
mutualism, in which hosts’ ability to sanction noncooperative symbionts
and recognition of symbiont signals are determined by separate loci, as
are symbionts’ cooperation and expression of signals. In the model,
variation persists without destabilizing the interaction, in part because
coevolution of symbiont signals and host recognition is altered by the
coevolution of sanctions and cooperation, and vice versa. Individual-based
simulations incorporating population structure strongly corroborate these
results. The dual systems of sanctions and partner recognition converge
toward conditions similar to some economic models of mutualistic
symbiosis, in which hosts offering the right incentives to potential
symbionts can initiate symbiosis without screening for partner quality.
These results predict that mutualists can maintain variation in
recognition of partner signals or in the ability to sanction
noncooperators without destabilizing mutualism, and they reinforce the
notion that studies of mutualism should consider communication between
partners as well as the exchange of benefits.