Why mutualist partners vary in quality: mutation–selection balance and incentives to cheat in the fig tree–fig wasp mutualism

Mutualisms between species are ecologically ubiquitous but evolutionarily puzzling. Host discrimination mechanisms that reduce the fitness of uncooperative symbionts can stabilise mutualism against collapse, but also present a paradox – if discrimination is effective, why do uncooperative symbionts persist? Here, we test whether mutations or fitness benefits of cheating best explain the prevalence of uncooperative wasps in the fig tree–fig wasp mutualism. By combining theory with field‐collected data we demonstrate that the proportions of pollen‐free wasps of strongly discriminating hosts are reached with reasonable mutation rates. In contrast, in weakly discriminating hosts, the required mutation rates, assuming a single locus, are untenably high, but the required cheater advantages fall within expected ranges. We propose that when discrimination is weak, uncooperative symbionts proliferate until they reach the equilibrium proportion that balances costs and benefits of cheating. Our results suggest that mechanisms that resolve the paradox of uncooperative symbionts differ among host species.