Mullerian mimicry, in which both partners are unpalatable to predators, is often used as an example of a coevolved mutualism. However, it is theoretically possible that some Mullerian mimics are parasitic if a weakly defended mimic benefits at the expense of a more highly defended model, a phenomenon known as 'quasi-Batesian mimicry'. The theory expounded by Muller and extended here for unequal unpalatability, on the other hand, suggests that quasi-Batesian mimicry should be rare in comparison with classical, or mutualistic Mullerian mimicry. Evolutionarily, quasi-Batesian mimicry has consequences similar to classical Batesian mimicry, including unilateral 'advergence' of the mimic to the model, and diversifying frequency-dependent selection on the mimic which may lead to mimetic polymorphism. In this paper, theory and empirical evidence for mutual benefit and coevolution in Mullerian mimicry are reviewed. I use examples from well-known insect Mullerian mimicry complexes: the Limenitis-Danaus (Nymphalidae) system in North America, the Bombus-Psithyrus (Apidae) system in the north temperate zone, and the Heliconius-Laparus (Nymphalidae) system in tropical America. These give abundant evidence for unilateral advergence, and no convincing evidence, to my knowledge, for coevolved mutual convergence. Furthermore, mimetic polymorphisms are not uncommon. Yet classical mutualistic Mullerian mimicry, coupled with spatial (and possibly temporal) variation in model abundances convincingly explain these apparent anomalies without recourse to a quasi-Batesian explanation. Nevertheless, the case against classical Mullerian mimicry is not totally disproved, and should be investigated further. I hope that this tentative analysis of actual mimicry rings may encourage others to look for evidence of coevolution and quasi-Batesian effects in a variety of other Mullerian mimicry systems.
