THE MAINTENANCE OF NON-MIMETIC FORMS IN A DIMORPHIC BATESIAN MIMIC SPECIES.

Abstract

The females of the butterfly Papilio glaucus are dimorphic. A black form mimics the unpalatable butterfly Battus philenor; a yellow form is similar to the monomorphic male coloration. The locus controlling this pattern is Y-linked. The close correspondence of the frequency of the mimic form and the abundance of the model indicate that mimicry may account for the existence of the dimorphism (Brower and Brower 1962; Platt and Brower 1968). Burns (1966) has demonstrated by counting spermatophores that males mate preferentially with the non-mimetic females. From this he infers that if a single mating is insufficient to fertilize all of the female eggs then a selective advantage will accrue to the non-mimic. If the mating advantage balances the mimetic advantage of the black form the dimorphism will be maintained in the population. Recently Levin (1973) has investigated the breeding biology of the different female forms of Papilio glaucus. He concluded that although preferential and multiple mating may occur, one insemination is sufficient to fertilize all of a female's eggs. There are no fertility or fecundity differences between the two female morphs. Prout (1967) has argued that frequencydependent selection for the mimic and a mating preference for the non-mimic may give rise to a stable polymorphism. But he warns of the dangers of interpreting the maintenance of polymorphisms by a system of opposing fitness components. Levin (1973) asserts that the mating preference must be frequency-dependent as, . .Such a conclusion is necessarily an outgrowth of the mathematical This paper describes some models which may be applicable to the resolution of this argument. If there is no mating preference for the yellow form then the only net selection will be the frequency-dependent selection for the mimiic. Let the population size of the model be N and the maximum population size of the females of the mimic species capable of being supported by the food resources be n; let m be the number of the mimic form. As the mimic increases in frequency relative to the model, the predator begins to associate the model pattern with the palatability of the mimic. When the predator starts to accept individuals with the model pattern more often than it rejects them, on average, the advantage of the mimicry is lost. There is, therefore, a limit to the number of mimics which will gain protection from their resemblance to the model. Let the limiting number be mmax.