Mimetic Versus Disruptive Coloration in Intergrading Populations of Limenitis arthemis and Astyanax Butterflies

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Abstract
1) The North American butterflies, Limenitis arthemis and L. astyanax undergo phenotypic intergradation in a relatively narrow zone of approximately two degrees of latitude across the northeastern United States and southern Ontario. 2) Three adult color-pattern characters (white bands, dorsal blue-green iridescence, and dorsal red-orange spotting) were analyzed quantitatively in seven populations from Quebec to Virginia. Phenotypic stability occurs in the south for all three characters, but in the north for only the white bands. All three characters exhibit extreme variance in the intergradation zone, and the iridescence and spotting continue to be highly variable in the north. The mean values for all three characters shift from broad white bands, extensive red-orange spotting, and extreme reduction of iridescence in the north, to a southern population which is uniformly dark, lacking both the white bands and red-orange spotting, but being highly iridescent. 3) Hardy-Weinberg analyses of population samples from areas within the intergradation zone show that breeding is at random. Laboratory crosses show no evidence of heterogametic inviability, and studies of the male genitalia indicate complete overlapping in all characters, there being only a clinal increase in size from northern to southern populations. The two butterflies therefore are conspecific, and their relationship to each other is considered one of primary intergradation. 4) The color-pattern of L. astyanax and the dark female form of another North American butterfly, Papilio glaucus, closely resemble Battus philenor, and it is widely held that both are Batesian mimics of this highly unpalatable troidine butterfly. Both mimics are extensively sympatric with the model. In the zone where L. astyanax intergrades to L. arthemis, the black form of P. glaucus gives way to a monomorphic yellow population. In this same area, B. philenor becomes increasingly rare, and is absent to the north where, however, L. arthemis and the non-mimetic form of P. glaucus remain common. 5) It is proposed that selection favoring mimicry in the south gives way to selection favoring disruptive coloration to the north as the model becomes increasingly rare to absent. The intergradation zone of the two Limenitis butterflies thus appears to represent an area of phenotypic reversal from one adaptive peak to another. 6) It is likely that an arthemis-like ancestor spread eastward and southward, and evolved the astyanax coloration when once inside the range of B. philenor, the latter being of neotropical origin. 7) Selection is probably favoring the evolution of clear-cut dominance for the white band character, but not for the iridescence and red-orange spotting, since variation per se in the latter two probably enhances the disruptive color-pattern of the northern form. It is therefore unlikely that the three characters will ever come under the control of a super-gene.

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