Evolution of sex-limited mimicry in swallowtail butterflies
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Many organisms are sexually dimorphic for ecologically and socially important traits. One of the major foci of biology is to understand the evolution of such sexually dimorphic traits. Here I present my work on the evolution of a dimorphic trait, female-limited Batesian mimicry, in Papilio swallowtail butterflies. I begin by developing a character state path network to study the diversity of mimicry types and directionality of trait change during the evolution of female-limited mimicry. My phylogenetic analysis showed that female-limited mimicry has evolved independently in several groups of swallowtails, mainly via single-step character changes from monomorphic non-mimetic ancestors to female-limited mimetic descendents. Mimetic polymorphism has evolved in tandem with female-limited mimicry, the two being tightly correlated among mimetic species. Most traditional explanations of female-limited mimicry and mimetic polymorphism invoke sexual selection. In reviewing these hypotheses, I show that their key assumptions and predictions remain untested, and that sexual selection cannot maintain female polymorphism under some conditions. Sexual selection hypotheses are also unable to explain community ecological aspects of mimicry rings. Hence, I developed a novel model of female-limited mimicry based on sex-specific, frequency- and density-dependent advantages of mimicry. This model shows that both-sex mimicry, female-limited mimicry and mimetic polymorphism are favored along a gradient of relative mimic frequency. My ecological data from south Indian mimicry rings support a key prediction of this model. Finally, I employ the patterns of female-limited mimicry among swallowtail butterflies to highlight the contrast between Darwin’s sexual selection model and Wallace’s natural selection model of sexual dimorphism. I show that most of the sexual dimorphism in swallowtail wing color patterns is a product of natural selection for protective female coloration, predominantly in the form of female-limited mimicry. Thus, swallowtails support Wallace’s model of sexual dimorphism, underlining the importance of natural selection.