Global biogeography of warning coloration in the butterfly Danaus chrysippus

Abstract

Warning coloration provides a textbook example of natural selection, but the frequent observation of polymorphism in aposematic species presents an evolutionary puzzle. We investigated biogeography and polymorphism of warning patterns in the widespread butterfly Danaus chrysippus using records from citizen science (n = 5467), museums (n = 8864) and fieldwork (n = 2586). We find that polymorphism in three traits controlled by known mendelian loci is extensive. Broad allele frequency clines, hundreds of kilometres wide, suggest a balance between long-range dispersal and predation of unfamiliar morphs. Mismatched clines for the white hindwing and forewing tip in East Africa are consistent with a previous finding that the black wingtip allele has spread recently in the region through hitchhiking with a heritable endosymbiont. Light/dark background coloration shows more extensive polymorphism. The darker genotype is more common in cooler regions, possibly reflecting a trade-off between thermoregulation and predator warning. Overall, our findings show how studying local adaptation at the global scale provides a more complete picture of the evolutionary forces involved.

Keywords: aposematic; citizen science; clines; dispersal; polymorphism.

Figure 1.

Colour pattern genotype frequency distributions (a–c) and number of records (d–f) based on 17 094 records. Each square represents a 4 × 4 degree grid cell and its colour represents the frequency of the homozygous recessive genotype (a–c) or number of records (d–f). Heterozygous and dominant homozygous genotypes were combined as their phenotypes are often indistinguishable (electronic supplementary material, figure S1).

Figure 2.

Transects and allele frequency clines. Each point along the transect represents a grid cell within 450 km from the transect, with the size of the point drawn relative to the number of records. The solid line is the best-fit model, and the surrounding grey region represents the 95% credible fuzzy cline region. Two solid vertical lines represent the cline centre confidence interval (details in electronic supplementary material, table S2). In all cases, the frequency represents the recessive allele (a, b or c), except for the ground colour locus in panel b, where the frequency of the dominant allele (B) is shown for convenience. Map inserts show the grid cells included and their corresponding frequencies. The confidence interval for cline centres are indicated by blue boxes.