Interference competition and sexual selection promote polymorphism in Colias (Lepidoptera, Pieridae)

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Abstract ‘Alba’ is a genetic polymorphism in butterflies of the genus Colias (the sulphurs) which confers on its female carriers white (A–) rather than yellow (aa) wings. Previous studies of ‘alba’s’ resource‐based effects in Colias alexandra and Colias scudderi suggested the rare morph should be eliminated in each species. Here the selective regime acting on ‘alba’s’ wing colour effect is examined to determine if it can maintain polymorphism. Field experiments and observations reveal species‐specific differences in morph fecundity resulting from selection on wing colour. ‘Alba’ female and male Colias lose flight time, an index of Colias fitness, to unproductive courtship with pierines (white butterflies in the subfamily Pieridae), a form of interference competition. Flight time losses are greater in C. alexandra, which co‐occur more extensively with pierines than C. scudderi. ‘Alba’ females also spent less time being harassed by courting male Colias than yellow females; this form of sexual selection on morph wing colour is particularly strong in C. alexandra. Loss of male and ‘alba’ female Colias flight time and the degree of male Colias preference for the yellow morph correlate with the extent each Colias species co‐occurs with pierines, suggesting interference competition influences Colias courtship decisions and ‘alba’ frequency. Both interference competition and sexual selection are density dependent in their effects; these effects were modelled using Holling’s equation. Modelling results show temporal variation in Colias and pierine densities over the past 20 years caused morph fitness to vary such that in at least one year each morph in each species had greater relative fitness with respect to colour‐based selective pressures. Although this selective regime does not maintain polymorphism under temporal models of fitness variation, trade‐offs between these colour‐based effects and ‘alba’s’ temperature and resource‐based effects in a spatially heterogeneous environment could maintain polymorphism.