Evolution of signal diversity : predator-prey interactions and the maintenance of warning colour polymorphism in the wood tiger moth Arctia plantaginis

Abstract

Aposematic organisms avoid predation by advertising defences with warning signals. The theory of aposematism predicts warning signal uniformity, yet variation in warning coloration is widespread. The chemically defended wood tiger moth Arctia plantaginis shows both geographic variation and local polymorphism in warning coloration. In this thesis, I studied whether predation by local avian predators is driving the evolution of wood tiger moth warning colours. The close relatives of the wood tiger moth designated here to genus Arctia do not show similar colour polymorphism. The wood tiger moth is thus apparently under evolutionary radiation and provides a natural laboratory for observing current selection and studying the mechanisms leading to population divergence. We found evidence of positive frequency-dependent selection as predicted by aposematic theory, but the direction and strength of selection varied geographically. Variation in predator behaviour and the quality and abundance of alternative prey affected selection on wood tiger moth warning colour. Experiments with wild-caught blue tits (Cyanistes caeruleus) and great tits (Parus major) indicate, that colour is of foremost importance in prey discrimination and avoidance generalization. Birds did not generalize their learned avoidance among morphs, but could generalize between vaguely similar prey species showing similar warning colours. I conclude, that a) the evolution of wood tiger moth warning coloration is driven by predation of local avian predators, b) the direction of selection is affected by variation in predator and prey qualities at different levels (from individuals to communities), which can contribute to the maintenance of polymorphism, and that c) the strength of selection can vary spatially and temporally, and be counterbalanced or overruled by other evolutionary processes, promoting variation in warning colour.