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item Sword, Gregory

Submitted to: Proceedings of the Royal Society of London B
Publication Type: Proceedings
Publication Acceptance Date: 4/24/2002
Publication Date: 7/18/2002
Citation: Sword, G.A. 2002. A role for phenotypic plasticity in the evolution of aposematism. Proceedings of the Royal Society of London B. 269:1639-1644.

Interpretive Summary: The evolution of warning coloration (aposematism) has been a long-standing mystery in biology. How can warning coloration initially evolve when conspicuous colors attract predators whom have yet to learn that prey are advertising their distastefulness? Recently we have come to understand that some insects can express warning coloration only when they become locally abundant. This flexibility reduces the danger to prey of being conspicuous when they are rare. The ability to shift between crypsis and warning coloration with changes in local population density is known as density-dependent warning coloration (or density-dependent aposematism). This study demonstrates that the ability to change color and express density-dependent warning coloration has evolved differently between closely-related palatable and unpalatable Schistocerca emarginata (=lineata) grasshopper populations. This difference suggests that the ability of insects to express density-dependent color changes can evolve in response to the relative costs and benefits of being conspicuous. These findings may help to explain the evolution of warning coloration because they suggest that phenotypic plasticity in coloration can provide an adaptive intermediate stage during the evolutionary transition from cryptic to conspicuous color patterns.

Technical Abstract: The evolution of warning coloration (aposematism) has been difficult to explain because rare conspicuous mutants should suffer a higher cost of discovery by predators relative to the cryptic majority while at frequencies too low to facilitate predator aversion learning. Traditional models for the evolution of aposematism have assumed conspicuous prey phenotypes to be genetically-determined and constitutive. In contrast, we have recently come to understand that warning coloration can be environmentally-determined and mediated by local prey density, thereby reducing the initial costs of conspicuousness. The expression of density-dependent colour polyphenism is widespread among the insects and may provide an alternative pathway for the evolution of constitutive aposematic phenotypes in unpalatable prey by providing a protected intermediate stage. If density-dependent aposematism can function as an adaptive intermediate stage for the evolution of constitutive aposematic phenotypes, differential reaction norm evolution is predicted among related palatable and unpalatable prey populations. Here I present empirical evidence which suggests that (i) the expression of density-dependent colour polyphenism has differentially evolved between palatable and unpalatable populations of the grasshopper, Schistocerca emarginata (=lineata) (Orthoptera: Acrididae), and (ii) variation in plasticity between these populations is commensurate with the expected costs of conspicuousness.