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Can aposematic signals evolve by gradual change?

Abstract

Aposematic species, which signal conspicuously of their unprofitability to predators, have puzzled evolutionary biologists for over a century1,2. Although conspicuousness of unpalatable prey improves avoidance learning by predators3,4,5, it also involves an evolutionary paradox: with increasing detectability4,6,7,8 the deviant aposematic prey would suffer high predation initially from naive predators. Here we test a neglected idea7,8,9,10,11 that aposematic coloration may evolve by gradual change rather than by major mutations. Weak signals did not suffer high initial predation, but predators (great tits, Parus major) did not learn to separate them from cryptic palatable prey. Furthermore, enhanced avoidance of more conspicuous signals occurred only if predators had previously encountered relatively strong signals. Thus, the gradual-change hypothesis does not provide an easy solution to the initial evolution of aposematism through predator learning. However, the possibility remains that cost-free step-wise mutations over the range of weak signals could accumulate under neutral selection to produce effective strong signals.

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Figure 2: The mean relative predation risk of different signals in the generalization experiment.
Figure 1: The mean relative predation risk for all four aposematic signal types on five consecutive days in the learning experiment.

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Acknowledgements

We thank C. Rowe and T. Guilford for insightful discussions; J. A. Endler, V. Kaitala, E. Koskela, K. Kärkkäinen, A. Lyytinen, T. Mappes and S. Parri for help in clarifying the manuscript; and the Konnevesi Research Station, Helinä Nisu and ‘the academic hobby crafts club’ for immense help. The study was supported by the Academy of Finland. Authors after senior author are in alphabetical order.

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Correspondence to Leena Lindström.

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Lindström, L., Alatalo, R., Mappes, J. et al. Can aposematic signals evolve by gradual change?. Nature 397, 249–251 (1999). https://doi.org/10.1038/16692

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