Letters to Nature

Nature 397, 249-251 (21 January 1999) | doi:10.1038/16692; Received 14 August 1998; Accepted 9 November 1998

Can aposematic signals evolve by gradual change?

Leena Lindström1, Rauno V. Alatalo1, Johanna Mappes1, Marianna Riipi1 & Laura Vertainen1

  1. University of Jyväskylä, Department of Biological and Environmental Science, Konnevesi Research Station, PO Box 35, FIN-40 351 Jyväskylä, Finland

Correspondence to: Leena Lindström1 Correspondence and requests for materials should be addressed to L. Lindström (e-mail: Email: lilema@tukki.jyu.fi).

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.