Can aposematic signals evolve by gradual change?

Aposematic species, which signal conspicuously of their unprofitability to predators, have puzzled evolutionary biologists for over a century(.1,2). A lthough conspicuousness of unpalatable prey improves avoidance learning by predators(3-5), it also involves an evolutionary paradox: with increasing d etectability(4,6-8) the deviant aposematic prey would suffer high predation initially from naive predators. Here we test a neglected idea(7-11) that a posematic coloration may evolve by gradual change rather than by major muta tions. Weak signals did not suffer high initial predation, but predators (g reat tits, Parus major) did not learn to separate them from cryptic palatab le prey. Furthermore, enhanced avoidance of more conspicuous signals occurr ed only if predators had previously encountered relatively strong signals. Thus, the gradual-change hypothesis does not provide an easy solution to th e initial evolution of aposematism through predator learning. However, the possibility remains that cost-free step-wise mutations over the range of we ak signals could accumulate under neutral selection to produce effective st rong signals.