Abstract

Warning coloration signals are a familiar and conspicuous phenomenon in nature. However, the fundamental question of how warning signals initially evolved remains unanswered. For an unpalatable prey to evolve a signal to indicate its unprofitability, a rare and conspicuous mutant in a population of unpalatable cryptic prey must overcome a double disadvantage: a greater risk of being detected (as a result of being more conspicuous) and of being attacked (because its rarity results in a decreased association with aversion) by a predator,. Although the prior evolution of prey gregariousness may help warning signals to evolve, such an evolutionary order may not always be the case,. Here we present a theoretical model that describes a mechanism for the evolution of warning signals without having to invoke gregariousness. Specifically, a predator's generalization of stimulus in associative learning, with a `peak shift' towards greater conspicuousness,, allows a warning signal to evolve when the prey population density exceeds a threshold. Once a warning signal starts to evolve, it continues to grow; the resulting, evolutionarily stable conspicuousness of prey is discontinuously greater than that of the original cryptic prey, drawing an unambiguous distinction in their appearance.