Signal-Based Frequency-Dependent Defense Strategies and the Evolution of Mimicry

Apostatic selection (for divergence of aspect) and aposematism (convergence or mimicry) can explain the universality of defensive signaling in animals necessarily exposed to perception by predators, but only if these two theories are synthesized. A hypothetical model of optimal predator behavior when faced with highly diverse prey appears to be adequate to explain, in very general terms, the coevolutionary relationship between apostasy and mimicry. The basis of the model is that predators rely on searching and avoidance images induced by the receipt, and continuous reinforcement, of threshold dosages of reward or punishment, which can be associated with prey aspect. Contact frequency with the prey is an important component of the dosages. Predation pressure will select for the evolution of signals in prey species which will have the effect, when correlated with the latter's individual defense capabilities and contact frequency, of helping to place the prey in either one of the two least dangerous zones of the predators' abstract field of perception, or classification. These are the zone of the avoidance image and the zone of ambiguity or novelty (of no image at all). This effect is achieved by adjusting the frequency of given signals through a coevolved divergence or convergence of aspect between coexisting individuals and species. That is to say, predator selection controls the relative frequencies of aspects, rather than of species. Patterns of variation in signal aspect cannot be completely understood unless the whole prey spectrum of generalized predators is considered simultaneously. The model is applied to the two most puzzling phenomena involving defensive signals: density-dependent phases in plague insects and mimicry complexes involving polymorphic models.