The evolution of imperfect mimicry

Examples of imperfect resemblance between Batesian mimics and their models appear widespread in the natural world, but so far few quantitative models have been proposed to explain the phenomenon. I used a simple signal detection model to show that the relationship between model-mimic similarity and mimic effectiveness is typically nonlinear. In particular, I found that there will be little or no further selection to improve model-mimic resemblance beyond a certain level if the model species is costly to attack, if the mimic species is not particularly profitable (e.g., hard to catch), or if the mimic is relatively rare. When there are two different sympatric model species, then mimics should usually evolve a phenotypic similarity to one or the other model species, but not to both. In contrast, when several model species occur in different areas (or emerge at different times) and individual mimics use each Of these areas, then the optimal phenotype should be a jack-of-all-trades intermediate phenotype that does not closely resemble any particular model species. Somewhat surprisingly, the theory predicts that if mimics spend an equal amount of time with each model species, then the optimal intermediate phenotype should more closely resemble the least numerous and least noxious model. This phenomenon arises because a vague similarity to all extremely noxious species is usually sufficient to guarantee significant protection, whereas a much closer resemblance to a mildly noxious model species is necessary to afford a similar level of benefit.