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Keywords:

  • Frequency-dependence—;
  • natural selection—;
  • ‘green beard’—;
  • warning colour

With a view to clarifying some recent controversies, the behaviour of a predator confronted with a mixture of models and mimics has been simulated by a simple Monte Carlo computer program.

When learning and forgetting are described by a stochastic model in which learning depends on experience, and forgetting on the passage of time, then the ‘spectrum of palatability’ (that is, the continuous variation in the desirability of prey to a predator, from the delectable, through the neutral, to the downright revolting), is converted into a spectrum of protection, in which palatable prey are heavily consumed and distasteful prey are largely avoided.

In such a system, a palatable (Batesian) mimic is protected, but causes an increase in the rate of predation on its model (compared with an identical but un-mimicked control); but when both model and mimic are distasteful (Muellerian mimics), both species gain in protection. The mimic in a Muellerian system can be regarded as the species which would be the less protected, and which therefore gains the more from the mimicry. When mimicry evolves by a single large mutation, it is this species which will change: the model will not alter in pattern at this stage. It is useful to recognize a further category, effectively neutral mimicry, in which the mimic, because it is rare or nearly neutral in palatability, has virtually no effect on the model.

The fitness of mimics is density-dependent: positively for Muellerian mimics (more protection when common) and negatively for Batesian mimics (more protection when rare). The computer model thus gives results which conform with some of the traditionally accepted differences between Batesian and Muellerian mimicry.

It is suggested that warning colour will usually evolve in the form of Muellerian mimicry, or perhaps as Batesian mimicry in species whose behaviour makes them conspicuous, and that distastefulness will usually evolve in cryptically-coloured species. The usual evolutionary pathway to full aposematism is predicted to be distastefulness first and warning colour second.