One of the most striking properties of many mimetic species is the spectacularly diverse array of forms (morphs) produced. One of the challenges in understanding the origin and evolution of such systems is to explain how such diversity can arise and be maintained, particularly when the various forms are near-perfect mimics of many different animals.
In Batesian mimicry a non-defended (edible) species gains protection by copying one or more defended (inedible, toxic or bad-tasting) species. This is only effective if there is a strong likeness between model and mimic. The mimic is parasitic on the model(s) because the presence of the mimic causes the model’s warning colours to be less effective, so if predators encounter and eat one of the mimics, they will associate the bright colours with edibility, not with toxicity.
The advantage gained by mimicry is related to the proportions of model and mimic – the more mimics there are, the higher the chance that predators will not quickly learn that the colour pattern is a warning and the greater the number of individuals (of both species) which will be lost to predation.
We therefore expect selection in favour of evolving polymorphism (multiple colour patterns) in species which are Batesian mimics to spread the ‘cost’ of mimicry across many different models. Many Batesian mimetic butterflies are highly polymorphic with diverse arrays of different wing patterns.
Polymorphisms like this are very obvious, but polymorphism is much more widespread that you might think – examples such as the human blood groups are examples of exactly the same phenomenon. This makes the study of these butterfly wing patterns very important for helping us to understand the evolutionary biology of lots of different systems.