The invisible moth: a beautiful illustration of natural selection

Moth camouflage Ecuador cloudforest Bellavista

This moth, found in the cloudforest in Ecuador (at Bellavista, which I have written more about here), blends in perfectly with its background, even down to the lichen spot mimics on the tips of its wings. The purpose of colour patterns such as these is to make the outline of the moth hard to detect, thus making it difficult for predatory birds to distinguish it against the bark of the tree. The success of the camouflage is a question of life or death for the moth. A moth that is a poor match is more likely to get eaten, and one that is a good match is more likely to live. Therefore the next generation will be dominated by descendents of moths that were successful at blending in. This is natural selection, famously stated (and often misunderstood) as ‘survival of the fittest’, with the most camouflaged moths in this example being the most fit. The predatory birds provide the ‘selection pressure’ on the moth population, which has evolved in response to this pressure to be so well camouflaged.

However, the process is dynamic: the birds are also responding to the moths. A bird who is better able to pick out a camouflaged moth against the tree will be better fed, and better able to raise a brood. So again those most successful (the fittest) will contribute more to the next generation than those less fit. Greater visual acuity will in turn select for better camouflaged moths, and so on, resulting in an ongoing coevolutionary relationship. In evolutionary biology, the ‘Red Queen hypothesis’ relates to cases such as these. The name refers to Alice in Through the Looking Glass, by Lewis Carroll, where Alice is told by the Red Queen that ‘here, you see, it takes all the running you can do, to keep in the same place’.

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