Coloration evidence for natural selection

Animal coloration provided important early evidence for evolution by natural selection, at a time when little direct evidence was available. Three major functions of animal coloration were discovered in the second half of the 19th century, and subsequently used as evidence of selection: camouflage (protective coloration); mimicry, both Batesian and Müllerian; and aposematism.

Charles Darwin's On the Origin of Species was published in 1859, arguing from circumstantial evidence that selection by human breeders could produce change, and that since there was clearly a struggle for existence, that natural selection must be taking place. But he lacked an explanation either for genetic variation or for heredity, both essential to the theory. Many alternative theories were accordingly considered by biologists, threatening to undermine Darwinian evolution.

Some of the first evidence was provided by Darwin's contemporaries, the naturalists Henry Walter Bates and Fritz Müller. They described forms of mimicry in butterflies that now carry their names. These highly specific patterns of coloration are readily explained by natural selection, since predators such as birds which hunt by sight will more often catch and kill insects that are less good mimics of distasteful models than those that are better mimics; but the patterns are otherwise hard to explain. Darwinists such as Alfred Russel Wallace and later Hugh Cott and Bernard Kettlewell sought evidence that natural selection was taking place. Wallace noted that snow camouflage, especially plumage and pelage that changed with the seasons, suggested an obvious explanation as an adaptation for concealment. Cott described many kinds of camouflage, and in particular his drawings of coincident disruptive coloration in frogs convinced other biologists that these deceptive markings must have been selected for. Kettlewell experimented on peppered moth evolution, showing that the species had adapted as pollution changed the environment; this provided compelling evidence of Darwinian evolution.

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