Coloration evidence for natural selection

Some of the first evidence was provided by Darwin's contemporaries, the naturalists Henry Walter Bates and Fritz Müller. They described forms of mimicry that now carry their names, based on their observations of tropical butterflies. 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

Snow camouflage

In his 1889 book Darwinism, the naturalist Alfred Russel Wallace considered the white coloration of Arctic animals. He recorded that the Arctic fox, Arctic hare, ermine and ptarmigan change their colour seasonally, and gave "the obvious explanation", that it was for concealment.^[8][a] The modern ornithologist W. L. N. Tickell, reviewing proposed explanations of white plumage in birds, writes that in the ptarmigan "it is difficult to escape the conclusion that cryptic brown summer plumage becomes a liability in snow, and white plumage is therefore another cryptic adaptation." All the same, he notes, "in spite of winter plumage, many Ptarmigan in NE Iceland are killed by Gyrfalcons throughout the winter."^[11]

More recently, decreasing snow cover in Poland, caused by

In the words of camouflage researchers

Cott explained, while discussing "a little frog known as Megalixalus fornasinii" in his chapter on coincident disruptive coloration, that "it is only when the pattern is considered in relation to the frog's normal attitude of rest that its remarkable nature becomes apparent... The attitude and very striking colour-scheme thus combine to produce an extraordinary effect, whose deceptive appearance depends upon the breaking up of the entire form into two strongly contrasted areas of brown and white. Considered separately, neither part resembles part of a frog. Together in nature the white configuration alone is conspicuous. This stands out and distracts the observer's attention from the true form and contour of the body and appendages on which it is superimposed".^[14]

Cott concluded that the effect was concealment "so long as the false configuration is recognized in preference to the real one".^[14] Such patterns embody, as Cott stressed, considerable precision as the markings must line up accurately for the disguise to work. Cott's description and in particular his drawings convinced biologists that the markings, and hence the camouflage, must have survival value (rather than occurring by chance); and further, as Cuthill and Székely indicate, that the bodies of animals that have such patterns must indeed have been shaped by natural selection.^[7]

Industrial melanism

Between 1953 and 1956, the geneticist Bernard Kettlewell experimented on peppered moth evolution. He presented results showing that in a polluted urban wood with dark tree trunks, dark moths survived better than pale ones, causing industrial melanism, whereas in a clean rural wood with paler trunks, pale moths survived better than dark ones. The implication was that survival was caused by camouflage against suitable backgrounds, where predators hunting by sight (insect-eating birds, such as the great tits used in the experiment) selectively caught and killed the less well-camouflaged moths. The results were intensely controversial, and from 2001 Michael Majerus carefully repeated the experiment. The results were published posthumously in 2012, vindicating Kettlewell's work as "the most direct evidence", and "one of the clearest and most easily understood examples of Darwinian evolution in action".^[15]

Mimicry

The butterfly Papilio polytes (left) mimics the unpalatable Pachliopta aristolochiae (right).

Batesian

In 1867, in a letter to Darwin, Wallace

Edward Bagnall Poulton's 1890 book, The Colours of Animals, renamed Wallace's concept of warning colours "aposematic" coloration, as well as supporting Darwin's then unpopular theories of natural selection and sexual selection.^[18] Poulton's explanations of coloration are emphatically Darwinian. For example, on aposematic coloration he wrote that

At first sight the existence of this group seems to be a difficulty in the way of the general applicability of the theory of natural selection. Warning Colours appear to benefit the would-be enemies rather than the conspicuous forms themselves, and the origin and growth of a character intended solely for the advantage of some other species cannot be explained by the theory of natural selection. But the conspicuous animal is greatly benefited by its Warning Colours. If it resembled its surroundings like the members of the other class, it would be liable to a great deal of accidental or experimental tasting, and there would be nothing about it to impress the memory of an enemy, and thus to prevent the continual destruction of individuals. The object of Warning Colours is to assist the education of enemies, enabling them to easily learn and remember the animals which are to be avoided. The great advantage conferred upon the conspicuous species is obvious when it is remembered that such an easy and successful education means an education involving only a small sacrifice of life."^[19]

Poulton summed up his allegiance to Darwinism as an explanation of Batesian mimicry in one sentence: "Every step in the gradually increasing change of the mimicking in the direction of specially protected form, would have been an advantage in the struggle for existence".^[19]

The historian of biology Peter J. Bowler commented that Poulton used his book to complain about experimentalists' lack of attention to what field naturalists (like Wallace, Bates, and Poulton) could readily see were adaptive features. Bowler added that "The fact that the adaptive significance of coloration was (sic) widely challenged indicates just how far anti-Darwinian feeling had developed.^[d] Only field naturalists such as Poulton refused to give in, convinced that their observations showed the validity of selection, whatever the theoretical problems."^[20]

Notes