Alfred Russel Wallace - Theory of Evolution - Natural Selection and Darwin - Warning Colouration and Sexual Selection

Warning Colouration and Sexual Selection

In 1867, Darwin wrote to Wallace about a problem he was having understanding how some caterpillars could have evolved conspicuous colour schemes. Darwin had come to believe that sexual selection, an agency to which Wallace did not attribute the same importance as Darwin did, explained many conspicuous animal colour schemes. However, Darwin realised that this could not apply to caterpillars. Wallace responded that he and Henry Bates had observed that many of the most spectacular butterflies had a peculiar odour and taste, and that he had been told by John Jenner Weir that birds would not eat a certain kind of common white moth because they found it unpalatable. "Now, as the white moth is as conspicuous at dusk as a coloured caterpillar in the daylight", Wallace wrote back to Darwin that it seemed likely that the conspicuous colour scheme served as a warning to predators and thus could have evolved through natural selection. Darwin was impressed by the idea. At a subsequent meeting of the Entomological Society, Wallace asked for any evidence anyone might have on the topic. In 1869, Weir published data from experiments and observations involving brightly coloured caterpillars that supported Wallace’s idea. Warning colouration was one of a number of contributions Wallace made in the area of the evolution of animal colouration in general and the concept of protective colouration in particular. It was also part of a lifelong disagreement Wallace had with Darwin over the importance of sexual selection. In his 1878 book Tropical Nature and Other Essays, he wrote extensively on the colouration of animals and plants and proposed alternative explanations for a number of cases Darwin had attributed to sexual selection. He revisited the topic at length in his 1889 book Darwinism. In 1890, he wrote a critical review in Nature of his friend Edward Bagnall Poulton's The Colours of Animals which supported Darwin on sexual selection, attacking especially Poulton's claims on the "æsthetic preferences of the insect world".