Darwinian and Wallacean evolution both explain sexual dichromatism in avian iridescence.
One of the longest-standing arguments in evolutionary biology is that between Darwin and Wallace on the evolution of sexual dichromatism, and which sex changed colour over time. While Darwin argued that males became more colourful over time, Wallace argued that females became less colourful, and numerous studies have lent support to each hypothesis over time. Iridescent colouration provides a particularly interesting system for studying sexual dichromatism because its conspicuousness is often associated with sexual selection and it has evolved multiple times. Here, using a dataset on iridescent colouration of 25,432 patches from 3,151 bird species, we provide one of the most comprehensive tests to date of the evolutionary origins of sexual dichromatism. We show that both hypotheses explain the evolution of iridescent sexual dichromatism in birds, but that Darwin's hypothesis is more strongly supported with males contributing stronger to evolutionary changes in dichromatism. Furthermore, our analyses support Darwin's prediction that sexual selection is associated to the evolution of dichromatism, but also Wallace's prediction that safer nests facilitate more colourful females (and males), and mostly in patches exposed during incubation.