Darwin's views on sexual selection were opposed strongly by his co-discoverer of natural selection, Alfred Russel Wallace, though much of his "debate" with Darwin took place after Darwin's death. Wallace argued against sexual selection, saying that the male-male competition aspects were simply forms of natural selection, and that the notion of female mate choice was attributing the ability to judge standards of beauty to animals far too cognitively undeveloped to be capable of aesthetic feeling (such as beetles).[43]
Wallace also argued that Darwin too much favoured the bright colours of the male peacock as adaptive without realising that the "drab" peahen's coloration is itself adaptive, as camouflage. Wallace more speculatively argued that the bright colours and long tails of the peacock were not adaptive in any way, and that bright coloration could result from non-adaptive physiological development (for example, the internal organs of animals, not being subject to a visual form of natural selection, come in a wide variety of bright colours). This has been questioned by later scholars as quite a stretch for Wallace, who in this particular instance abandoned his normally strict "adaptationist" agenda in asserting that the highly intricate and developed forms such as a peacock's tail resulted by sheer "physiological processes" that were somehow not at all subjected to adaptation.
Apart from Wallace, a number of scholars considered the role of sexual selection in human evolution controversial. Darwin was accused of looking at the evolution of early human ancestors through the moral lens of the 19th century Victorian society. Joan Roughgarden, citing many elements of sexual behaviour in animals and humans that cannot be explained by the sexual selection model, suggested that the function of sex in human evolution was primarily social.[47] Joseph Jordania suggested that in explaining such human morphological and behavioural characteristics as singing, dancing, body painting, wearing of clothes, Darwin (and proponents of sexual selection) neglected another important evolutionary force, intimidation of predators and competitors with the ritualised forms of warning display. Warning display uses virtually the same arsenal of visual, audio, olfactory and behavioural features as sexual selection. According to the principle of aposematism (warning display), to avoid costly physical violence and to replace violence with the ritualised forms of display, many animal species (including humans) use different forms of warning display: visual signals (contrastive body colours, eyespots, body ornaments, threat display and various postures to look bigger), audio signals (hissing, growling, group vocalisations, drumming on external objects), olfactory signals (producing strong body odors, particularly when excited or scared), behavioural signals (demonstratively slow walking, aggregation in large groups, aggressive display behaviour against predators and conspecific competitors). According to Jordania, most of these warning displays were incorrectly attributed to the forces of sexual selection.[48]
While debates on the subject continued, in January 1871 Darwin started on another book, using leftover material on emotional expressions, which became The Expression of the Emotions in Man and Animals.
In recent years controversy also involved the peacock tail, the most famous symbol of the principle of sexual selection. A seven-year Japanese study of free-ranging peafowl came to the conclusion that female peafowl do not select mates merely on the basis of their trains. Mariko Takahashi found no evidence that peahens expressed any preference for peacocks with more elaborate trains, such as trains having more ocelli, a more symmetrical arrangement or a greater length.[49] Takahashi determined that the peacock's train was not the universal target of female mate choice, showed little variance across male populations, and, based on physiological data collected from this group of peafowl, do not correlate to male physical conditions. Adeline Loyau and her colleagues responded to Takahashi's study by voicing concern that alternative explanations for these results had been overlooked and that these might be essential for the understanding of the complexity of mate choice.[50] They concluded that female choice might indeed vary in different ecological conditions. Jordania suggested that peacock's display of colourful and oversize train with plenty of eyespots, together with their extremely loud call and fearless behaviour has been formed by the forces of natural selection (not sexual selection), and served as a warning (aposematic) display to intimidate predators and rivals.[51]
Effect on society
In January 1871, Thomas Huxley's former disciple, the anatomist St. George Mivart, had published On the Genesis of Species as a critique of natural selection. In an anonymous Quarterly Review article, he claimed that the Descent of Man would unsettle "our half educated classes" and talked of people doing as they pleased, breaking laws and customs.[52] An infuriated Darwin guessed that Mivart was the author and, thinking "I shall soon be viewed as the most despicable of men", looked for an ally. In September, Huxley wrote a cutting review of Mivart's book and article and a relieved Darwin told him "How you do smash Mivart's theology... He may write his worst & he will never mortify me again". As 1872 began, Mivart politely inflamed the argument again, writing "wishing you very sincerely a happy new year" while wanting a disclaimer of the "fundamental intellectual errors" in the Descent of Man. This time, Darwin ended the correspondence.
