Evolution by Sexual Selection

Charles Darwin published his second book “Sexual selection and the descent of man” in 1871 150 years ago, to try to explain, amongst other things, the evolution of the peacock’s train, something that he famously thought was problematic for his theory of evolution by natural selection. He proposed that the peacock’s train had evolved because females preferred to mate with males with more elaborate trains. This idea was very controversial at the time and it wasn’t until 1991 that a manuscript testing Darwin’s hypothesis was published. The idea that a character could arise as a result of a female preference is still controversial. Some argue that there is no need to distinguish sexual from natural selection and that natural selection can adequately explain the evolution of extravagant characteristics that are characteristic of sexually selected species. Here, I outline the reasons why I think that this is not the case and that Darwin was right to distinguish sexual selection as a distinct process. I present a simple verbal and mathematical model to expound the view that sexual selection is profoundly different from natural selection because, uniquely, it can simultaneously promote and maintain the genetic variation which fuels evolutionary change. Viewed in this way, sexual selection can help resolve other evolutionary conundrums, such as the evolution of sexual reproduction, that are characterised by having impossibly large costs and no obvious immediate benefits and which have baffled evolutionary biologists for a very long time. If sexual selection does indeed facilitate rapid adaptation to a changing environment as I have outlined, then it is very important that we understand the fundamentals of adaptive mate choice and guard against any disruption to this natural process.

Contextualizando The descent of man, de Charles Darwin: debates calorosos persistem após 150 anos de sua publicação

O presente artigo faz uma discussão do livro de Charles Darwin (1809-1882) The Descent of Man and Selection in Relation to Sex, de 1871, cuja primeira edição completa 150 anos de publicação em 2021. Embora tão famoso, e importante, quanto A origem das espécies, o Descent é, contudo, menos lido e o mais controvertido livro de Darwin, desde seu lançamento até os dias atuais. Os objetivos são o de recolher aspectos do contexto em que o livro foi escrito e problematizar algumas das questões polêmicas que o cercam. Para isso, inicialmente, por aproximações aos estudos de Darwin publicados a partir dos anos 1980, a abordagem historiográfica adotada é caracterizada como pós-positivista, contextualista e enriquecida por teorias multiculturais do conhecimento. O escopo e objetivos do Descent são apresentados, tendo em vista seu autor como representante da elite intelectual inglesa do século XIX. A seleção das polêmicas vivas hoje ocorreu em dois fóruns acadêmicos de 2021, uma disciplina sobre Darwin e um congresso internacional de estudos metacientíficos da biologia. As polêmicas foram reunidas em três grupos: 1) a escola craniométrica e a hierarquia das raças e civilizações; 2) a seleção sexual e os estereótipos culturais de gênero; 3) a seleção natural no âmbito humano e os movimentos eugênicos. As conclusões são desenhadas em convergência com os achados da historiografia recente, reconhecendo que a construção da teoria evolucionista de Darwin se deu na interação de mão dupla entre a ciência e a cultura, como é da natureza da construção de todo conhecimento científico. O seu trabalho teórico reflete elementos da sociedade vitoriana, com a qual o naturalista compartilhava as virtudes e os vícios.

The late ascent of Darwin's Descent: exploring human evolution and women's role for a new China, 1927–1965

Darwin's ideas held sway among Chinese intellectuals by the early twentieth century. Yet the usual emphasis was a Spencerism instead of Darwin's original ideas. As a result, translations of The Descent of Man in the early twentieth century quickly fell into oblivion. When the embryologist Zhu Xi (1900–62) eventually decided to give all evolutionary theories a comprehensive examination, he nevertheless found the idea of sexual evolution inadequate, as expressed in his volume Biological Evolution (1958). Only in the 1950s did serious efforts to translate Descent gather momentum, thanks to eugenicist and sociologist Pan Guangdan (1899–1967). Such efforts were not only responses to a renewed interest in Darwinism under the socialist regime, but also expressions that synthesized both scholars’ earlier paths in wrestling with schemes of human evolution and the roles of women in China's survival and renewal. Trained in different scientific and cultural milieus and holding almost oppositional views, the two scholars nevertheless converged in finding new meanings in Darwin's Descent.

Descent of Darwin: race, sex, and human nature

In 1871, Charles Darwin published Descent of Man and Selection in Relation to Sex, a text that extended, elaborated and completed his On the Origin of Species (1859). When he had published Origin, Darwin sought, albeit unsuccessfully, to skirt controversy; in Descent he waded into the fray on near-innumerable issues. Readers could find explicit the claim that humans had descended from apes, in addition to explorations of the similarities and apparent gulfs between ‘man’ and other animals. They also found Darwin's opinions on issues ranging from the origin and hierarchy of races to the question of women's education, from the source of altruistic bravery to the biological importance of aesthetic judgement, from his views on what his cousin would term ‘eugenics’ to the history of monogamy. In the last 150 years these ideas have been variously contested, rejected and recovered, so that the shadow of Descent extended into debates over the development of languages, the evolution of human sexualities, the ongoing possibilities of eugenics and the question of women's equality. In this volume, appearing during the sesquicentennial of the text's first appearance, one finds papers dedicated to all of these themes and more, laying out the roots and fruits of Darwin's Descent.

The evolution of Darwinian sexualities

Charles Darwin's Descent of Man was suffused with questions of courtship, mating and sex. Following in his footsteps, biologists throughout the twentieth century interrogated the sexual behaviour of humans and animals. This paper charts the fate of evolutionary theories of sexuality to argue that – despite legal and social gains of the past century – when biologists used sexual selection as a tool for theorizing the evolution of homosexual behaviour (which happened only rarely), the effect of their theories was to continuously reinscribe normative heterosexuality. If, at the end of the nineteenth century, certain sex theorists viewed homosexuality as a marker of intermediate sex, by the late twentieth a new generation of evolutionary theorists idealized gay men as hyper-masculine biological males whose sexual behaviours were uncompromised by the necessity of accommodating women's sexual preferences. In both cases, normative assumptions about gender were interwoven with those about sexuality. By the twenty-first century, animal exemplars were again mobilized alongside data gathered about human sexual practices in defence of gay rights, but this time by creating the opportunity for naturalization without recourse to biological determinism.

Evolutionary antagonisms and the progress of three categories of traits

Darwin in The Descent of Man deliberates over the question of progress in relation to three categories of traits – aesthetic, moral and intellectual – attending to their interplay. The later formulations of Thomas Henry Huxley and Alfred Russel Wallace shift and reframe the terms for weighing together progress and the relationship across these traits, downplaying the role of aesthetic assessments. Huxley and Wallace invoke ‘antagonisms’ countering, respectively, ‘ethical progress’ and ‘cosmic process’, ‘humanity – the essentially human emotion’ and ‘physical and even intellectual race-improvement’. Thereafter, evolutionary antagonisms reappear – whether to endorse, dismiss or overcome them – and they remain relevant in evolutionary arguments, whether made explicit or left implicit. Following a thread of ongoing appeals to this interplay of traits and corresponding antagonisms invoking Huxley's 1893 lecture ‘Evolution and ethics’, implicit differences appear in the treatment of aesthetic, moral and intellectual development. These treatments maintain the progress that their own ethical systems represented, even while granting moral variation and conceding independent/alternative notions of the beautiful. They generally took as granted the uniformity of intellectual judgements, where evolutionary progress was both ethical and intellectual/scientific, even when speculating on the development of different types of mind. As characteristic of future-oriented visions of progress by the first decades of the twentieth century, sexual selection was subsumed under natural selection.

Of lice and men: Charles Darwin, Henry Denny and the evidence for the human races as varieties or species

Charles Darwin never doubted the common ancestry of the human races. But he was open-minded about whether the races might nevertheless be so different from each other that they ought to be classified not as varieties of one species but as distinct species. He pondered this varieties-or-species question on and off for decades, from his time aboard the Beagle through to the publication of the Descent of Man. A constant throughout was his concern with something that he first learned on the Beagle voyage and that, on the face of it, seemed to favour the species ranking: the different races, he was told, play host to distinct species of lice. This paper reconstructs the long run of Darwin's reflections and interactions on race, lice and history, using his extended correspondence with Henry Denny – curator of the scientific collections of the Leeds Philosophical and Literary Society, and Britain's leading expert in the natural history of lice – as a window onto the social world whose imprint is everywhere in the pages of the Descent.