Biologists could have revived sexual selection in the 1930s by building upon Fisher's work. If they had, the benefits to the behavioral sciences would have been enormous. Anthropologists could have studied real mate choice in primitive cultures instead of concentrating on incest taboos and inter-tribal marriages. Psychotherapists might have rejected Freud's Lamarckian theories about our ancestors inheriting acquired memories of sexually competitive patricide and incest. Psychologists might have overcome the Behaviorist obsession with maze learning by rats, and found a more fruitful way to study human nature. The pioneering sex researchers Alfred Kinsey, William Masters, and Virginia Johnson could have interpreted their questionnaire studies from a richer evolutionary perspective. Archeologists interested in human evolution might not have been so concerned with hunting and warfare, and so baffled by cave paintings and Venus statuettes. Yet none of this happened.
Sexual selection's modern neglect owed more to scientific problems than to ideological biases. One problem is that sexual selection is hard to model mathematically. When a species is adapting to a fixed environment through natural selection, it is possible to predict how a given gene with a given survival effect will spread through a population. With sexual selection, however, the pressures come from other members of the species, which are themselves evolving. It is hard to know where to begin an analysis of sexual selection, because the feedback loops between sexual preferences and sexual ornaments make evolution hard to model and hard to predict. Only in the 1980s did some brilliant mathematical biologists finally start to develop workable models of sexual selection.
Also, the biologists of the Modern Synthesis were consumed by the problem of speciation—how a lineage splits into two distinct species that no longer interbreed. Sexual selection was seen as a possible explanation for speciation, rather than as an explanation for ornamentation. Mate preferences were viewed as nothing more than a way of making sure that individuals mate only with members of their own species. The boundaries of the species were defined by mate preferences, but these preferences were not viewed as ranking individual attractiveness within the species. For many biologists, such as Ernst Mayr, this led to the assumption that most sexual ornaments were nothing more than marks showing what species an animal is. Following Wallace, they were considered to be "species recognition signals."
Sexual selection also suffered at the hands of the early 20th-century doctrines of behaviorism in psychology and reductionism in the sciences generally. These warned against attributing any mental capacities to animals, and this made biologists feel uncomfortable talking about the evolution of female choice mechanisms. Even animal behavior researchers such as Konrad Lorenz and Niko Tinbergen viewed copulation as a stereotyped behavior that is "released" by a few simple stimuli. They did not view mate choice as a complex strategic decision with high stakes. Behaviorist psychologists were not willing to credit even humans with free will or the capacity for choice, so it seemed unscientific to talk about "mate choice" in animals rather than "sexual stimuli." The mid-20th century was the era of B. F. Skinner's manifesto Science and Human Behavior, in which people were portrayed as robots driven by conditioned associations. Only with the rise of cognitive psychology in the 1970s did it once again become intellectually respectable in psychology to talk about judgment and decision-making in humans or animals. By then, most psychologists had forgotten all about Darwin. When they thought of sex, they thought of Freud and his theories of subconscious drives and neurotic complexes. Human sexuality, with its alleged existential intricacies, had been set apart from animal sexuality, with its supposedly stereotyped copulation reflexes. A science of mate choice applicable to both animals and humans seemed an absurd conceit.
Moreover, many evolutionary biologists before the 1970s had a very limited concept of adaptation. To them, evolution basically solved problems of survival posed by the external environment. Evolution was supposed to be about the survival of the fittest and the good of the species. Sexual selection was neither progressive nor respectable. Certainly, runaway sexual selection was a theoretical possibility, but bizarre ornaments were not considered to be real adaptations. They impaired individual survival and predisposed species to extinction. Mere ornamentation was not a proper role for a genuine adaptation.
This narrow definition of adaptation was perhaps reinforced by 20th-century aesthetics, which held conspicuous, costly ornamentation in low regard. The modernist reaction against Victorian ornamentation may have spilled over into a reaction against Darwin's sexual selection theory. The Modern Synthesis coincided with the peak of an austere, modernist machine aesthetic. In the 1920s Walter Gropius and other theorists of the Bauhaus movement in Germany had argued that, in a socialist Utopia, working people would not waste time and energy hand-decorating objects for purchase by the rich, merely so the rich could show how much wasteful ornamentation they could afford. Form should follow function. Ornament was viewed as morally decadent and politically reactionary, while simplicity and efficiency were considered progressive. This anti-ornament aesthetic seems to have spilled over from culture into nature, leading 1930s biologists to express their contempt for sexual selection's baroque excesses. For example, the socialist biologist J. B. S. Haldane suggested that with sexual selection, "the results may be biologically advantageous for the individual, but ultimately disastrous for the species." In one of his 1938 papers, Julian Huxley declared sexual selection a selfish process because it may "favour the evolution of characters which are useless or even deleterious to the species as a whole." Similar views were held by leading biologists such as Konrad Lorenz, George Simpson, and Ernst Mayr right through to the 1960s. They believed that evolved adaptations, like modernist design, should serve their economic purposes simply, efficiently, and plainly. Sexual ornamentation served no legitimate species-benefiting purpose, so must be ignored or derogated.
Darwin's sexual selection theory was kept in exile by these five factors: mathematical difficulties, an overemphasis on ornaments as species-recognition markers, a mechanistic view of animal psychology, a narrow definition of biological adaptation, and a modernist machine aesthetic. In other words, Darwin's favorite idea was not ignored because there was evidence against it. On the contrary, the mountain of evidence presented in The Descent of
Man was never seriously challenged. Sexual selection was ignored because biology was not ready—ideologically, conceptually or methodologically—to deal with it.
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