WE’D LIKE TO THINK of science as objective and apolitical, but of course it is also a human institution shaped by the biases of its participants. April 2017’s “March For Science” inadvertently put its claims of objectivity under a public spotlight when organizers openly grappled with accommodating competing visions and defining their goals. “Science is universal, international, inclusive, nonpartisan, a-political, a-gender, a-race, & a-ideological,” intoned science historian Michael Shermer on Twitter, reflecting a view widely held by older (mostly white, mostly male) academics. Many younger researchers disagreed, reflecting the increasing influence of critiques from the humanities and social sciences. They argued that science writ large should engage with its own failings and the politics of the day. As Shay-Akil McLean put it in a widely read essay: “[W]e cannot separate knowledge from the contextual connected histories and conditions from which it is produced.”

Richard Prum’s new book The Evolution of Beauty springs into this fray like the explosive erection of a duck penis, to borrow one of its own more memorable images. Explicitly political, it synthesizes decades of research into the evolution of animal sexuality to argue for the importance of female reproductive autonomy. “[W]e will discover that reproductive freedom of choice is not merely a political ideology invented by modern suffragettes and feminists,” writes Prum in the introduction. “Freedom of choice matters to animals, too.” This is a compelling argument, and a welcome examination of gender bias in evolutionary biology. Yet it also raises the thorny question of whether scientific consensus, with its checkered history on issues of race and gender, can ever be a solid foundation for a “just” society.

But first, Prum has a story to tell us about the natural world.

An evolutionary biologist who primarily studies birds, he necessarily begins with the works of Charles Darwin. Though On the Origin of Species triggered a scientific revolution and made Darwin a household name, Prum joins the growing chorus of scientists and historians who argue that The Descent of Man, and Selection in Relation to Sex (1871) is his most radical work. With the public’s renewed appetite for scientific perspectives on gender, the book’s central insight fits our times: female sexual preferences have shaped how animals have evolved, and they may be responsible for much of what we consider beautiful in the tree of life.

To explain the science behind this theory, Prum primarily draws on gaudy sexual displays in birds. “The male Argus Pheasant acquired his beauty gradually through the preference of the females during many generations,” he writes, referring to a Malaysian species known for its peacock-like ornamentation. In other words, female desire for longer tails — whether arbitrary or initially reflecting some adaptive value — became stuck in a positive feedback loop. Males with the longest tails have more offspring, who themselves have long tails and are more successful in attracting mates, increasing the overall frequency of long tail genes in the population. Here, the evolutionary mechanism at play is not increased fitness for a particular environment due to natural selection, but increased fitness due to sexual selection — to the likelihood certain offspring will themselves reproduce because of their desirability to other members of their species.

In both Beauty itself and promotional interviews, Prum suggests sexual selection by arbitrary mate choice is rejected by most biologists. This is an exaggeration verging on a straw man: the bones of the theory have been a feature of introductory evolution textbooks for at least the past two decades. What is true, however, is that this wasn’t the case in the early years of Prum’s career, and even today the finer details of the relationship between natural and sexual selection remain controversial. Descent initially met with wide criticism, in no small part because it challenged the rigid sexual mores of Victorian England. Critics sought to explain away the existence of beauty using Darwin’s other ideas. The examples of sexual selection by mate choice in Descent were “more conveniently reckoned” as cryptic cases of natural selection, wrote the 19th-century biologist St. George Mivart. Alfred Russel Wallace, famous for independently discovering evolution by natural selection, went even further, claiming that his strongly worded rebuttal of Descent made him “the advocate of pure Darwinism” — or, as Prum puts it, more Darwinian than Darwin himself. Both Mivart and Wallace were promoting what is today known as the “honest signaling” theory, whereby sensory extravagances associated with reproduction in animals — in a word, beauty — exist solely as “honest” signals of an individual’s genetic worth. A male peacock’s stunning tail feathers, for instance, indicate its superior strength, vitality, resistance to disease, and general fitness. How else, goes the theory, could it survive in the face of its obvious disadvantages?

The enduring appeal of birds and the tension between these two explanations (for their great diversity of sexual ornaments and displays) drives the remainder of the book. Prum gives us a tour of avian mating habits worldwide, beginning with the family of birds that were the focus of his PhD thesis in the 1980s, the manakins. A group of approximately 60 species found across Central and South America, manakins are small songbirds known for their showy mating behaviors, which occur in a communal territory known as a lek. Here, numerous males compete for female attention via extravagant displays of their plumage. The male golden-headed manakin, for instance, slides backward across branches in a convincing pantomime of Michael Jackson’s moonwalk. The club-winged manakin creates a haunting, electronic-sounding song with its deformed wing feathers. As Prum emphasizes, these traits range from useless to actively detrimental to an individual’s ability to fly. As such, they’re a convincing example of how female manakins’ arbitrary aesthetic preferences shaped evolution. Or, to use the catchphrase he coins: “Beauty Happens,” and sexual selection through female mate choice is, he argues, its cause.

Providing an intimate view into the lives of creatures a world away from New Haven, Beauty’s lovingly detailed descriptions of birds in their natural habitat are full of an infectious sense of wonder. They represent Prum at his best. Somewhat ironically, however, these passages reminded me of a famous quote from Alfred Russel Wallace, in The Malay Archipelago (1869), referencing another group of gaudily plumaged birds featuring a lek mating system (New Guinea’s birds of paradise):

I thought of the long ages of the past, during which the successive generations of this little creature had run their course — year by year being born, and living and dying amid these dark and gloomy woods, with no intelligent eye to gaze upon their loveliness […] This consideration must surely tell us that all living things were not made for man. Many of them have no relation to him. The cycle of their existence has gone on independently of his, and is disturbed or broken by every advance in man’s intellectual development; and their happiness and enjoyments, their loves and hates, their struggles for existence, their vigorous life and early death, would seem to be immediately related to their own well-being and perpetuation alone.

Despite their strident differences in opinion over sexual selection, not to mention the century and a half that separates them, Prum and Wallace appear to share a fundamental appreciation for the role of an organism’s subjective experience. Both break from mainstream science in suggesting that something fundamentally unmeasurable may play a significant role in determining biological reality. For Prum, what Wallace terms a species’ “happiness and enjoyments […] [its] loves and hates” can shape its evolutionary destiny. And though another being’s experience of the world may be unknowable, the demonstrable ability in nonhuman animals for detecting differences among potential mates and then using these differences to make reproductive choices is a powerful argument for the importance of their subjectivity. As Prum puts it plainly in the introduction: “[A]nimals can play a distinct and vital role in their own evolution.”

Even abstracted to animals in general, it’s a loaded idea. Though never explicitly referenced in Beauty, a contemporary discussion over whether evolutionary biology needs a new, “extended” synthesis of its main principles treads similar ground in its discussion of “niche construction.” Both this debate and controversies over mate choice theory are at their core philosophical arguments over whether organisms are passive objects that selection acts on, or active subjects that can shape their own selective environments. Given these broad premises, it’s unsurprising that Beauty eventually turns its argument for desire as a motor of evolution back on us: if the aesthetic preferences (and subjective experiences) of animals can shape their evolutionary trajectories, then surely it happens in humans, too.

Prum is quick to acknowledge that his arguments on the evolution of Homo sapiens are speculative, extrapolating from but not concretely grounded in his own research on birds. As in his earlier case studies, he compares physical and behavioral traits among closely related species to generate his evidence. The absence of human female secondary sex characteristics such as curved hips, large breasts, and ornamental hair patches in chimpanzees are, he argues, the product of selection by aesthetic mate choice, having “arbitrarily coevolved with male sexual preferences.” This is not in and of itself particularly controversial. But Prum then goes on to apply the same evolutionary logic to demonstrate how female mate choice shaped both the social qualities and the anatomy of males, whose features have predictably received far less scrutiny. Male social traits, he argues, have been selected in part due to the traditional role of females (on evolutionary timescales) as primary caregivers, invested in qualities that “indicate the potential for relationship endurance.” He discusses the human male in terms of a single sexual ornament, the penis, noting how its odd structure compared to other great apes strongly indicates selection for something perennially controversial: female sexual pleasure.

It’s here that Prum introduces his second catchphrase, “Pleasure Happens.” The remainder of the book explores the diversity of human sexuality and partnership through the lens of mate choice and aesthetic preferences. The female orgasm, for instance, owes its current form to the fact that “females have preferred to mate, and remate, with males who stimulated their own sexual pleasure”; human same-sex behavior “might have evolved through female mate choice as a mechanism to advance female sexual autonomy.” In other words, many features of feminist, socially liberal politics are not the biological or historical aberrations that conservatives think they are. Instead, they’re the predictable outcome of millennia of evolution driven by both sexual and natural selection, as tightly linked to what it means to be human as anything in our society.

For someone like me who shares Prum’s politics, this makes for emotionally satisfying reading. It also fits neatly into the broader liberal media ecosystem, with its refrain that liberal politics simply reflect factual truths about the world. His takedowns of the field of evolutionary psychology, which draws on adaptive explanations for human behavior (and so can easily be criticized for being overly reductive), are similarly satisfying. “There is never any doubt what the conclusion of any evolutionary psychology study will be,” he writes, referring to the preordained conclusion that natural selection is the only evolutionary mechanism at play. “The only question is how far the study will have to go to get there.”

And yet.

For all its power as a corrective to the pervasive male bias in science, it’s hard not to wonder: what would all this mean if Prum were wrong? The very nature of his metanarrative of scientific change, not to mention his commentary on the transience of many theories, suggests that today’s affirming conclusions about the driving force of female autonomy in evolution are as vulnerable to upset as the “honest signaling” dogma of Prum’s early career.

The history of science is littered with tales that should caution us against political readings of evolutionary biology, perhaps especially when they support our current beliefs. Though Darwin’s views on the biological basis of race were mixed, the publication of Origin informed the rise of so-called scientific racism, based on imagined evolutionary hierarchies. Several decades later, the advent of genetics theory led to support for eugenics programs to “improve” the human population by sterilizing (in the United States) and euthanizing (in Germany) individuals with undesirable traits. In our own era of debate over the extended evolutionary synthesis, it’s easy to imagine a shift to a far less feminist theory of biological change. If, as his book argues, progress is nonlinear and worldviews are repeatedly overturned even within the cloistered world of academic science, then outside its precincts, the process is necessarily messier and potentially far more dangerous.

As it turns out, many of the book’s own anecdotes about the evolution of sexual behavior show darker trajectories. Among our close primate relatives, for instance, female mate choice is nearly nonexistent and sexual coercion is the norm. (As Prum admits, “It cannot be that fun to be a female monkey.”) Or to elaborate on those duck penises the evolution of the explosive reproductive anatomy of male ducks and convoluted, corkscrew vaginas in female ducks has been driven by competing desires for sexual control and sexual autonomy. If we had the evolutionary history of baboons or mallards, would we want to lean on it to justify a kinder, more accepting world?

For this reason, there’s an unwritten but necessary caveat to Beauty: while evolutionary biology and science are inherently political and can provide a framework for understanding political issues, the hard work of convincing others should rely on moral, not empirical, arguments. Female reproductive autonomy matters not because it is an important force in evolution, but because it is a fundamental human right in the world we want to live in.

In other words, Beauty Happened, and Pleasure Happened. But they easily might not have, and we easily can lose them.

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Ethan Linck is a PhD candidate in biology at the University of Washington. His writing has appeared in Jacobin and The Stranger.