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The Evolution of Beauty
How Darwin's Forgotten Theory of Mate Choice Shapes the Animal World - and Us
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'Female love of beauty has got nothing to do with functionality: it is pure aesthetic evolution'
'THE sight of a feather in a peacock's tail... makes me sick,' wrote Darwin, worrying about how structures we consider beautiful might come to exist in nature. The view nowadays is that ornaments such as the peacock's stunning train, the splendid plumes of birds of paradise, bowerbirds' love nests, deer antlers, fins on guppies and just about everything to do with the mandarin goby are indications of male quality.
In such species, females choose males with features that indicate resistance to parasites (shapes go wonky, colours go flat if a male isn't immunologically buff) or skill at foraging (antlers need lots of calcium, bowers lots of time).
But in other cases, the evolutionary handicap principle applies, and the fact it's hard to stay alive while possessing a huge or brightly coloured attraction becomes the reason for the visual pizzazz. And when this process occasionally goes a bit mad, and ever bigger or brasher becomes synonymous with ever better, then the object of female fixation undergoes runaway selection until physiology or predation steps in to set limits.
What unites these explanations is that they are all generally credited to Darwin and his book The Descent of Man, and Selection in Relation to Sex. Here, biologists say, having set out his adaptationist stall in On the Origin of Species, Darwin proposed female choice as the driving force behind much of the animal world's visual exuberance.
And then along comes Richard Prum to tell you there's more to it than that. Prum is an ornithology professor at Yale University and a world authority on manakins, a group of sparrow-sized birds whose dazzling males perform mate-attracting gymnastics on branches in the understories of Central and South American forests. Years of watching the males carry on until they nearly collapsed convinced him that much of the selection is linked to nothing except a female love of beauty itself, that the only force pushing things forward is female appreciation. This, he says, has nothing to do with functionality: it is pure aesthetic evolution, with 'the potential to evolve arbitrary and useless beauty'.
As Prum recounts, this idea has not found the greatest favour in academic circles. But, as he makes plain, he's not alone. Once again, it seems Darwin got there first, writing in Descent that 'the most refined beauty may serve as a sexual charm, and for no other purpose'. The problem is, it seems, that we all think we know Darwin. In fact, few of us go back to the original, instead taking for granted what other people say he said. In this case, it seems to have created a bit of validation by wish fulfilment: Darwin's views on sexual selection, Prum says, have been 'laundered, re-tailored and cleaned-up for ideological purity'.
Clearly Prum is, to put it mildly, bucking a trend, even if he is in good company. But his career has been diverse and full, so that reading this fascinating book, we learn about the patterning of dinosaur feathers, consider the evolutionary basis of the human female orgasm, the tyranny of academic patriarchy, and the corkscrewed enormity of a duck's penis. Combining this with in-depth study of how science selects the ideas it approves of and fine writing about fieldwork results in a rich, absorbing text.
Not all of Prum's analogies or counterexamples worked for me, and the attacks on the prevailing view often seemed strident. However, the book deserves to be read, just as the idea of pure beauty evolving unallied to selection and unalloyed by function deserves to be examined and considered. You may not end up agreeing with the reason for its existence, but the dance Prum performs to convince you to take him on as an intellectual partner is beautiful and deserves to be appreciated on its own terms.
Prum (Ornithology/Yale Univ.), the head curator of vertebrate zoology at the Yale Peabody Museum of Natural History, argues that natural selection is not the only evolutionary mechanism at work in nature.
Beauty and desire in nature are also dynamic forces, and those features in males that females prefer in choosing mates evolve rapidly. In a nutshell, each species evolves its own standard of beauty by which it chooses mates.
After a brief discussion of the early and continued opposition to Darwin's aesthetic theory, the author illustrates the role of beauty in bird mating by taking readers to Borneo to observe the rituals of the Great Argus, a species of pheasant known as 'one of the most aesthetically extreme animals on the planet,' and to Suriname, to see the displays of male manakins, which must meet the 'very high standards' of potential female mates.
In other chapters, Prum reveals the intricate machinery involved in female bowerbirds choosing their mates. Female ducks, it seems, may not have such autonomy. Readers may be in for a shock when Prum turns to duck sex, which can be violent, involving what humans would call gang rape, and the illustrations of record-setting duck penises are eye-opening.
The author, who charmingly reveals his lifelong fascination with birds, does not base his argument solely on avian evolution, however. In later chapters, he explores the role of female mate choice in primate evolution, a challenging subject that he views as warranting further study. Throughout, the narrative is well-documented and wholly accessible, enriched by the author's warm personal touches.
Not long ago, a physicist at Stanford posed a rhetorical question that took me by surprise.
'Why is there so much beauty?' he asked.
Beauty was not what I was thinking the world was full of when he brought it up. The physicist, Manu Prakash, was captivated by the patterns in seawater made as starfish larvae swam about. But he did put his finger on quite a puzzle: Why is there beauty? Why is there any beauty at all?
Richard O. Prum, a Yale ornithologist and evolutionary biologist, offers a partial answer in a new book,'The Evolution of Beauty: How Darwin's Forgotten Theory of Mate Choice Shapes the Animal World - and Us.' He writes about one kind of beauty - the oh-is-he/she-hot variety - and mostly as it concerns birds, not people. And his answer is, in short: That's what female birds like.
This won't help with understanding the appeal of fluid dynamics or the night sky, but Dr. Prum is attempting to revive and expand on a view that Charles Darwin held, one that sounds revolutionary even now.
The idea is that when they are choosing mates - and in birds it's mostly the females who choose - animals make choices that can only be called aesthetic. They perceive a kind of beauty. Dr. Prum defines it as 'co-evolved attraction.' They desire that beauty, often in the form of fancy feathers, and their desires change the course of evolution.
All biologists recognize that birds choose mates, but the mainstream view now is that the mate chosen is the fittest in terms of health and good genes. Any ornaments or patterns simply reflect signs of fitness. Such utility is objective. Dr. Prum's - and Darwin's - notion of beauty is something more subjective, with no other meaning than its aesthetic appeal.
Dr. Prum wants to push evolutionary biologists to re-examine their assumptions about utility and beauty, objectivity and subjectivity. But he also wants to reach the public with a message that is clear whether or not you dip into the technical aspects of evolution. The yearning to pick your own mate is not something that began with humans, he says. It can be found in ducks, pheasants and other creatures.
'Freedom of choice matters to animals,' he said recently on a birding trip to a beach near his office in New Haven. 'We've been explaining away desire rather than actually trying to understand or explain it. That's one of the biggest shifts that the book is about.'
The book ranges from hard science to speculation, and he does not expect his colleagues to agree with him on all of his ideas. In fact, he gets a twinkle in his eye when he anticipates intellectual conflict.
'I do'’t know anybody who actually agrees with me,' he said with a frank smile. 'Even my own students aren't there yet.'
To grasp his view, a little bit of history is in order. Darwin famously proposed the idea of evolution by natural selection, what is often called survival of the fittest. To put it simply, living things vary in their inherited traits, from speed to color to sense of smell. The traits of the individuals who survive longer and have the most offspring become more common. So, over time, the faster antelope have more young, the fastest of them have more offspring, and antelope end up very speedy.
But reproduction isn't just about surviving and staying healthy long enough to mate. You have to find a mate. And in many species, your mate must choose you. This process is sexual selection. Female birds are often the ones choosing. And their choices can produce male birds that are incredibly colorful, and some that are elaborate dancers or designers of striking boudoirs - like the bower birds. If, for example, females like males with long tails, then long-tailed males have more offspring, and the longest-tailed of those offspring reproduce more. In the end, that species becomes known for its long tails.
Maydianee Andrade, an evolutionary biologist and vice dean at the University of Toronto, Scarborough, who studies sexual selection and teaches evolution, said that 'the question is basically this. You can think of females when they are choosing a mate as foraging. So what are they looking for?'
'If you're dragging a giant tail behind you, that might tell the female something,' she said. 'A male that survives carrying a large heavy tail is more impressive than a male that survives with a short tail.'
But survival might not have anything to do with it. Some female finches use white feathers to line their nest, perhaps to camouflage white eggs. In one experiment, they also liked males with white feathers stuck on their heads better than other males. This seemed to be an aesthetic choice, and also proved that there is no accounting for taste.
Darwin contended that selection-based mate choice was different from natural selection because the females were often making decisions based on what looked good - on beauty, as they perceived it - and not on survival or some objective quality like speed or strength. Scientists of that era reacted negatively, partly because of the emphasis on females. 'Such is the instability of vicious feminine caprice that no constancy of coloration could be produced by its selective action,' wrote St. George Jackson Mivart, an English biologist who was at first a great supporter and later a critic of natural selection.
Alfred Russell Wallace, who came up with the theory of evolution at the same time as Darwin, preferred the idea that the colors and patterns meant something - either they were signs that this was a male of the right species, or they indicated underlying fitness. Perhaps only a strong, healthy male could support such a big, beautiful tail.
At the very birth of evolutionary theory, scientists were arguing about how sexual selection worked. And they kept at it, through the discovery of genes and many other advances.
Fast forward to the 1980s, when Dr. Prum was in graduate school at the University of Michigan, sharing an office with Geoffrey Hill, now a professor at Auburn University.
At that time, mainstream evolutionary thought took a big swing toward the idea that ornaments and fancy feathers were indications of underlying fitness. 'Animals with the best ornamentation were the best males,' Dr. Hill said. This was called 'honest signaling' of underlying genetic fitness. The idea, he said, 'almost completely ran over what was the old idea of beauty.'
Dr. Hill, for one, was completely convinced. 'I was pretty sure I could explain all ornaments in all animals as honest signaling.' But, he added, he has since reconsidered. There are some extreme forms of ornamentation that he thinks don't signal anything, but rather are a result of the kind of process Dr. Prum favors. 'You can't explain a peacock's tale with honest signaling,' Dr. Hill said.
But, he said, he thought Dr. Prum had taken an important idea and gotten 'a little bit carried away with it.' The book, he said, 'was a great read, and I could tell he put his heart and soul into it.' But, he said, he found it 'scientifically disappointing.'
Darwin himself, Dr. Hill said, 'was completely unsatisfied with his work on sexual selection.' And the mainstream of evolutionary biology is not hostile to a partial role for arbitrary female choice. Dr. Hill has recently argued for combining several different processes to explain sexual selection.
Dr. Prum is indeed given to enthusiasm, and to intellectual contention. He has been on the winning side of initially unpopular ideas before.
As a graduate student, he sided with researchers who wanted to change the way animals are classified, to emphasize their evolutionary descent. The new idea was called cladistics and it is now the established idea. He has done groundbreaking research on both the physical structure and the evolution of feathers, and he was an early supporter of the notion that birds descended from dinosaurs, another new idea that is now the mainstream view.
In neither case was he a lone voice. But he is nothing if not confident, and not only in his science. Take the question of pizza.
In New Haven, pizza is something akin to a religion, and there are different sects. When I asked Dr. Prum who makes the best pizza in town, thinking he would pick one of the rival pizzerias, he didn't hesitate.
“I do,” he said. He uses an outdoor grill with a special attachment, and he described his pursuit of the perfect pizza in some detail. When I raised an eyebrow he offered me a reference, a friend and writer who had consumed the Prum pies.
He also acknowledged that he approaches many things with single-minded intensity.
“I’m given to obsessions,” he said. Bird watching was the first and most long-lasting. Evolutionary biology may be the deepest. Cooking, opera, gardening and politics (left-wing) are others.
He has disagreed with the dominant view of sexual selection since graduate school and sees his new book, which he hopes will reach beyond scientists, as a kind of manifesto. It has too many parts to summarize. He takes a chapter, for instance, to speculate that same-sex attraction in humans evolved in our ancestors through female choices that undermine male sexual coercion. For a full account, you need to read the book.
But one particular aspect of his argument is his distress at the idea that almost all evolutionary change is assumed to be adaptive, contributing to fitness. In other words, if a fish is blue, it must be blue for a reason. The color must help it escape predators or sneak up on prey, or be otherwise useful in some way. Beauty, therefore, must be adaptive, or a sign of underlying qualities that are adaptive. Pick a behavior or an ornament or a physical trait, and it is useful until proven otherwise.
That’s backward, says Dr. Prum. Take beauty. Since animals have aesthetic preferences and make choices, beauty will inevitably appear. 'Beauty happens,' as he puts it, and it should be taken as nonadaptive until proven otherwise.
In proposing this so-called 'null hypothesis,' he draws on the work of Mark A. Kirkpatrick at the University of Texas, Austin, who studies population genetics, genomics and evolutionary theory and had read parts of 'The Evolution of Beauty'
'I'm very impressed that Rick is taking on this crusade,' Dr. Kirkpatrick said. He is not convinced that all aspects of sexual selection are based on arbitrary choices for perceived beauty, but, he said, if Dr. Prum can convince some other scientists to question their assumptions, 'he will do a great service.'
For Dr. Prum, at least, there is a partial answer to the question posed by Dr. Prakash. Why are birds beautiful?
'Birds are beautiful because they're beautiful to themselves.'
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