Tuesday, February 13, 2018

The Evolution of Beauty by Richard O. Prum

For most of us, the idea of heading out into the wilderness in hope of catching sight of some interesting birds likely seems strange and inevitably boring.  However, a new book, while probably not convincing many readers to head for the woods, will convince most that birds are pretty damned interesting.  Richard O. Prum is an ornithologist at Yale University who mines his knowledge of various bird species to provide some fascinating insights into the true nature of evolution in The Evolution of Beauty: How Darwin's Forgotten Theory of Mate Choice Shapes the Animal World - and Us.  You will never think of evolution in the same way again after reading this book.  You will probably never think of sex in the same way either.

Prum’s thesis centers on the mistake evolutionary theorists make in focusing on the conclusions of Darwin’s first landmark book On the Origin of Species by Means of Natural Selection, and ignoring the conclusions produced in his second great volume The Descent of Man and Selection in Relation to Sex. 

Focusing on the first book has led to the standard prescription of survival of the fittest.  From this perspective, natural selection enhances characteristics that best ensure the propagation of a given species.  It does this by providing a better means of adapting to whatever environmental conditions are operative.  Prum claims that adherents to this limited view of evolution call themselves Darwinists, but they have betrayed Darwin who concluded that such a view could not explain all features of evolution.  Prum refers to such people as neo-Darwinists, or adaptionists.

“I am convinced that most of those who think of themselves as Darwinians today—the neo-Darwinists—have gotten Darwin all wrong.  The real Darwin has been excised from modern scientific hagiography.”

The weakness of a strict adaptionist approach is that it does not consider that an animal with really great genes has to get them propagated by finding an animal of the opposite sex willing to mate with it.  We know from our own experience as human animals that mate selection is a complicated venture, one that does not necessarily focus merely on producing survivable offspring.  Generally, we will select and consider mating with someone to whom we feel a physical attraction—someone we consider “beautiful.”  Considerations of fitness as a parent of a subsequent child may or may not arise later.  In fact, humans have evolved to a state where sex and mating are essentially two separate considerations.

Are we so confident in our uniqueness that we can’t consider that animals may be driven by similar considerations?  Darwin wasn’t.

“Natural selection cannot be the only dynamic at work in evolution, Darwin maintained in Descent, because it cannot fully account for the extraordinary diversity of ornament we see in the biological world.”

“It took Darwin a long time to grapple with this dilemma.  He famously wrote, ‘The sight of a feather in a peacock’s tail, whenever I gaze at it, makes me sick!’  Because the extravagance of its design seemed of no survival value whatsoever, unlike other heritable features that are the result of natural selection, the peacock’s tail seemed to challenge everything that he had said in Origin.  The insight he eventually arrived at, that there was another evolutionary force at work, was considered an unforgivable apostasy by Darwin’s orthodox, adaptionist followers.  As a consequence, the Darwinian theory of mate choice has largely been suppressed, misinterpreted, redefined, and forgotten ever since.”

The neo-Darwinians of his era were also influenced by a bit of misogyny.  The obvious explanation of the peacock’s tail is that it evolved because the female peahen thought beautiful tail feathers were sexually attractive.  Peacocks with the best tail feathers were the most likely to be selected as a mate by a peahen.  There are three crucial considerations that follow from that conclusion.  The first is that the female of the species had attained autonomy in mate selection, something that a patriarchal society of male scientists might consider to be too outlandish to be true.  The second is that female mate selection has driven a pattern of coevolution in which females’ evolving choices have produced the evolution of the desired characteristics in the male.  The third thing that follows is that mate selection, and its consequences, need not lead to improvement in survivability of a species.  In fact, it can lead to the opposite.

Prum refers to evolution by mate selection as “aesthetic” evolution.

“Aesthetic evolution by mate choice is an idea so dangerous that it had to be laundered out of Darwinism itself in order to preserve the omnipotence of the explanatory power of natural selection.  Only when Darwin’s aesthetic view of evolution is restored to the biological and cultural mainstream will we have a science capable of explaining the diversity of beauty in nature.”

To fully grasp Darwin’s conception of mate choice as an evolutionary process, one must accept that there is innate sexual conflict between males and females of a given species.  One must also accept that females will strive to have freedom of choice as to who they will mate with.  That is not too difficult a concept from a human perspective, but for the Victorian males Darwin had to contend with, applying it to animals was what Prum refers to as “a bridge too far.”

Each species seems to resolve this conflict in its own way.  At one extreme there are species where female autonomy is essentially nonexistent.  That does not mean that sex is necessarily coercive on the part of the male.  In these cases, males will often contend against each other for dominance and the associated access to females.  At the other extreme are species where female mate selection is dominant and males must audition for females if they wish to copulate with them.  Most bird species fall into this category and provide the prototypes that drove Darwin to his conclusion and set off Prum to write his book.  However, there are also species where this sexual conflict has not been successfully resolved.  Prum provides an example that illustrates the potential power of female sexual autonomy and demonstrates that sexual conflict can drive evolution in directions that are maladaptive in terms of species survivability.

Prum devotes an entire chapter to the fascinating story of “duck sex” in which males have spent millions of years evolving the tools to allow them to coercively inseminate a female against her will.  Meanwhile, female ducks have spent millions of years evolving tools to allow them to be inseminated only by the male of their choice.  Many females are injured or even killed in the annual mating process as they try to resist undesired males.  It makes a fascinating story that needs recounting in detail.

So, mate choice is a potent force in evolution.  Once one sex develops a fancy for some characteristic in the opposite sex, and that sex has some degree of autonomy, possessors of that characteristic will become more successful at mating and that characteristic will be enhanced in subsequent generations.  The desire for that characteristic will also be enhanced by natural selection.  In fact, the initial degree of autonomy will also be enhanced.  An aesthetic choice on the part of members of a species alters the evolution of that species—for better or worse.

Except for the unfortunate duck situation, most bird species have settled into situations where the females exercise sexual autonomy, leaving the males to compete for the ladies’ favor.  This has resulted in males developing exotic ornaments such as feather displays and fancy color patterns as means of attraction.  Many also practice physical displays consisting of flight gymnastics or suggestive posturing.  In some cases, male birds will congregate in the equivalent of arenas maintained for auditioning before any female who wanders by.  Members of a species that become geographically isolated will develop regional peculiarities in ornamentation and in their mating rituals.

Curiously, although birds evolved from species that possessed a penis, most birds have lost their penis somewhere along the way (the unfortunate ducks are an exception).  Those who might be uncomfortable in a situation where females have absolute power over sex and reproduction might note that it is extremely difficult to coerce a female into having sex without a penis.  Copulation without one requires the cooperation of the female in the transfer of sperm.  Could the bird penis have disappeared as a result of female desire to eliminate the threat from forced copulations?

Prum does not hesitate to speculate.

“Birds originally inherited the penis from their dinosaurian ancestors, but then it was lost some sixty-six to seventy million years ago in the most recent common ancestor of the group known as the Neoaves, which includes over 95 percent of the world’s species of birds.”

“It’s possible that the neoavian penis was lost because females explicitly preferred males without penises.  Why?  If one of the primary functions of the penis is to subvert female mate choice through forced copulations, as it is in waterfowl, then female mating preferences against intromission could have evolved to reduce the threat to female sexual autonomy.”

Mere wishing cannot make the penis disappear.  There must be a mechanism that renders it of little or no value to the male.  Selection via female choice could lead to ever-smaller penises until they became ineffective.

“In the more than 95 percent of bird species that are penis-free, females can eject/reject unwanted sperm.  For example, barnyard hens can eject sperm after coerced copulations with unwanted males.  Attempts at sexual harassment and intimidation do still exist in birds without a penis, and the female birds may still suffer injuries by resistance, but the loss of the penis has resulted in a nearly complete end to forced fertilizations.  Through the loss of the penis, female neoavian birds have essentially won the battle of sexual conflict over fertilization.”

Birds have become the most differentiated of species as they generated an explosion of variations of plumage and other ornamentations.  Prum speculates that it was this complete victory of females resulting in the loss of a penis that forced males to evolve ever more effective means of attracting females, leading to an “aesthetic evolutionary extravaganza” among birds.

Birds are interesting, but humans are too.  Mate choice and the conflict over sexual autonomy are also part of human evolutionary history.  Prum devotes a significant section of his book to trying to address the extent to which aesthetic choices have had, and continue to have, impacts on human evolution.  That must be a topic for another day.

The interested reader might find the following articles informative:

Bonobos, Christians, Scandinavians, Atheists, and Government

Are Humans Inherently Warlike?

Change in Human Brain Size, Natural Selection, and Evolution

Brains, Energy, and Humanity’s Most Important Achievement: Learning to Cook

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