Darwin's Peacock

September 30, 2016

A year after publishing The Origin of Species in 1859, Charles Darwin wrote to botanist Asa Gray and complained, “The sight of a feather in a peacock’s tail, whenever I gaze at it, makes me sick!”  The trouble was, the peacock’s tail was entirely too beautiful, and that threatened to upend his beautiful theory of evolution by natural selection.  Darwin’s theory was entirely utilitarian. Only traits that aided the survival of the species were supposed to be passed on to succeeding generations.  Peacock feathers appeared to be entirely ornamental, which made them a useless extravagance in evolutionary terms.  If anything, they made peacocks more visible to predators and hindered their flight to safety.  A peacock’s tail clearly wasn’t made to fly; it was made to strut.

Darwin eventually accounted for the peacock’s tail by proposing an alternative path to reproductive fitness that he called sexual selection.  In effect, a peacock could ensure that his genes would be passed on to future generations by strutting his stuff and thereby impressing the ladies. It didn’t seem to matter that the traits making peacocks irresistible to peahens also made them vulnerable to predators.  Cherchez la femme! 

Darwin’s theory of sexual selection has proven to be no less controversial than natural selection, even among some evolutionists.  Alfred Russel Wallace, who is mainly remembered as an also-ran in the development of evolutionary theory, dismissed sexual selection, arguing that peahens were incapable of making aesthetic judgments.  The theory went into eclipse for nearly a century but has recently been revived by theorists who believe female preference for peacocks with the most beautiful tails may be an instinctive response to their supposed greater reproductive fitness.  The debate has recently been joined by advocates of intelligent design who argue that the iridescent colors and intricate patterns of a peacock’s tail are of such dazzling complexity that they could not possibly have arisen by random genetic variation.

Even if mutations alone could account for the peacock’s tail, you also need a genetic predisposition in peahens to choose mates with beautiful plumage.  There is no inherent reason why females should be attracted to colorful feathers per se, particularly if they make peacocks more vulnerable to predators.  Wouldn’t reproductive success be better served by some sort of camouflage regalia that would enable the peacock to blend in with its surroundings? 

In the Origin of Species, Darwin had puzzled over the fact that “a great number of male animals, as all our most gorgeous birds, some fishes, reptiles, and mammals, and a host of magnificently coloured butterflies, have been rendered beautiful for beauty’s sake…”  He later hypothesized that females exercised an “aesthetic faculty” in choosing a mate, resulting in the evolution of ornamental traits in animals.  Evolutionary biologists were slow to accept sexual selection generally, but now some are even coming around to the view that creatures other than ourselves may also be capable of appreciating beauty for beauty’s sake.  Yale ornithologist Richard Plum, head curator of vertebrate zoology in the Yale Peabody Museum of Natural History, has studied the elaborate mating dance of South American manakins and concludes “there is no biological reason to assume that non-human organisms lack the sensory capacity for aesthetic experience.”

As to how non-human organisms developed a “taste for the beautiful” (to use Darwin’s phrase), we do not know.  Presumably some primordial peahen with a genetic anomaly favoring fancy feathers chanced upon a singular peacock with stuff to strut.  What are the odds?  No matter.  Their offspring would be more inclined to pair up with others who are similarly disposed. Generation upon generation, the peacock’s tail grows longer and its patterns more intricate. Eventually you reach a tipping point where mating rituals have evolved into a beauty pageant. Likewise with all those gorgeous birds, fishes, reptiles, mammals, and magnificently colored butterflies that Darwin puzzled over.  Strict Darwinists may balk at any evolutionary purpose beyond the propagation of the species.  Yet Plum notes that “beauty is the overwhelmingly predominant aesthetic property in both biotic and human aesthetics.”  The conclusion appears inescapable: whether by chance or by design, the natural world has embarked on a continual beauty makeover.