Are display traits in animals always honest signals of quality? Or can beauty happen purely for beauty’s sake?

These are the questions that Dr. Richard Prum, Yale professor of Ornithology and award winning author, contended with in his recent Hann Endowed Lecture at UMBS. They are also the questions he seeks to answer in his popular 2017 book, The Evolution of Beauty.

Spoiler alert: Beauty happens.

Or so argues Dr. Prum, a scientific powerhouse and modern champion of Charles Darwin’s theory of sexual selection. In his lecture, Prum explained that despite Darwin’s inability to nail down the mechanism of heritability, he understood that mate choice and sexual selection exert a unique evolutionary power. Darwin posited that competition within one sex for mating opportunities and the mate choice of one sex for the other has the potential to irrevocably shape the entire species. Darwin explained this idea in explicitly aesthetic terms, believing that animals possess “an aesthetic faculty,” “a taste for the beautiful,” and “standards of beauty.” Most provocatively, he believed that “refined beauty may serve as a sexual charm, and for no other purpose.”

At the time, these ideas were broadly panned in the scientific community. One of the most scathing critiques came from Alfred Russel Wallace, the co-discoverer of natural selection. Wallace maintained that the limitations of females’ capacity for choice and animal psychology rendered Darwin’s theory completely unviable. He preferred the adaptive explanation of beauty as an honest signal of an individual’s ability to bear the cost of expensive display traits, and through influence and force of will, he successfully pushed sexual selection to the backseat of mainstream thought for the next century. Through extensive research, thoughtful observation, and hard empirical data, Prum hopes to prove that Wallace was wrong to dismiss sexual autonomy and mate choice as powerful—and distinct—drivers of evolution.

“We have been denied—both the science, and the culture at large—the actual richness of Darwin’s own view,” said Prum.

Prum explained that he was first struck by sexual selection as an explanation for the stunning aesthetic diversity in his vertebrates of choice: birds. Though not in the market for an overarching worldview, he found one in sexual selection, and, in his words, the “beauty happens” school of thought.

Dr. Prum stands before a picture of a Clubwinged Manakin.

Dr. Prum used the Clubwinged Manakin as an example of evolutionary decadence based on aesthetic preference in mate choice.

“Beauty is a coevolved attraction in which desire and the objects of desire have shaped one another,” said Prum. “Birds are beautiful because they are beautiful to themselves, and because they are active agents in their own evolution.”

One of Prum’s most powerful examples comes from the Clubwinged Manakin, a bird native to South America. True to their name, Clubwinged Manakins have evolved solid, bulbous ulnas in their secondary feathers that allow them to produce an attractive song – not with their voices, but through the rapid vibration of their wings. The strange structure of the Clubwinged Manakin’s ulnas make flying and evading predators unnecessarily difficult, but allow for the wing stridulation songs that females strongly prefer.

But could the high cost of display be interpreted as an honest, informative handicap? Prum thinks not. For one, female Clubwinged Manakins have developed the same strange ulnas as their male counterparts. In other words, through a strong preference for the beautiful song produced by males with bulbous ulnas, females now bear the cost of a feature from which they will never reap the benefits. Prum calls this the “evolution of decadence,” defined by the tradeoff in having attractive sons who are more likely to reproduce, and having daughters burdened with a maladaptive handicap. This kind of extremity and complex beauty is particularly difficult to explain through the adaptive model.

Said Prum: “Freedom of choice begets beauty in nature.”

With implications spanning the entire animal kingdom—including humans—Prum continues his crusade to expand the intellectual framework of modern evolutionary biology.

Video of Prum’s full lecture is available below.

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