With the publication of his book The Evolution of Beauty (subtitle: How Darwin’s Forgotten Theory of Mate Choice Shapes the Animal World—and Us), Yale ornithologist Richard Prum gained an extraordinary amount of publicity in the popular press.  His theme was that “beauty”—that is, the evolution of extreme and stunning displays and ornamentation in male birds—results from a form of “runaway sexual selection” in which females’ random preference for extreme male traits produces amazing sexual dimorphism that has nothing to do with natural selection. (The peacock is perhaps the most famous example.) Prum’s book got two separate reviews in the New York Times, at least one other notice, and two big reportorial pieces, including recent the one below. The book was also nominated for a Pulitzer Prize for nonfiction, though it didn’t win.

Prum’s book is worth reading for two reasons. First, it presents a strong defense of the “runaway” model of sexual selection Prum calls it the “beauty happens” model, in which random female preferences lead to the exaggeration of male traits up to the point at which those traits actually hurt the male’s reproductive success (a peacock with a bigger tail would presumably not only be unable to fly, but would be a target for predators and find it hard to get around). Second some of Prum’s writing is very good, and his examples of exaggerated male behaviors and plumage engrossing and yet unknown to many laypeople.

But the book, as I’ve written before (see posts here), is tendentious. It ignores other models of sexual selection (except to denigrate them), it ignores the weaknesses of his own favored runaway model, and it misrepresents the views of evolutionary biologists (many of whom agree that the runaway may be important, but won’t buy into Prum’s view that it’s ubiquitous).  Prum claims that the runaway model is universally rejected by biologists in favor of “good genes” models (male traits indicate their genetic endowment). But that claim isn’t true: we just don’t have much data to distinguish all the competing models we have for how sexual selection works.

Further, Prum ties his model to progressive politics, saying that female choice in animals should hearten us because it shows that female “sexual autonomy” is natural. But such autonomy isn’t always present: many animals, for instance, have forced copulation. Bedbugs, for example, exhibit “traumatic insemination”, in which males bypass copulation by simply injecting sperm through the female body wall, with that sperm finding its way to the female eggs. Females don’t get to choose their mates, and copulation can actually kill them.

And there are many cases of forced and unwanted copulation by males, as well as male-male competition (viz., elephant seals) in which females are simply constrained to mate with whichever male wins a contest. Prum’s evocation of politics therefore demonstrates the “naturalistic fallacy”: that what happens in nature is what we should emulate. However, a lot of what happens in nature is stuff we shouldn’t emulate.

Prum also ties other models of sexual selection, including those in which a male’s traits indicate his vigor, health, or presence of “good genes”, to eugenics, and Nazi genocide, tarring the theories he doesn’t like with the social-justice cry of “Nazi”.  This is unconscionable. I can’t help but think, though, that Prum’s tying sexual selection to feminism was partly responsible for the book’s popularity and its Pulitzer nomination.

As I’ve written before, however, while Prum’s book received public approbation and good reviews—mostly from reviewers with no science background)—the reaction of the scientific community itself has been tepid and mostly critical for reasons I gave above. The three reviews I’ve read in scientific journals, including one by Gerald Borgia and Gregory Ball and another by Doug Futuyma, both highlight serious problem’s with Prum’s presentation, including the ignoring of alternative theories, the misrepresentation of the “beauty happens theory”, and the unwarranted connection between women’s rights and mate choice in birds. A more recent and much longer review, by Patricelli, Hebets, and Mendelson, published in Evolution (click on screenshot below for free access), was severely critical, and rightly so, though the authors did their best to be evenhanded and polite:

I’ve discussed this review before (full disclosure: I gave the authors some suggestions on a draft of their piece), and so won’t go over its contentions here. But if you want to read a review of Prum’s book—and one that is objective but critical—Patricelli et al. is the one to read. It is a good palliative for the publicity Prum gets repeatedly about his book.

That aside, several readers sent me the link to Ferris Jabr’s NYT piece above, suggesting that I write about it. I intended to, but was in Hawaii where I was having too much fun to work. Now that I’m back, I’ll summarize it as briefly as I can. (The piece is very long, and appeared in the NYT Sunday Magazine, an indication of how important the editors deemed the topic.)

Upshot:  Jabr’s piece is a mixed bag. (He’s a contributing writer to the New York Times and and often writes about science.)

The good bit is that Jabr at least indicates, as many writers haven’t, that the scientific community is lukewarm about The Evolution of Beauty and that Prum is somewhat dogmatic and dismissive of his critics. For example:

Despite his recent Pulitzer nomination, Prum still stings from the perceived scorn of his academic peers. But after speaking with numerous researchers in the field of sexual selection, I learned that all of Prum’s peers are well aware of his work and that many already accept some of the core tenets of his argument: namely that natural and sexual selection are distinct processes and that, in at least some cases, beauty reveals nothing about an individual’s health or vigor. At the same time, nearly every researcher I spoke to said that Prum inflates the importance of arbitrary preferences and Fisherian selection to the point of eclipsing all other possibilities. In conversation, Prum’s brilliance is obvious, but he has a tendency to be dogmatic, sometimes interrupting to dismiss an argument that does not agree with his own. Although he admits that certain forms of beauty may be linked to survival advantages, he does not seem particularly interested in engaging with the considerable research on this topic. When I asked him which studies he thought offered the strongest support of “good genes” and other benefits, he paused for a while before finally responding that it was not his job to review the literature.

Of course it was Prum’s job to review the literature, and especially to weigh his favored theory against alternatives, including “good genes” models and “sensory bias” models, in which female preference are not random but the byproduct of natural selection based on the species’ environment. How could it not be an author’s duty, when defending a theory, to review the literature for and against that theory?

Jabr also says this:

Like Darwin, Prum is so enchanted by the outcomes of aesthetic preferences that he mostly ignores their origins. Toward the end of our bird walk at Hammonasset Beach State Park, we got to talking about club-winged manakins. I asked him about their evolutionary history. Prum thinks that long ago, an earlier version of the bird’s courtship dance incidentally produced a feathery susurration. Over time, this sound became highly attractive to females, which pressured males to evolve adaptations that made their rustling feathers louder and more noticeable, culminating in a quick-winged strumming. But why, I asked Prum, would females be attracted to those particular sounds in the first place?

To Prum, it was a question without an answer — and thus a question not worth contemplating. “Not everything,” he said, “has this explicit causal explanation.”

Here Prum simply dismisses something that scientific reviewers mentioned repeatedly—where do female preferences come from? Prum assumes they are random, but there is a thriving field of sexual selection studying female preferences, showing how they might result from natural selection instead of just being “random” (i.e., aspects of neuronal wiring that have nothing to do with natural selection for the preference). Jabr also says, properly, that not all biologists have dismissed the runaway model, as Prum contends they have, but see it as one of a competing panoply of models that are hard to resolve. (Getting this kind of data from nature or even the lab is very difficult, and we weren’t there to see how sexual selection operated in the past.)

But in the rest of the article, Jabr seems to buy a lot of Prum’s contentions without properly evaluating the criticisms of other scientists. For example:

1.) The runaway model is not “Prum’s theory.” This model was first suggested by Ronald Fisher and elaborated and developed by scientists like Russ Lande and Mark Kirkpatrick. Yet Jabr repeatedly refers to the “beauty happens” model as “Prum’s theory”, as when he says that “Prum’s indifference to the ultimate source of aesthetic taste leaves a conspicuous gap in his grand theory.” (That statement is correct except that it’s not Prum’s grand theory.) This misleading attribution of the theory happens repeatedly. Let us be clear: Prum’s book is about presenting, defending, and applying a theory developed by other scientists.

2.) Jabr buys into Prum’s contention that sexual selection is fundamentally different from natural selection. Most biologists, I think, would disagree, seeing sexual selection as a subset of natural selection. That is, sexual selection is a form of selection based on female mate choice rather than other factors. But both sexual and natural selection involve enhancing those traits that affect reproductive success. (Jabr seems to mistake natural selection as a form of selection that enhances survival rather than reproductive success, but in fact the currency of all selection is the number of offspring that survive to spread your genes.). This may seem a semantic question, but both Jabr and Prum use this distinction to suggest that the runaway theory is a big and revolutionary improvement over previous notions of natural selection. This further inflates the runaway theory into something that it’s not.

In fact, natural and sexual selection blend into each other, and in some cases you can’t distinguish them. If a male produces sperm that swim faster than the sperm of other males in his species, and thus he gets more offspring, is this natural or sexual selection? It’s not based on mate choice, but does involve reproductive success. This is a form of male/male competition, analogous to those bull elk who butt horns during mating season, with the winner getting a harem of females. No female choice is involved in either case, but both could be seen as sexual selection. But they also represent natural selection—selection based on some individuals having traits (horns, fighting ability) that enables them to leave more genes.  My own judgment is that sexual selection is simply a subset of natural selection that involves mate choice, and not something fundamentally different.

3.) Jabr leaves out some aspects of Prum’s views that scientific critics have homed in on. Jabr doesn’t mention, for example, that Prum views the runaway model as the “null model” of sexual selection. That is, Prum deems it the model that we should accept unless we have good evidence for other models. But the runaway model isn’t null in that way: it does carry its own assumptions that themselves have to be justified and tested, such as female preference being “random” and not itself initially the result of natural selection or subject to stabilizing selection. The runaway assumes that male traits and female preferences are genetically correlated, and so on. No single model of sexual selection can be regarded as a “null model” to be regarded as a default option in the absence of any evidence.

4.) Jabr doesn’t fairly summarize the extent of scientific criticism of Prum’s book. While he does cite Borgia and Ball’s criticism, he neglects those of Futuyma and especially the thorough paper of Patricelli et al., and thus leaves out some important problems with Prum’s views (see below). Further, Jabr seems to have consulted critics at only the University of Texas at Austin, including my colleagues and friends Gil Rosenthal, Molly Cummings, and Mike Ryan. These people generally work on the sensory bias model of sexual selection, and thus emphasize theories different from Prum’s, but it would have been good to consult others who work on Prum’s model itself. These would include both Mark Kirkpatrick (also UT Austin!) and Russ Lande. I have talked to several “runaway” modelers, and their take is different from Prum’s: while they think the theory can operate, they are wary of its ubiquity in the absence of empirical evidence. This view, by the very proponents of Prum’s favorite model, shows a scientific caution far more admirable than Prum’s dogmatism.

5.) Jabr doesn’t mention at all an important aspect of Prum’s book: Prum’s view that because in some species females have “sexual autonomy” in choosing males, that hearten feminists who, rightfully, are against sexual coercion by human males. This omission by Jabr is a mistake, for this part of Prum’s message is one of its selling points, and surely explains some of the book’s popularity. But we shouldn’t buttress our morals by looking for parallels in nature, for, as I’ve said repeatedly, doing that makes our morality subject to revision via new information about nature. While some moral judgement can depend on empirical information (abortion may be one example), arguments about human rights and autonomy should be independent of how other species behave.

Jabr further ignores Prum’s invidious use of eugenics and comparisons to Nazis and genocide to tar models of sexual selection based on “good genes”. Ball and Borgia explicitly mention this, as do Patricelli et al. in the section of their review called “Birds and bedbugs make bad politics” (all three authors of that review are women).

My view then, is that Jabr’s summary of Prum’s work and the “beauty happens” theory is better than that of any of the summaries in popular venues, but still suffers from a general laziness manifested in contacting only scientists at UT Austin and in failing to summarize much of the criticism leveled by scientists at The Evolution of Beauty. Jabr didn’t do his scientific homework. The definitive popular critique of Prums’s views, as opposed to those that have already appeared in scientific journals, has yet to be written.