More about sexual selection in the New York Times

January 21, 2019 • 9:45 am

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.

The results of sexual selection: male and female Manadrin Ducks (Aix galericulata). Photo from Wikipedia.


59 thoughts on “More about sexual selection in the New York Times

  1. I have always wondered why some want to consider sexual selection as separate from natural selection when the opposite sex is probably the most important part of the environment and natural selection is about reproductive success.
    I also have a question. If the runaway model results in extreme traits that impact survivability, doesn’t this suggest that individuals surviving with such traits likely hold better genes (compensation for the exaggeration)?

    1. Another thing, so what if the sexually selected trait adversely affects the male’s ability to survive? In the final measure the only criterion that matters is leaving more offspring than the other guys. If female peacocks prefer to mate with males that have the biggest tail it doesn’t matter if smaller tailed peacocks are better at surviving and therefore live longer than large tailed peacocks. If the ladies don’t want to breed with them then they aren’t going to be passing on their genes. Even if big tailed peacocks typically mate once and then die they would still win the selection competition if female peacocks simply don’t breed with males with small tales. Evolution doesn’t produce best results, not even close. It just produces things that work at the moment. Or not. I’m sure there are plenty of examples of species evolving themselves to extinction.

  2. I have not understood why the different models (good genes, runaway, etc) are described as separate. It seems plausible that different combinations could be at work in some cases.
    Also I don’t fully buy that a trait that seems involved in runaway selection should have arisen in some ‘random’ way. It seems likely that it would be based on a species-recognition character.

    1. While I have no expertise or education beyond my amateur interest (I have a history, not science degree), might I suggest that while a trait might arise randomly, the female interest might be due to the innate interest in novelty? Perhaps this has been studied and suggested before, I don’t know, but since many animals show a natural curiosity towards novel stimulus (thinking of ravens specifically here, from books by B. Heinrich, for example) then perhaps any behavior or physical attribute that helps a male stand out from the crowd, if they are a strong, healthy specimen already, might tilt the scales in his favor in the eyes of the female.

      1. There is experimental support for ‘novelty’ in at least some cases (putting a feather on the head of a male zebra finch is the best example that I know). But there seem other cases where it appears to be a trait in both sexes, only it is very augmented in males. Red color in cardinals is what comes to mind.
        Anyway, a lot of this has to do with what it seems to us.

    2. I was thinking this too as I read this post. Any unique sound produced by a creature can help others of the same species find a mating partner. Feather ruffling playing this role, as opposed to song or brightly colored plumage, might be a result of the predatory environment and the species’ hearing. Colors and songs may attract predators more effectively than feather ruffling.

  3. I always have a problem with adjectives if their antonym makes no sense. eg:

    unnatural selection or supernatural selection.

    Regarding sexual selection … is this somehow not natural?

    Just playing with words 🙂

    1. I’m sorry but the pedant in me can’t help it…in this case, the opposite of “natural” is “artificial”.

          1. I know … and?

            So when someone builds a house is that natural or is it artificial?

            My point to cut a long story short:

            Humankind is part of nature, and there is something kind of strange in thinking human actions are artificial.

            1. I think you’re missing the point rom. The terms are used as antonyms precisely for the purpose of explication. Darwin was saying – in nature we see a process similar in many ways to how we breed our plants snd animals. The terms are used to distinguish them – because even though they are similar in some ways they are profoundly different in others. One is “natural” because it happens without supervision by humans and the other “artificial” because it does.

            2. Words are complicated. Although “artificial” can mean “man-made”, we can still distinguish between evolutionary pressure applied by humans naturally (eg, construction and agriculture taking habitat) from deliberate manipulation of other species’ genome (eg, dog breeding). I guess these are degrees of artificial but, on the other hand, the latter kind might be considered natural.

    2. I’ve always thought of the antonym as being “imposed” selection. Natural selection is then so named because it occurs automatically, without any additional action required. Reproductive success just “naturally” produces more of the genes that contributed to the success.

      1. But the all those little critters are imposing selection on one another all the time.

        Darwin’s breeding experiments were not automatic? In what sense was the chemistry and physics of Darwin’s actions not “automatic”?

  4. Male chauvinism and feminism in a nutshell:

    Male chauvinist: “The male Mandarin duck is much more magnificent than the female.”

    Feminist: “The female selection is what made him that way.”

    1. Or to put it another way, female Mandarins will only mate with the most gorgeous males, but male Mandarins will mate with any [even non-gorgeous] female. [I don’t think this rule always applies to humans, though.]

  5. Dr Prum seems familiar – a smart guy with a bloated ego and little humility. My bet is he understands all the criticism (the Patricelli review was thorough and fair, as has been yours) but he just doesn’t care. He has figured out what sells. Again I think you’re right; it is the appeal to a Natural Fallacy that is currently in favor that accounts for the books sales. His ego protects both his sales and his scientific self-esteem. It’s a winning combination, for those with a damaged set of professional ethics.

    1. Indeed. As a former science PR flack, I’ve observed that taking a too-strong, oversimplified position and backing it dogmatically with ego is a successful strategy for publicity. Journalists love it, as do publishers of popular books. SJ Gould also fits this mold.

  6. I found this quote in the “Birds and Bedbugs Make Bad Politics” of the linked paper

    ” Indeed, male self‐sacrifice and subsequent sexual cannibalism has evolved in multiple spider species, but we suspect no man would use this as a justification to encourage such behavior in humans.”

    I would agree, no man would encourage behavior like that, but I suspect that there are feminists who would, albeit without the cannibalism.

    1. I’d also offer than female humans probably would not find it appealing if I were to repeatedly ram into them from behind, block their forward progress while bobbing my head up and down, or chasing them while biting their ankles, yet this seems to do the trick for many female tortoises. Perhaps if I grew my fingernails out really long and vibrated them in front of the ladies’ faces at a dance club? I mean, look at how successful red-eared sliders are!

  7. … 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).

    Is that the live fast, die young, and leave a good-lookin’ corpse theory of selection?

  8. Thanks for the very good review. I wonder how important it is to science to decide whether sexual selection “is” or “is not” a case of natural selection. Is there some other important issue that depends on how we decide to categorize sexual selection?

    I did wonder why Prum (per Jabr’s account) talked to various people at University of Texas but never got around to talking to Mark Kirkpatrick. They’re in the same building.

    1. I think that the distinction between natural selection and sexual selection is a semantic one. There are certainly cases that fall on the borders, but I’m not that worried about the semantic problem. What bothers me about its use in Prum’s book is his claim that he’s making some kind of revolutionary leap by showing that natural and sexual selection are different. One could make a claim that they’re not that different. What I object to, mostly, is the rhetorical inflation and self promotion.

      1. IIRC, the random model of runaway sexual selection was first proposed by Darwin, and Fisher gave it more thorough statistical oomph. Note, I always was suspicious of it being completely random at the start. As you point out, it is very difficult to make sure either way.

    2. I find it curious that sexual selection rarely if ever happens with monogamous mating. Do monogamous females have poor taste? Or do they simply favor traits that won’t get their partner killed prematurely? If the male is expendable, then the female can choose “dangerous” traits that reveal his fitness. This is, in my mind, the best argument that sexual selection is simply natural selection disguised by a few fancy feathers.

      1. In monogamous species of birds, both sexes seem to be colorful and they establish and reinforce the bonding with frequent courtship rituals. In these species, it can be hard to tell the two apart, as in penguins and albatross. I think the consensus there is that sexual selection is working on both sexes, so both have been modified by selection from the opposite sex.
        Overall, intersexual selection (this is where one sex chooses another based on the traits they display) runs along a continuum in the animal kingdom. On one end, females choose males and males compete to be chosen. On the other end of the spectrum it is the other way around, as seems to be the case in a bird known as the phalerope. Then there are species that seem to do sexual selection in both directions.

        1. My point is that dimorphism in birds coorelates with the degree of paternal input in child rearing. I think Robert Trivers noticed this. I do not know of a species of birds that is truly monogamous and dimorphic. These monogamous species may exhibit sexual selection but it tends towards traits that improve survival of both and not to a decrement in survival of one or both.

      2. Yes, a good point. Species are monogamous when both parents are needed to get the more successful reproductive outcomes. They obviously do have sexual selection, but I cannot imagine it to be random (admittedly a weak argument), and it would indeed not ‘run away’ easily for the reasons you mention.

      3. Certainly sexual selection is taking place among monogamous species, with the most fit females pairing with the most fit males, on down the line. To take one example from nature, look at Tom Brady and Gisele Bündchen.

        IIRC, studies have found that in monogamous species, less desirable individuals tend to pair up later in the mating season, thus lowering the chances of survival for their offspring.

  9. “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.”

    Is Jabr perhaps not fully on-board with the gene as unit of selection? Cuz an individual surviving is only of value in that it extends the chances of that individual eventually reproducing the genes inside it.

    1. There is a lot Mr Jabr is not aware of. He is not a scientist, but a pop writer. That being said, he’s not the worst of them either.

  10. I have had some friends post about Prum’s book on Facebook. I think that this idea appeals to non-scientists because it has to do with sex/mating, and the totally unscientific concept of “beauty.” It comes on at the same time as other assaults in the media on natural selection, like eugenics.

    I find everything that natural selection produces “beautiful” (i.e. intellectually appealing, fascinating, amazing). I was enthralled by the video of the fire ant structures and find it as “beautiful” as a peaccock’s tail.

  11. It is odd to link bird and human behavior. Using that mindset, if one were to look around here in rural America it would appear that the local human females prefer beer bellies, missing teeth and double-digit IQs. These are all traits that are detrimental to the males but they still exhibit disturbingly high reproductive fitness. The display behavior of “rolling coal” (intentionally blowing black smoke from truck exhaust stacks) is common but it’s unclear if this is aimed at prospective mates or rival males and its biological underpinnings remain unresolved. Research continues.

  12. Further, Prum ties his model to progressive politics…

    I have come to believe that tying naturalistic events to any form of political model is a category error.

    Even writers that I admire (Pinker, for instance) seem to work from their political views back to data, and I can’t help feeling that this invites biases to creep in.

      1. I can see A C Harper’s point here but “inviting biases” is does not prove actual bias. This observation can be applied to all writers. After all, emotion likely plays a role in choosing their subject. In Pinker’s case, he seems to have gone to great lengths to choose data that is accurate and fair. AFAIK, the accusations of cherry picking have not been substantiated. I suspect the accusers of having a gut feel that things are not as rosy as Pinker says and they conclude that he must have cherry-picked his data. It’s up to them to show us how or shut up about it.

        1. Then there’s the confirmation bias of those who reject Pinker’s claims. For, if your overriding goal is to fix the world, required is a world in need of fixing.

  13. Maybe I’m misinterpreting, but it seems to me that a concept like “beauty happens” is either:

    1) So vague that it lacks any particular meaning (As in say “Red happens” or “hydrogen happens”. I mean yes, obviously, their existence attests to that, but it’s not a particularly informative statement.) To say that females selected beautiful traits because they found them beautiful is like saying they selected for red traits because they found them red.

    2) Necessarily smuggling another concept in with it. Universal standards of beauty, instinctively finding beauty in higher levels of organization (I think it’s been proposed that this is why people find fractal beautiful), the divine feminine manifesting or something along those lines.

      1. PS. I understand that this is one of those gray -intermediate- examples Dr PCC(e) mentions so not, strictly speaking, a direct response to your query, Graham. But it is a sexually dimorphic trait that I bet could be construed as being sexually selected, at least in part. Besides I just couldn’t resist the gag*. A thousand pardons.

        *another meaning is apropro

    1. Not ugly to others of their species but definitely ugly to us. Turkey wattles and the like are pretty ugly. I assume, but don’t know, they are the result of sexual selection. Those varicolored backsides on some apes are pretty ugly.

      1. In fact it is striking that so much of the results of sexual selection are so appealing to us, a completely different species. (no, I’m not referring to ape butts*, but to say, birds).
        The fact that bird results of sexual selection are so often more appealing to us may be that we are quite visual as mammals go, like birds.

        *The beautiful bright red penis and powder blue balls of a male vervet monkey undermines my thesis, though 😁

        1. Human’s attraction to the beauty of other species does seem to be more than mere curiosity. I wonder if it is just a side-effect of our attraction for human ornamentation.

    2. You could also perhaps ask if there is sexual selection for drab characteristics. Do female crows prefer regular black male crows over other variants? Humans will of course focus on the cases where the male coloration and features are striking to us.

      1. It is well known that some species’ drab coloration helps them hide from predators. The selection mechanism is obvious. But is sexual selection ever involved? Do they ever choose mates for their drabness?

  14. Thanks for this post. I read the NYT Magazine article and have been checking whyevolutionis true hoping for this kind of help.

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