Book Review: The Evolution of Beauty

The Evolution of Beauty: How Darwin's Forgotten Theory of Mate Choice Shapes the Animal World--And Us
Richard O. Prum
Natural history

[WARNING: long]

The Evolution of Beauty is a very good book. It's well-written. It has an interesting idea at its core and a leavening of entertaining personal content. It may even change the way you look at the world. It's also overstated, and in parts quite probably wrong--but intriguingly so.

Very briefly summarized, the theory that Prum is championing--originating in Charles Darwin's second great book, The Descent of Man--is that species can evolve features that don't help them survive at all, if those features help them attract mates. The peacock's tail is an example: it doesn't help the peacock fly, fight, or feed, but it surely attracts the chicks (rim shot). Prum's own field of study provides numerous fascinating examples of birds that have evolved some astonishing ornaments and behaviors, none of which seem to be "adaptive" in the survival-of-the-fittest sense. These features persist not because they have any use, but because mates prefer them--and mates prefer them not for any adaptive reason, but just ... because.

To take an well-known example, consider the sneetch (Seussius sneetchis).

Image hosted at Wikimedia

Quite at random, a few male sneetches have stars on their bellies. The stars have no function; they're just a random mutation. Nonetheless, some female sneetches think the stars are sexy--again, not for any particular reason, but just because sneetches make value judgments about stuff. Generation after generation . . . 

1. Male star-bellied sneetches attract more mates than plain-bellied sneetches, because some sneetch-ettes just prefer them. 
2. These couples have more children than plain-bellied sneetches.
3. The children inherit stars.
4. The females inherit a preference for stars.

Over time, then, the both the stars and the preference will tend to spread through the population, displacing the plain-bellied sneetches. Within broad limits, it doesn't matter whether the stars are adaptive, maladaptive, or neutral. If females preferentially chose males with stars, the star-bellied gene will win out.

For a slightly more subtle point, imagine that what the sneetch-ettes inherit is not just "a star is better than no star," but "more stars are better". In this case, the even-rarer two-star sneetches have an advantage over their one-star brethren. Once the one-star trait becomes common, the female preference for more stars preferentially benefits the two-star sneetches. And similarly, once two stars are common, the sneetch-ettes start flocking to the three-star sneetches . . . and so on, and so on . . . In this way, even a trait that does begin as a positive adaptation can turn negative, by being exaggerated into uselessness.

Returning to Richard Prum, his point is that a purely aesthetic preference is perfectly capable of driving evolution, survival value be damned. This is (according to him) something of a heresy within the evolutionary-biology world. Apparently there are those--Stephen Jay Gould called them "hyperadaptationists"--who insist that no trait can be inherited unless it contributes to genetic fitness, i.e. competitive advantage. This view explains the peacock's tail as a signal, advertising the genetic health of the bearer. An extreme version holds that the very uselessness of the tail is part of the signal--look at me, it's saying, I'm so genetically overendowed that I can carry this useless ornament around and still prosper.

Prum takes vigorous issue with these views. I think he's right to do so. The evidence for them seems weak, and their internal logic is less than compelling. Under the extreme view, for instance, shouldn't organisms with extreme deformities attract more mates? If surviving with a long useless tail signals fitness, then surely surviving with a long useless tail and intestinal parasites and only one wing signals more fitness. Ask for it by name!

Another of Prum's targets is the idea that animals simply can't form arbitrary, aesthetic preferences. Evidently there are a good many scientists who believe this; these scientists have never owned cats. (Consider, for example, the laser pointer. One of ours ignores it, one evinces occasional interest, and one will chase it endlessly. Nor is it just a matter of different neurological wiring, because all three were equally excited when they first encountered the Red Dot.) The idea that humans are just so very special in this regard is nothing more than a warmed-over search for the soul. Humans' ability to form aesthetic preferences came from somewhere; simple scientific parsimony suggests that it came from our animal ancestors.

At the same time, I wonder a little bit whether the prevailing wisdom is quite as prevalent as Prum makes out. Certainly his theory is one that I--a complete outsider--had heard before, including the fact that a trait need not be adaptive if it's sufficiently preferred. It's clear, however, that Prum is heavily emotionally invested in his theory. It may be that he's a bit hyper-focused on the opposition. It's almost certain that he's overstating his case.

One issue is that Prum mingles factual truth and normative truth. He points out, correctly, that--in the common case where females chose the mates--mate choice is consistent with female sexual agency, choice, and empowerment (which I'll call FSACE, for short). It's also true that we, societally, consider FSACE ethically desirable--indeed, mandatory. Those two things, however, are not connected. FSACE is not ethically mandatory because it's consistent with mate choice, and mate choice isn't necessarily true because it's consistent with FSACE. Put another way, FSACE is an ethical must even if someone proves that Prum's ideas are false.

In this regard, Prum also doesn't see that the evidence he cites against competing theories can also be used against his own. For example: in many societies women's sexual pleasure has always been marginalized, denigrated, or just ignored. This is evidence against the idea that female sexual pleasure is adaptive, true enough. But it's also evidence against the idea that female sexual pleasure is a consequence of women selecting sexually-compatible mates.

Another overreach comes when Prum tries to explain some behavioral differences between humans and our closest evolutionary relatives. Among apes, females generally can't select mates; the dominant male controls mating. So why do human relationships (cross-culturally, even in small tribal groups) tend towards pair-bonding?

Prum's proposal is that female mate choice explains this. Women, he suggests, prefer males who allow them to exercise their preferences. That is, proto-women preferred to hook up with proto-men who were inclined towards pair-bonding, rather than dominance/polygyny, which in turn spread the pair-bonding genes through the population--not because they provide any particular survival advantage, but because they offer more mating opportunities.

Well, sure, that might be true. There's no evidence of it. Furthermore, it suffers from a bit of a chicken-and-egg problem: women promote free mate choice by exercising their free mate choice to select males who will permit them free mate choice, which raises the question of where the free mate choice came from in the first place.

There are plenty of other equally-credible explanations. Here's one alternative, in dialogue form:

#1 MALE: I hereby invoke my maleness to take away your Female Sexual Agency, Choice, and Empowerment!

FEMALES: Let's kill him.

#2 MALE: Yeah!

#3 MALE: I'm in.

#1 MALE: Wait, what? Argh, gurgle.

FEMALES: Good riddance.

FORMER #2 MALE: Aha! Now I am #1 Male, and I--

FEMALES and FORMER #3 MALE: Ahem.

FORMER #2 MALE: . . . that is . . . ah . . . things will be a bit different . . . mistakes were made.

Richard Prum doesn't present any more evidence for his idea than I have for mine. If anything, I think mine is more plausible, because it explains one thing that's unique to humans (our sexual arrangements) with another thing that's unique to humans (our ability to communicate, plan, and cooperate).

On the other hand, Prum's description of his own work with birds is tremendous. He goes into deep detail (which could have gotten dull, but doesn't) about, for example, bower birds' elaborate constructions and manakins' elaborate dances. Here I think his evidence is very strong and his arguments sound. It's no coincidence that the bird-oriented chapters are a lot lighter on the "it is possible"s and the "it could well be"s and the "might have"s.

That's not to say that I discount Prum's ideas entirely when it comes to humans. I do mistrust the single-cause fallacy, particularly when it comes to something as multifarious as human behavior--but that applies to Prum's opponents as well as to Prum himself. If you're at all interested in this sort of thing, do read The Evolution of Beauty. It'll keep you thinking for quite some time.

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There are a number of good books on evolution, adaptation, and related topics. High among them, though also speculative, is Richard Wrangham's Catching Fire. Daniel Lieberman's The Story of the Human Body is extremely convincing, if not quite as daring. A good deal older, but still very much worth reading, is Melvin Konner's The Tangled Wing.