Here’s the latest Radiolab podcast, this time about sexual selection in birds and about Richard Prum’s revival of the idea of “runaway sexual selection,” which he calls the “beauty happens” theory. (It’s in his book, The Evolution of Beauty: How Darwin’s Forgotten Theory Shapes the Animal World—and Us.) I’ve written about Prum’s book before, and about my criticisms of his ideas—or rather, how he presents other people’s ideas—so I won’t go into that here.
I was interviewed by the Radiolab hosts last summer about sexual selection, but the show has just appeared today (click on screenshot below and then on the “listen” button to hear the 45-minute podcast). Note that the show claims to “present another way of looking at evolution.” One could construe “another way of looking at evolution” as simply the widely accepted view that sexual selection (rather than “survival of the fittest”) explains sexual dimorphism, or, alternatively, as Prum’s view that runaway sexual selection based on “random” female aesthetic preference is the best explanation for sexual dimorphism of color and behavior instead of “good genes” models positing that such traits indicate a male’s genetic quality.
When I talked to the hosts last summer, I got the idea that they had bought into Prum’s idea of runaway sexual selection as the best explanation of sexual selection, although, of course, there are other models, as described in the review of Prum’s book in Evolution by Gail Patricelli, Eileen Hebets, and Tamra Mendelson (if you’ve read Prum’s book, you must read that review).
It turns out that this show is not entirely—or even largely—about runaway sexual selection, though that process figures heavily in the last half. Much of the show is simply about the wonders of sexual selection and the traits it creates, and features interviews with scientists working on duck penises (the show begins with a discussion of that issue, which is of course a way to attract ears) and on bowerbirds (Patricelli), and it’s not bad.
The discussion of sexual selection and Prum’s “beauty happens” model begins about halfway in, with Prum calling evolutionists’ acceptance of “good genes” models, in which male traits indicate their genetic fitness, a “flattened, dumb down and ideologically purified version of Darwin’s actual richness.” Well, that’s grossly unfair, as all of us recognize that there are a variety of models for how sexual selection works, with Prum’s “runaway” model being just one of them. (Behavioral ecologists tend to concentrate more on good genes, though.)
But, as I said, there are more than just the runaway model and “good genes” models. There are, for instance, “sensory bias” models, in which female preference is not a random phenomenon that gives the female “aesthetic” preferences, but that such preferences themselves are a product of evolution. Female may prefer certain traits or behaviors of males because (as I say in the show), those preferences are either a byproduct of evolution on their sensory system, or a direct result of selection. (Females may be attuned to some sounds more than others, for instance, because those sounds give them useful information about their environment.) And if that’s the case, then there may be natural selection operating on female preference by itself.
And if female preferences are subject to this kind of selection, Prum’s “beauty happens” model runs into trouble, for it assumes there are no selective constraint on female preference. (This is taken up in the Patricelli et al. review). If such selection occurs, the runaway often doesn’t work.
Further complicating attempts to distinguish the models is the fact that they can work together. The paper below, for instance, shows that in many cases—at least according to theory—runaway sexual selection can create a situation in which females are also choosing males with good genes, so experiments to distinguish the runaway vs. the good-genes model for the origin of sexual selection would be hard or impossible. (Click on screenshot to go to the paper.)
When I talked to Krulwich et al. in our 90-minute conversation (they managed to insert in the podcast the noises of me sucking on a coughdrop before the interview began!), I had two aims:
1.) To point out that Prum’s “beauty happens” model was not NOT a “null model” of sexual selection that should be assumed to be true in the absence of other information. Further, I wanted to point out that there were problems with this model itself, as it makes assumptions that may not be true (i.e., that there’s no direct selection on female preference itself). While I think the runaway model is certainly plausible, and must have played some role in the evolution of male traits and female preferences, there are other plausible models as well.
2.) To point out that hard data on which model explains a given case of sexual selection are sorely lacking. It’s hard to distinguish the various models, especially because selection happened in the past and because the models can operate together. To assert, as Prum does, that we know one model explains nearly all sexual dimorphism for ornaments and calls, is to make an unwarranted claim. It’s not that we know Prum is wrong; it’s that we don’t know much about how any of these systems evolved.
How did I do? Well, my bit begins at about 35:30; you tell me. I was surprised that the show let some of my more critical remarks about the book appear. But I think it’s good that the public knows how scientists can disagree on matters where there is no dispositive data.
In the end, Krulwich goes into a soliloquy in which he seems disappointed that we don’t know the answer, and almost depressed because future research may show that different models may explain different cases of sexual dimorphism. This would mean that we don’t have a “rule” for how sexual selection works, but a series of anecdotes that give us statements about the relative frequencies of different processes.
So be it: this is evolution, not physics, and evolution works in multifarious ways. I had a few pithy statements about this issue in my phone interview, but, sadly, these didn’t make it onto the show.
As a whole, I’m not sure how well the show hangs together. I can’t listen to it as if I were a nonscientist hearing about this for the first time, so give your take below. I think the Radiolab folks will be reading this, so be civil but also be honest.