On evolution and beauty

A sort-of review of The Evolution of Beauty by Richard Prum

Why has evolution produced so many beautiful things? And does the answer tell us anything about what beauty is?

1.

Beauty can evolve if it signals fitness. Economists as well as biologist talk about signaling, and the concept is maybe easier to understand in that context. A company will use signaling when the quality of its product is not easy to observe. Company A makes a better air conditioner than company B, and wants shoppers to know this; but the units look pretty much the same, and plastering "the better AC!" on the box won't be believed. A solution: offer a money-back guarantee on the product. This will of course cost company A (some of their air conditioners will fail), but a similar guarantee would cost company B way more (their air conditioners are crap)—and so B offers no guarantee, or a weaker one. The guarantee signals to shoppers that A's ACs are better.

In nature, organisms are the customers, and what they "want" is to mate with highly fit organisms: a fitter mate means fitter offspring, which means a better chance of spreading their genes through future generations (of course organisms "want" this only in the sense that natural selection will push the species toward behaviors that have this effect). But fitness, at least some aspects of it, is hidden. If A and B are male yaya birds (a species I made up), which of them has the stronger immune system? Hard to tell, if neither is sick. If only males could signal to females the quality of their immune system! The signal must be something (i) expensive, (ii) easily observable, and (iii) much more expensive, if you have a weak immune system. If pretty tails on males have these features (and other conditions hold, for example immune system quality must partly heritable), then natural selection will favor the propensity, among males, to try to grow a pretty tail, and also the propensity, among females, to prefer to mate with pretty-tailed males. (Prum writes that when a pretty tail evolves because it signals fitness, it "functions like a birdie Internet dating profile," which is exactly wrong; a signal, in the technical sense used here, must be hard to fake, but making yourself look good on an internet dating profile even if you're a louse is—I have been told—not hard at all.)

Prum's core thesis, which he says was Darwin's originally, is that beauty can evolve even when it does not signal fitness, and that this non-signaling process explains a lot of the beauty in the animal world. If the females of a species get to choose their mates (rather than being forced to mate with the alpha), and if the females prefer more beautiful mates, then males will evolve to be more beautiful, even if that beauty does not signal anything else, because beautiful males will have more descendents (in this cutthroat world, ugly males won't get to mate at all).