The Mate Selection Trapdoor

Tracing the evolution of hidden sexual preferences

By Michael J. Ryan

The evolution of sexual beauty is an ongoing experiment in every species, including our own. Variation in courtship traits arise and are judged by choosers who quickly relegate most of them to the dustbin of history. But new attempts at beauty that pass muster, that trigger a hidden preference, hit the evolutionary jackpot.

Hidden preferences are often lurking in animals’ sexual aesthetics, masked to others because there are yet no sexual traits to elicit them. But if that trait evolves, one that matches or exploits a particular sexual aesthetic, then it is immediately deemed to be sexually beautiful, and, all else being equal, it should soon evolve to be common among males. This notion of how sexual beauty can evolve was virtually unknown before 1990 until a few other researchers and I developed this theory. Now it is thought to be one of the major factors driving the evolution of sexual beauty.

To appreciate the evolutionary importance of hidden preferences, it pays to realize there are no free lunches in the sexual marketplace. Regardless of how traits and preferences evolve, they incur costs as well as reap benefits. It is the cost-benefit ratio, and how this changes through time, that determines their legacy. Given the cost-benefit ratio involved in sensory exploitation of hidden preferences, this might be an especially easy process to trigger.

A hallmark of sexually attractive traits is that they are costly. Whether they are the showy tail of a peacock or the bright colors of a guppy, these traits usually take more energy to produce, more time to maintain, and are more conspicuous to predators than other types of traits. Attractive red badges evolve in red-winged blackbirds. Yet a mutation that causes red badges can quickly disappear from the population if it is attractive to predators but not yet attractive to females. This must happen often: A mutation gives rise to a conspicuous sexual trait, but it goes extinct while waiting around for a mutation in the preference gene that will deem this trait…

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