The brain, as you may have noticed, can be a strange thing. It allows us to do a lot of things that are obviously adaptive: find food, avoid bears, and so on. At the same time, it is responsible for traits that don't have obvious advantages, such as dreaming or a propensity to enjoy skydiving. As Chapter 5 explains, not all traits are adaptive. Some traits that aren't adaptive can get dragged along by the traits that are. Pre-existing preferences could fall into this category: in this scenario. females have a pre-existing preference for the trait. Something about how the female brain is wired leads them to favor the trait just because.
Scientists have tested ideas about pre-existing preferences in the laboratory, iff using fish closely related to swordtails. Swordtails are small freshwater fish common in home aquariums. Males have a long pointed tail — hence, the name swordtail. Females, as you've probably guessed, have a mating preference for males with long tails.
The fish used in the studies, although otherwise very similar to the swordtails, differed from them in a couple of key ways: The males lack the long pointed tails, and the females obviously don't choose mates based on tail length, because none of the males has a long tail. To find out what would happen if the males did have long tails, researchers attached fake long pointed tails to the male fish. And what did the females do? They preferred to mate with the males wearing the fake tails.
This experiment was important because scientists were able to demonstrate that pre-existing preferences exist. These preferences — in combination with the data showing that the reproductive success of sons is decreased if their fathers were less flashy — suggests that the runaway-selection model is a very real possibility even if the showy trait never had any ecological advantage.
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