The runaway-selection hypothesis says that, if females choose mates based on some random showy character simply because they're attracted to that character, that character becomes more pronounced in each successive generation until the disadvantages of the having the trait (it makes you less likely to survive) outweigh the advantages (it makes the ladies like you).
Here's how runaway-selection works: Females choose males with, for example, bigger tails for no other reason than that they like big tails. Because the biggest-tailed birds father the most offspring, the average male in the next generation will have a slightly bigger tail. When the next generation's females choose among these males, they select for even longer tails. In this way, male tail length increases until it reaches the point where the increased risk of predation, starvation, or some other unfortunate outcome of having an enormous tail outweighs any additional attractiveness to the females.
Two questions come to mind in considering this model: How does it benefit the females to mate with showy males, and what makes them prefer one particular trait over others? The next sections provide the answers.
The runaway-selection hypothesis seems as though it shouldn't be right simply because . . . well, evolution shouldn't work in such a strange way. But it's important to remember that our emotional reaction to any particular hypothesis rarely has anything to do with whether it's correct.
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