Enough just-so stories! I have been mostly saying this part for the last several years:

But one particular aspect of his argument is his distress at the idea that almost all evolutionary change is assumed to be adaptive, contributing to fitness. In other words, if a fish is blue, it must be blue for a reason. The color must help it escape predators or sneak up on prey, or be otherwise useful in some way. Beauty, therefore, must be adaptive, or a sign of underlying qualities that are adaptive. Pick a behavior or an ornament or a physical trait, and it is useful until proven otherwise.

That’s backward, says Dr. Prum. Take beauty. Since animals have aesthetic preferences and make choices, beauty will inevitably appear. “Beauty happens,” as he puts it, and it should be taken as nonadaptive until proven otherwise.