When evolutionary biologists encounter a trait in nature, they perform a process known as reverse engineering to understand why that trait existed in the past and continues to exist in the present.
Take, for example, the peacock’s tail.
Evolutionary theory is based on the idea that every adaptation must increase the organism’s reproductive fitness or it would long ago have been bred out of existence. On the face of it, the peacock’s tail poses a problem to the theory. It’s big, heavy and impractical to the point of being downright counterfunctional. The recent theory is that the tail’s very cumbersomeness advertises the peacock’s high level of overall reproductive fitness. The tail announces to peahens, “look at me, I can schlep around all this excess plumage, I must be a pretty impressive peacock.”