Photo: MrClean1982 (flickr)
One common misconception people have regarding evolution is that the gradual changing of physical forms always benefits the species in question.
A good counter-example to this can be found in a particular evolutionary model known as Fischer’s Runaway Selection model.
Named after a British geneticist, the Fischerian Model of Runaway Sexual Selection suggests that evolving traits can continue to reinforce themselves well past the point of efficiency. Take the case of birds. A short tail is the most useful kind for flying. However, in many species of bird the females show a preference for longer, gaudier appendages in males. That means the females who choose longer-tailed males have just upped their chances of passing on their genes.
Why? A long-tailed mate ensures that their sons will also have long tails, and thus be found attractive by future females. Furthermore, their daughters will also be attracted to long-tailed males, and also produce long-tailed sons.
You can see that, once started, this cycle will just keep repeating itself indefinitely, entering what is called a “runaway” phase. Tails will continue to become longer and gaudier across the generations, simply because the bigger ones will always be favored over the smaller ones. Moreover, such a process can continue well past the point when the resulting tails become unusable for flying altogether and have no use other than attracting mates. Only when the tails become life-threateningly large will the cycle be broken.
We may see the end result of just this kind of thing in the peacock, for example, which sports a famously enormous and colorful plumage, but couldn’t possibly get off the ground.