The concept of “runaway” selection comes from the field of evolutionary biology, and to
explain it, biologists most often cite the peacock’s tail. That ornamental feature has grown
ever more flamboyant across the centuries thanks to a simple fact: Peahens show a preference
for large-tailed peacocks. In the earliest days of the species, this made sense. A showy tail was
a marker of a healthy male that knew how to feed himself. (Think of it as something like a
Ferrari—at least before easy credit.) Consequently, well-feathered males had more frequent
opportunities to breed and to pass along that trait. The next generation had, on average, larger
tails. Initially, this would have weeded out the weak; but after many generations, it created a
problem for the strong. That tail is expensive (again, like a Ferrari). It requires nutrients to
grow and maintain. And it’s heavy, slowing down its owner (OK, not so much like a Ferrari)
and making him easier prey.