Darwin's Other Theory

Darwin found a second engine of evolution running alongside survival of the fittest: sexual selection. A trait can spread through a species even when it makes staying alive harder. The peacock grows a tail so large and showy that it costs him dearly: it is metabolically expensive to produce and makes him far more conspicuous to predators. Peahens love it anyway, and that preference is enough to keep the tail growing.

Now it becomes a feedback loop. Females that pick the showiest males have showy sons, who get picked in turn. Each generation the bar creeps higher, and the tail balloons past anything survival would ever favor. Biologists call this a runaway process: a chase between display and preference with no built-in finish line.