Imagine a population of rhinoceros beetles where the average length of male horns is one-half inch. In this social environment, a few males stand out. A mutation increasing horn growth has given them three-fourths-inch horns. These males win battles the most frequently; they mate with the greatest proportion of female beetles; and they populate later generations with disproportionate numbers of their kind (including sons wielding three-fourths-inch horns). Over the next few dozen generations, the population shifts. It evolves in response to sexual selection so that now the average size of horns is three-fourths inches. … The benefits of a three-fourths-inch horn aren't so great anymore, because now everybody has horns this big. The evolutionary increase in horn length reset the standard for male competition. Into this new social milieu another mutation arises, this time leading to a one-inch horn. Males with these new alleles now have longer horns than their opponents, and they begin to win. So the new alleles sweep through the population as the largest horned males outcompete the earlier, three-fourths-inch rivals, until the population has evolved to this new weapon size. The scale has shifted once again. The population has ratcheted up to the new norm, and it stands poised and ready for the next mutation leading to still another increase in weapon size.

Emlen, Douglas J, Animal Weapons_ The Evoluti…, loc. 183