This is why we often find not just a single species with huge weapons but entire clades packed with species after species all armed to the teeth. Inherited characteristics like asymmetrical parenting tip the balance in favor of arms races for all descendant members of the clade. All that is needed is for the remaining two pieces to fall into place. If many of these species also share another one of the ingredients, say an inclination for using habitats such as burrows that result in defensible resources or choke points, the balance is tipped still further. The result can be explosions of animal diversity, as species after species within these clades launch onto trajectories of rapid weapon evolution.

Emlen, Douglas J, Animal Weapons_ The Evoluti…, loc. 269