Of course, every story has exceptions. But with sticklebacks, the exception proves the rule. Dan Bolnick has been studying sticklebacks in Lake Washington, where fish have much bigger weapons than in other lakes. Bolnick found that this shift in armor happened very recently—fish samples collected prior to the 1960s had reduced armor typical of other lake sticklebacks. Efforts to stem pollution in this lake resulted in a dramatic improvement in the transparency of the water, and in these especially clear waters, introduced trout began to feed in earnest on sticklebacks. More predators translated almost immediately into bigger weapons.

Emlen, Douglas J, Animal Weapons_ The Evoluti…, loc. 57