In the beginning (well, the first eighty thousand years of their one-hundred-thousand-year window), Nevada lake sticklebacks had almost no protective weapons (only one dorsal spine, rudimentary pelvic spines, and very few lateral plates). But then, eighty-four thousand years into the time sequence, this type of stickleback was replaced entirely by armored sticklebacks, meaning three long dorsal spines and full pelvic spines. Bell suspects that marine fish flooded into the lake around this time, because both forms co-occurred for about one hundred years before the early fish type disappeared. Remarkably, over the following thirteen thousand years, the defensive structures in this new fish regressed: in graded steps through time, the spines got shorter and shorter, until by the end of this period the new sticklebacks resembled the earlier form that they’d replaced. Lake-bound fish lost their weapons.

Emlen, Douglas J, Animal Weapons_ The Evoluti…, loc. 54