1. In lakes with dragonfly larvae, pelvic spines can be disadvantageous, allowing the predatory larvae to grab the fish.
2. In lakes where there are no large predatory fish, there is no advantage to having pelvic spines.
(Different environments can provide different selective pressures on an organism's morphology. In the threespine stickleback, pelvic spines provide a selective advantage in environments with large predatory fish but are a liability in environments with dragonfly larvae.)