What is stickleback evolution?
Stickleback fish can undergo modifications to their genes and undergo evolution (through several generation) depending on the lake environment that is favorable for the evolved species.
The genes in this case is PITX1 gene that is involved making armor plating (with spikes). The main environment that they live in contains different predator.
One environment contains dragonfly larvae that can catch the fish by latching on to the spikes. Hence, the environment allows stickleback to evolve with no armor plating.
Another environment contains big fish like trout that can consume stickleback with no armour plating. Hence, stickleback with no armor plating survive in this environment.