Answer:
Presence of predators.
Explanation:
Presence and absence of predators is the main cause of the presence of spines on stickleback fish populations because these spines helps in the protection against predators. The stickleback fish populations that lives in marine ecosystem have spines because there are many predators are present while on the other hand, stickleback fish populations of freshwater ecosystems have no spines due to no predator so they acquire the adaptation of having no spines.
Number one is D
Number 2 is b im pretty sure
