Marine biologists are studying stickleback fish populations. There are two distinct variations in the sticklebacks based on phenotypes. They are found with the three spines more commonly in marine ecosystems while they are more commonly found without the three spines in freshwater ecosystems. This has been observed in many marine and freshwater ecosystems across the globe. What explains the variation seen in the image here across different aquatic ecosystems?

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.

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