Final answer:
Some stickleback populations came to live exclusively in freshwater due to evolutionary mutation that allowed them to adapt to this new environment. Over time, different stickleback populations became isolated and evolved into distinct species.
Step-by-step explanation:
The three-spined stickleback (Gasterosteus aculeatus) is a freshwater fish that evolved from a saltwater fish to live in freshwater lakes about 10,000 years ago. This adaptation is an example of evolutionary mutation where the stickleback population developed genetic changes that allowed them to survive and reproduce in freshwater environments. Over time, different stickleback populations became isolated in different lakes, leading to the formation of distinct species. This isolation and divergence in species is made possible by the sticklebacks' use of different feeding areas in the lakes, called micro niches.