How did some stickleback populations become freshwater specialists?

• A. Through colonization of freshwater lakes after glacial melt and subsequent isolation
• B. Through rapid ocean voyage
• C. Through lab breeding experiments
• D. Through hybridization with unrelated species

Answer: (A)

Colonization of newly formed freshwater habitats after glacial retreat, followed by isolation from marine populations, drives freshwater specialization in sticklebacks. When glaciers melted, sticklebacks moved into lakes and streams that were cut off from the ocean. The freshwater environment imposed different selection pressures—predators, prey, and ion-rich conditions differ from the sea—so natural selection favored traits better suited to freshwater, such as reduced armor plating and changes in pelvic structures. Over generations, these isolated populations adapted to their new environment and diverged from their marine ancestors, becoming freshwater specialists. The other scenarios don’t fit natural stickleback history: a rapid ocean voyage wouldn’t lead to freshwater adaptation, lab breeding is artificial, and hybridization with unrelated species isn’t the typical path to freshwater specialization.