How did ancestral sea-run sticklebacks come to live exclusively in freshwater?

• A. They moved into freshwater due to river barriers
• B. They were born in freshwater and never returned
• C. They became trapped in lakes from the ice age
• D. They were introduced by humans

Answer: (C)

The key idea is how geographic isolation from glacial events can push a population from the sea into freshwater and keep it there. During the last ice age, advancing glaciers carved out and connected or blocked waterways, creating new freshwater lakes and separating fish from their marine populations. Sea-run sticklebacks that found themselves in these isolated freshwater habitats faced very different conditions. With limited or no opportunity to return to the ocean, natural selection favored traits suited to freshwater life, and gene flow back to the marine populations diminished. Over time, these freshwater populations persisted and spread, resulting in lineages that live exclusively in freshwater.

Other scenarios don’t fit the big-picture pattern as well. River barriers alone don’t account for the long-term isolation and broad freshwater colonization seen after glaciation, being born in freshwater and never returning isn’t the usual explanation for how sea-run ancestors ended up there, and human introduction doesn’t explain the widespread, ancient freshwater lineages formed by natural processes.