What role do glacial cycles play in stickleback population structure?

• A. Glacial cycles influence population structure by creating cycles of isolation and contact, shaping divergence and later gene flow.
• B. Glacial cycles promote constant gene flow across populations.
• C. Glacial cycles have no impact on sticklebacks.
• D. Glacial cycles have only ecological effects on plants.

Answer: (A)

Glacial cycles create shifting barriers that alternately isolate and reconnect populations. In sticklebacks, advancing ice and changing waterways fragment habitats, sending groups into refugia where they diverge due to drift and local adaptation to different freshwater conditions. When the ice retreats, newly available connections allow secondary contact and gene flow, mixing populations and sometimes reducing differences or creating admixed lineages. Repeated cycles of isolation and contact accumulate patterns of divergence and later gene flow, shaping the present-day population structure. That’s why the idea that glacial cycles create cycles of isolation and contact, driving both divergence and later gene flow, best captures the concept.