What role do glacial cycles play in stickleback population structure?

• A. Glacial cycles create cycles of isolation and contact, promoting divergence and subsequent secondary contact with gene flow.
• B. Glacial cycles promote constant gene flow across populations.
• C. Glacial cycles have no impact on sticklebacks.
• D. Glacial cycles cause immediate extinction.

Answer: (A)

Glacial cycles create alternating phases where freshwater habitats are fragmented and then reconnect as ice advances and retreats. During the isolated phases, stickleback populations diverge genetically because drift and local adaptation act without much gene flow. When the ice retreats and habitats reconnect, populations come into contact again and gene flow resumes, bringing in new genetic variation but often tempered by ongoing local adaptation. This pattern of cycles—divergence during isolation followed by secondary contact with some gene flow—produces a structured population landscape with locally adapted lineages and complex population dynamics.

That’s why this option fits best: glacial cycles don’t promote constant gene flow across all populations, they generate periods of reduced or no gene flow, followed by episodes of contact. They do have a clear impact, and they don’t cause universal immediate extinction of populations.