Measuring three to 10 centimeters, stickleback fish originated in the ocean but began populating freshwater lakes and streams following the last ice age. Over the past 10,000 years, marine and freshwater sticklebacks have evolved different physical and behavioral traits, making them ideal models for Darwin's natural selection theory.

"By testing the temperature tolerance of wild and lab-raised sticklebacks, we were able to determine that freshwater sticklebacks can tolerate lower temperatures than their marine counterparts," says lead author Rowan Barrett from the UBC Department of Zoology. "This made sense from an evolutionary perspective because their ancestors were able to adapt to freshwater lakes, which typically reach colder temperatures than the ocean."

To learn how quickly this adaptation took place, Barrett and colleagues from Switzerland and Sweden "recreated history" by transplanting marine sticklebacks to freshwater ponds and found that in as little as three generations (or three years), they were able to tolerate the same minimum temperature as freshwater sticklebacks, 2.5 °C lower than their ancestral populations.