They’re not the first of their kind to do so. Stickleback populate certain inland Alaska lakes, due to an event that cut the lake off from the sea 13,000 years ago. These survivors have accumulated genes and physical traits that allowed them to become full freshwater fish, but no one thought that they did so quickly.

A new study shows that some populations stranded in 1964 adapted within ten years. To all outward appearances, these two populations were stranded at the same time — even though they spent over 10,000 years having different life cycles.