Species normally evolve gradually in a process that unfolds over thousands -- sometimes millions -- of years. But scientists say they have discovered an Alaska fish population that appears to have transformed in the last 50 years -- a lightning-quick transformation, at least by evolutionary standards.

The changes happened after the 1964 earthquake, the most powerful ever recorded in the United States, rearranged terrain around Southcentral Alaska and abruptly changed the habitat of some of the region's threespine stickleback, a small, widely distributed fish.

When the quake lifted parts of Prince William Sound and the Gulf of Alaska by as much as several meters, it created freshwater ponds on several islands.

The stickleback that found themselves in those new ponds adapted rapidly when their habitat changed from saltwater to fresh, according to a new study by scientists from the University of Alaska Anchorage, University of Oregon and other institutions. The study was published in the Proceedings of the National Academy of Sciences.

The research focused on about 1,000 threespine stickleback collected between 2005 and 2011 from ponds that hadn't previously existed on three of the islands -- Middleton, Montague and Danger -- that were thrust upward by the earthquake, said Emily Lescak, the lead author and a postdoctoral fellow at UAA.

The stickleback in the newly formed ponds had the physical features of freshwater varieties, she said. They were smaller and more squat in shape, a contrast to the bigger and more elongated long-distance-swimming saltwater stickleback. And like typical freshwater stickleback, they had only a small amount of bony armor on their sides, compared to the full rows that are characteristic of ocean stickleback.

Still, despite their freshwater-like outward physical features, the fish from the post-earthquake ponds were "dramatically different" genetically from nearby stickleback in longtime freshwater bodies, Lescak said.

Instead, they were fairly closely related to the nearby ocean stickleback, indicating that they were the descendants of fish that came from the sea, she said.

"The most probable ancestors were oceanic fish," she said.

Threespine stickleback are small and abundant fish found in coastal waters of Eurasia, eastern Asia, North America, Greenland and Iceland, according to a species profile from the Alaska Department of Fish and Game. They live in habitats ranging from open ocean to freshwater inland lakes. Adults spawn at 1 to 2 years of age.

Some threespine stickleback live full time in saltwater, some full time in freshwater and some are anadromous, moving between ocean and freshwater environments and occasionally mingling with the freshwater populations, Lescak said. It is easy to tell the difference between ocean-dwelling and freshwater-dwelling stickleback, even when ocean stickleback make their forays into freshwater bodies.

The different populations around the world have their own genetic identities, which have been shown to be in flux over time. That makes the fish good material for evolutionary biologists.

"Stickleback have been studied extensively since the time of Darwin, so we know more about them than most vertebrates. They show independent and parallel evolution from oceanic to freshwater forms around the northern hemisphere, and the morphology, physiology and genetics of this diversification has been extensively studied," co-author Frank von Hippel, a UAA biology professor who has focused on stickleback, said in an email.

Some previous genetic research in Alaska, carried out by some of the co-authors of the new study, showed how stickleback in the Cook Inlet region evolved after the last ice age so that they could live in a freshwater environment.

One thing that makes the new study different from past work, Lescak said, is the broad range of genetic traits that were examined.

The study, which had National Science Foundation funding, addresses an important biological subject, she said.

"One of our big questions concerns how quickly evolution can occur in wild populations," she said.

On these three islands, at least, it appears that evolution happened within five decades.

It might have happened even faster than that, Lescak said.

"It's possible that it occurred over a few generations," she said.