Evolution at Lightning Speed: Fish Adapts in Just 3 Years

Aug. 11, 2010 — -- Within an incredibly brief period of three years, a small fish has evolved to survive temperatures that are cold enough to have killed its ancestors. The lowly stickleback, a fish no bigger than a human finger, has accomplished something that many scientists had thought impossible. It adapted genetically to a radically different environment and passed that change on to its progeny in just three generations.

At the end of a three-year experiment, scientists at the University of British Columbia, Vancouver, demonstrated that third-generation sticklebacks had increased their cold-water tolerance by a significant 2.5 degrees Celsius (4.5 degrees Fahrenheit.) The research, published in the current issue of the Proceedings of the Royal Society B, involved scientists from Switzerland, Sweden and Canada, and it built upon other work at several additional institutions.

Sticklebacks were saltwater fish until the end of the last ice age, about 15,000 years ago, when many were trapped in freshwater lakes behind receding glaciers. They didn't just adapt to that very different environment. They thrived, changing rapidly to meet changing conditions while abandoning traits, like body armor that was needed in the ocean but not in a lake, and producing radically different species in lake after lake after lake.

That rapid expression of natural selection probably would have stunned even Charles Darwin, and it has caught the attention of a wide range of leading scientists, including David M. Kingsley of the Howard Hughes Medical Institute, who laid the framework for understanding the genetically adaptive stickleback.

But the latest work raises many important questions, because the genes that made it possible for the Canadian sticklebacks to adapt to cold water so quickly must have been present -- although unexpressed -- in the wild fish that were captured and used in the experiment.

So does that mean many species probably have genes necessary to survive in very different conditions, like global climate change? Maybe, but no one knows at this point because relatively little attention has been paid to the question.

And if many species do have the necessary genes, is that a good thing? Maybe, if they are endangered species, but maybe not, if they are pests, or harmful bacteria, but again, nobody knows.

In fact, we wouldn't even know as much as we do had it not been for the pioneering work of biologist Dolph Schluter, who dug more than 30 ponds on the University of British Columbia's Vancouver campus to urge evolution along. He calls the ponds his "evolution accelerator," and for several years now, his students have been breeding sticklebacks and watching the results.

Evolution at Lightning Speed

Rowan Barrett, who received his doctorate from the university just last week, led the latest experiment and is the lead author of the study.

"We replicated what happened at the end of the last ice age," Barrett said in a telephone interview. Barrett and several colleagues collected wild saltwater sticklebacks along the Canadian coast and after a brief period of adjustment in the lab, the wild fish were introduced in three ponds. They started during the summer, when the water was reasonably warm, and progressed through colder and colder seasons over a three year period.

"The initial colonists produced thousands and thousands of babies, so we had quite a large population heading into the first winter," Barrett said. "There was massive mortality in the winter, and a lot of die-offs," he said, but enough survived for the experiment to continue. The second and third years were similar -- many died, but enough survived. Since the lifespan of a stickleback is one year, the researchers ended up with third generation progeny.

"We took a sample into the lab and ran them through a protocol which essentially just involved lowering the temperature (in the aquarium,)" he said. The temperature dropped so low that the fish became "ecologically dead because they couldn't forage for food and they couldn't escape predation."

At that point, the temperature was 4.5 degrees lower than what their ancestors -- just three generations before -- could have survived in the wild. So somehow, the last survivors had made a radical adaptation to a very different climate. How did they do it?

Natural selection is normally thought of as the acceptance of a genetic mutation that gives the organism an advantage over its cohorts.

"The time required for mutations to arrive that are beneficial for a particular trait, say cold tolerance, are so rare that it would normally take at least hundreds of years before you would see the right mutations arriving in a population," Barrett said. Yet these fish adapted in only three years.

"It's quite likely that the genes that are important for temperature tolerance almost certainly were brought into the ponds with the original colonists," he added. So the fish probably had the genetic equipment they needed all along, possibly left over from a previous epoch when they lived in a colder climate.

Evolution at Lightning Speed

Once in a cooler pool, those genes expressed themselves, thus adapting the fish to their new environment.

So is it possible that many creatures have genes that can help out during the difficult years ahead? Barrett cautions that no one knows yet.

"That's not to say that all species are going to be fine and we can do whatever we want because they will adapt to whatever we throw at them," because even in the best of circumstances many will die during evolutionary processes, and even if a few survive they may not be numerous enough to sustain the population. All of the sticklebacks in this experiment eventually died.

So at this point scientists are drowning in a pool of unanswered questions. Barrett argues that potential evolutionary changes need to be brought into the equation but those changes are understood in very few cases, at least for now. We don't really know which animals, and plants, are most likely to survive. They're left with an old battle plan - migration being the first line of defense - which Barrett outlined like this:

"You move, and if you can't move you adapt, and if you can't adapt you die."