This saltwater trout evolved to live in freshwater—in just 100 years

Although we tend to think of evolution as happening over thousands, if not millions, of years, critical changes can take little more than a century. That’s what happened with a group of steelhead trout transplanted from the salty seas of California to the fresh waters of Lake Michigan for game fishermen in the 1890s. A new study shows that the fish, which typically live part of their lives in the ocean like salmon, developed key genetic differences that allowed it to live wholly in freshwater—in little more than 100 years.

The discovery shows how quickly organisms can adapt to a new lifestyle—if they have some of the right genes to start with, says Michael Blouin, a geneticist at Oregon State University in Corvallis. “The work is a nice example” of how evolution can happen “over very short time periods.”

Steelhead already had a taste for freshwater. They hatch in rivers hundreds of kilometers from the Pacific, spend long periods as adults in the ocean, then return to their home rivers to spawn. And they even have a form—the popular rainbow trout—that lives out its whole life in freshwater streams. But that saltwater steelhead so readily made Lake Michigan their full-time home was surprising.

To find the genetic basis of this quick adaptation, a team led by evolutionary biologist Mark Christie from Purdue University in West Lafayette, Indiana, and his postdoc Janna Willoughby sequenced the genomes of 264 steelhead. Some came from the source waters in California that supplied the first Lake Michigan fish, while others were collected from the lake’s watershed in 1983 and 1998. By comparing those genomes, they reconstructed the steelhead’s struggles to adapt.

The first batch of transplants had a hard time, likely dying off by the hundreds. But the few that survived thrived, and between 1983 and 1998, their population started to rebound and even diversify, most likely because of interbreeding with newly introduced hatchery fish, Willoughby and Christie report this week in Molecular Ecology.

Three regions of DNA were quite different between the modern lake and saltwater steelhead. Two of those contain genes critical for maintaining the fish’s internal salt balance: Freshwater fish must take in extra salts, whereas saltwater fish must get rid of them. Moving salt in opposite directions requires different versions of the relevant genes. Another DNA region seems to affect wound healing. This may help the lake steelheads recover from parasitic lampreys, which are widespread in that freshwater.

So how did the genes change so quickly from one version to another? Intriguingly, there was no sign that steelhead had interbred with rainbow trout to get the genes they needed to thrive. They also didn’t have to mutate, Christie explains. Instead, there were likely a few steelhead among the first batch of transplants that already had the right versions of these genes—they simply survived and reproduced much more successfully than their peers. Eventually, the less well-adapted steelhead disappeared.

More work needs to be done to prove that the genetic changes are due to freshwater living and not chance, says Felicity Jones, an evolutionary biologist at the Friedrich Miescher Laboratory of the Max Planck Society in Tübingen, Germany. She and her colleagues have found that the small minnowlike fish called sticklebacks have also made the transition from saltwater to freshwater, and have undergone similar same genetic shifts. “The transition … is a major change,” Blouin explains. “It would not be surprising to see the same adaptation in multiple species.”