http://www.sciencemag.org/news/2018/06/saltwater-trout-evolved-live-fresh-water-just-100-years

Steelhead trout have proved quite capable of abandoning the sea for a life in fresh water.

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.