All kinds of animals, from rats and pigeons to weeds and mosquitoes, are evolving so they can survive and often thrive in cities. Now it has been shown that even city fish are evolving in unique ways.

“Even though urban fish are widely separated, they are consistently different from rural fish,” says Elizabeth Kern , formerly at North Carolina State University and now at Ewha Womans University in South Korea.

The big difference between urban and rural streams is the speed of water flow, she says. In rural areas, rain …