Aquatic weeds that choke waterways are an increasing plague as we humans spread them willy-nilly. Sometimes natural selection contributes to the problem, as this story from the Hartford Courant reports:
What scientists have found in Selden Cove and everywhere else along the 200 miles of the Connecticut River and its tributaries below Springfield, Connecticut is something different: A previously undiscovered, genetically unique and exceptionally robust strain of hydrilla that so far has not been found anywhere else in the world.