For worse or for better, humans are changing the course of evolution. On February 26, we talked about how fishing practices may be driving the evolution of smaller, harder-to-catch trout. This week brings news that urbanization has changed the way weeds make seeds. The results appear in the current online edition of the Proceedings of the National Academy of Sciences. Scientists in the South of France were studying the sort of weed you see cropping up around the base of trees planted along city sidewalks. The weed, called Crepis sancta, can make two different kinds of seed: one that’s light like a dandelion seed with a feathery little parachute that gets carried by the wind; and another that’s heavy and just drops to the ground.
The scientists compared weeds that were growing in these small urban patches to ones growing in an open field. They found that the urban weeds produced fewer of the fluffy seeds than their country cousins, which makes evolutionary sense because in a city setting, drifting seeds are more likely to hit pavement than soil. In the long run, though, that strategy might not be healthy. Because plants benefit from spreading their seeds. So that’s bad news for the weeds. But maybe not so bad news for the sidewalks.