If you spray herbicide (weedkiller) on your land, it kills the weeds. Until they fight back by becoming resistant and then your herbicide doesn’t get rid of them any more.
The International Survey of Herbicide Resistant Weeds watches what’s happening. Here’s its summary of the most recent reports of herbicide resistant weeds. Here’s its list of the number of resistant species to individual herbicides.
I’m grateful to Jeremy Cherfas at the Agricultural BioDiversity Weblog for drawing attention to this story.