*sigh* how many times does it need to be said? evolution is NOT this thing that magically happens to everything every so often just because. it's natural selection, the plant with the genetic code that gives it the upper hand to survival, becoming more prominant in the gene pool. weed is controlled and bred by humans now, and therfor we control it's evolution. it's not going to evolve into some useless plant unless we breed it to do so.

species don't evolve because one day all the dinosaurs said "lets all mutate now!"