Evolution is a biological thing in this context. The conversation was about humans and psychedelic plants evolving together. What other sort of evolution is applicable?
Evolution occurs every generation. Some changes are bad, and some turn out good. They're all entirely random though- if some random mutation is good, it'll be kept. If it's bad, the poor critter will perform poorly, not pass it's genes on and it'll disapear.
Things don't evolve for a reason. They evolve entirely by chance. That's why it takes so long. If every generation had the most appropriate mutations occuring, it'd happen very rapidly. That's just not the case, though.
Learning and evolution are not remotely similar. Learning occurs within a brain, within a liftetime. It can be passed down, but it has to be re-learned every time. Having it in a book instead of having to directly figure things out just makes it a hell of a lot easier.
Evolution is purely biological.
If corn wasn't known about until recently, how many generations does it take for evolution to occur?
Tens of thousands? Corn didn't evolve along with humans. It was almost exactly how it is today, just bigger (through human conducted selective 'breeding' efforts). Again, corn didn't evolve "because"- it happen by random chance, and it turned out that this worked well for it, so the ones that are the way it is, were kept, and others faded out. It turned out that animals ate it and passed the grains on, or if not, the seeds could still make it to the ground and grow again. This was long before humans.
Maybe I should have said adaptation due to the relatively short time scale.
No, you shouldn't have. Adaptation is the same thing as evolution in this context. You can apply whatever mystical overtones you want, but there's only one thing guiding the changes in a species from one generation to the next: the ability for those changes to improve the likelihood that it'll live and reproduce. Neutral changes will likely fade out over time, or continue on as a subspecies or perhaps a new species if the change was large enough, but they other species would still remain, since they have equal chances of passing on.
Having psychedelic drugs within a plant seems to be a bit of a neutral change- or an abberation in the biosynthetic proccesses. It doesn't seem to give any benefit to the plant to live onto the next generation, though some DMT containing grasses live on better because once a sheep eats some, it'll never eat it again.