Evolution occurs on a species level, not an individual one. Also, evolution isn't "directed" towards anything. It's random mutations, and those which end up being more beneficial are the ones that stick around.
In this case, it's entirely feasible to imagine multiple species developing a common scent that is avoided by all. It would after all improve the chances of survival for the _entire_ population. There are many species of insect and plant which have bitter tastes as well. Such features may not be beneficial at all to the individual plant, but to the population they discourage future meals.
Then again, it may have simply evolved in a common ancestor to them all.