Think about it this way, you can evolve a horse into a giraffe, you just have an environment where they have to eat leaves higher and higher up - that's like vaccines, synthetic antibodies, etc. But you can't evolve a horse into a turtle, they're just too different - that's like ivermectin.

Or, perhaps more accurately, if you castrate every horse you come into contact with, they're not going to evolve into anything at all. The only remaining selection pressure will be "Stay the fuck away from almond_activator".