None of that is how evolution works, just look at the Italian wall lizard to show how animals evolve to eat new food sources. It's not like suddenly plant A goes from toxic to edible, it's a progress.
Best examples are snakes that can eat salamanders that are extremely deadly. It's not like the salamander got this deadly poison that instantly kills everything but the snake. It got something deadly that could harm most animals, but the snake had a natural resistance, as the snakes resistance grew, so did the toxicity of the salamanders over time.
It's not the snake or the salamander trying to be more toxic or more resistant. It's that those salamanders that were more toxic, were better able to escape the snake, and the snakes that had mutations that mad them better able to survive the toxin, were better able to eat them and so on. Everything is evolving at all times, and within every population you will get a spectrum, of those more toxi, and those less toxic then the average. The mutations that are less get weeded out, while those that are more toxic on average are more likely to survive.