[uplifter]

>Thus, one may guess, that first distinctive poisonous mushrooms like the fly agaric developed, then most animals large enough to eat them developed an instinct to avoid all mushrooms, and then the non-poisonous freeloading mushrooms developed.

Just wanted to note that these phenomena are important enough in the study of mimicry in biology to have earned their own names:

Müllerian mimicry is when two species who are similarly well defended (foul tasting, toxic or otherwise noxious to eat) converge in appearance to mimic each other's honest warning signals.

Batesian mimicry is when a harmless or palatable species evolves to mimic a harmful, toxic, or otherwise defended species.