In a word: predators. Predators are very bad news, not just because they will kill an animals but because a dead animal can’t have babies and so continue its family line (which is what really matters for evolution, it’s what we call ‘fitness’). So animals have evolved all sorts of clever ways to stop themselves from being eaten, and one of these is to be toxic.
This works very well because predators usually want an easy meal, not one that might kill it (or at least make it ill). Many toxic animals (though certainly not all) will also use warning signals such as the bright colours of bees or the rattle of a rattlesnake to tell predators that they are toxic before they are even attacked. Also, because they are not at as high risk of predation, toxic prey can move around their environment better and so spend more time eating and things without having to worry about hiding from predators. So there are extra benefits of being toxic on top of not getting eaten by predators, but the evolution of toxins is still driven by predators.
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