system
Given the following post, respond as if you're commenting on Reddit:
human
Many poisonous and venomous vertebrates get their toxins from toxic arthropods that form part of their diets. Why can't they just form the toxins themselves the same ways their prey do?
gpt
Plenty of vertebrates do make their own, for example multiple snake species.

Most animals that have some level of toxicity due to diet likely have this due to the survival benefits conferred to their ancestors from that food source. For example, monarch butterfly caterpillars eat milkweed, which makes them toxic. This could be the result of a population of a species which prefers a food source which also happens to give them toxicity, giving them a survival advantage as predators will learn not to eat the tiny toxic bugs. 

Eventually, if the selective pressures are strong enough (in this case the pressure would be predation) the individuals who prefer the food source which gives them toxicity will pass on their genes so their offspring will also gain toxicity from their preferred food source. So on until the whole species is toxic. 

You can think of it more as their food source giving them toxicity rather than them eating a food source in order to gain toxicity. 

Most (I would say all but I’m not sure) animals which derive their toxins in this manner use them as a defence mechanism. Whereas animals that produce their own toxins tend to use them for either catching prey or defence.