Why is the manchineel tree so poisonous?
According to http://www.atlasobscura.com/articles/whatever-you-do-do-not-eat-touch-or-even-inhale-the-air-around-the-manchineel-tree, it evolved to be poisonous to discourage animals from digging into it. Here's one possible way that could have happened. It was once only a tiny speck more poisonous than other trees by random fluctuations in toxicity. By natural selection, animals evolved to prefer it less. They were more likely to dislike the taste of the more poisonous manchineel trees, which in turn caused it to evolve to be more poisonous. Maybe it was such an advantage to get more energy by eating some of it that some species evolved to be more resistant to the poison so that they could eat some of it. That caused the manchineel tree to evolve to be even more poisonous to discourage animals from eating it. It was an evolutionary disadvantage for the apples to be poisonous. That was just an accidental result of the rest of the tree being so poisonous. Indeed, they did evolve to be much less poisonous than the sap for that reason. Animals evolved to dislike the taste of the apples because they're poisonous. The less poisonous ones were more likely to get eaten and have their seeds spread. Since they couldn't evolve to be nonpoisonous, they instead evolved to taste like there's nothing wrong with them so that they could get eaten. Since iguanas did evolve to be immune to the poison apples, there wasn't much of an advantage in them evolving to be even less poisonous.