Some poisonous fruits are dispersed by species that are not affected by their toxins. (Either they have evolved resistance, or the toxins are directed at other kinds of animals.) In other species, only the seeds are poisonous while the fruit pulp is not. Birds swallow the fruits whole, and pass the seeds through without digesting them, and thus are not affected. Mammals, which may chew the fruits and break up the seeds, would release the toxins and are thus deterred.
I’ve already pointed out that these effects are fortuitous but not coincidental. Neurotoxins that are effective against insects in small doses have milder effects on large animals like humans.
What about more strange evolutions? The Cecrophia tree (Brazil) has evolved with local ant species. the tree provides a home for the ants; the ants reciprocate by attacking any animal that tries to eat the tree. This is a highly sophisticated adaptation-I wonder how long it took. Almost Lamarckian, in a way.
Likewise, one might ask, “Why did chocolate evolve in such a way as to make dogs sick when they eat it?” Something in the plant works in its favor. People like chocolate and it doesn’t make us sick. So, who cares?
I know the post I quoted is years old, but now that the thread is active again I thought I’d point out there are subcultures like some Jains that believe they should only eat things that were “intended” to be eaten, i.e. fruit designed to attract animals who will scatter the seeds inside the fruit after eating it. If you truly believe in not harming another living being, it’s a perfectly reasonable ethical stance.
It’s not good luck. The plants evolved toxins that affect animals or fungi that try to eat them. Different toxins work in different ways, the opiates that effect the human nervous system do the exact same thing to animals that try to eat plants with opiates. If you get too much opiate you’ll drop dead. An herbivore that ate poppies would sicken and die before it could eat enough to prevent starvation. If you only take a small amount you’ll screw up your nervous system, but not die. But the toxin didn’t evolve to protect against herbivores who occasionally eat a bit of the plant, the toxin evolved to deter herbivores who eat a lot of the plant. The opiates in a poppy don’t interfere with your nervous system by accident, they evolved to interfere with your nervous system.
Of course, some herbivore species have evolved mechanisms to detoxify some types of antiherbivory toxins. Monarch butterflies eat milkweed, which is toxic to most species, because they have adaptations that neutralize the toxins, the remaining toxins make Monarchs unpalatable to predators.
The question arose because I couldn’t see how producing nicotine benefits tobacco plants THC benefits hemp plants, etc. I assumed that producing this compound had some additional cost to the plant (probably in increased nutrient and energy consumption), and, since evolution selects against wasteful use of resources it must provide some benefit to the plant.
I’d previously read about why some venoms are so toxic to non-target species (iirc it was partly a quirk of biology on the victim’s part, partly a lack of pressure to evolve a defense), so I strongly suspected it’s affect on humans might be simply a side effect. Still, that begged the question of what IS the ‘intended’ effect.
