Obviously some plants are poisonous, and for good reason, they have evolved defense mechanisms in the form of toxins so that they can survive and avoid being eaten. Yet some plants are edible. Why? How does it serve a carrot, for example, or a turnip to be edible? Because by being edible the entire plant dies, often before it flowers and has a chance to reproduce itself.
I can understand why fruits are edible, to attract animals for seed dispersal, and also because an animal can eat fruit without killing the plant that made it. But why are plants themselves edible, especially when some aren’t? Wouldn’t it make better evolutionary sense for all plants to be poisonous, but obviously to sustain the food chain simply produce edible fruits, seeds, nuts, beans, grains, or flowers?
And one other thing, if fruits can be consumed without harming a plant and aid in seed dispersal, why would any fruit be poisonous and inedible (such as the tomato-like fruit of the potato plant)?
you seem to be asking many questions wrapped around one question, which may be more easily answered separately.
first, why are carrots edible?
the carrot that you see today at your supermarket is a monster. it is unnatural, a construct of human agriculture. go see what a wild carrot looks like–its roots are scawny, undeveloped, certainly unappetising. why would a carrot choose to fatten itself with starch so that it can be eaten? to survive. it made evolutionary sense for it to fatten itself, because in doing so, it got selected–not naturally selected by the elements of nature by by the hand of the farmer. after generations of choosing the carrots who were fatter, longer, and more orange in colour, the farmer ensured that these species survived over the less appetising. so imagine you are a wild carrot again, would you choose to be unnutritious, inedible and maybe even posionous, protecting yourself from advancing predators who would eat you but in doing so lose out to the protection agriculture gives to your other fellow carrots who choose to be fat, wholesome and delicious and hence get to produce offspring and pass down their lineage? (side note, obviously plants dont get conscious choice to all this, but the concept is the same, that of selection, whether arfiticial or natural)
second, why plant parts (other than the fruits for obvious reasons) edible?
it makes evolutionary sense for a plant to be inedible. but it also makes perfect evolutionary sense for animals to be able to consume plants. so simply (too simply) put, whose ‘evolutionary sense’ wins in the end? lets take a simple example. imagine a world where all plants have edible fruits and inedible other plant parts (i.e. leaves, stems, roots etc) and all animals only ate the edible fruits. sounds like a perfect world where each gets what so desires. Now one day in this world, one of the animals (who would ordinarily only eat the fruits) learnt how to eat the leaves of the plant, to digest the leaves and gain nutrition out of the digested leaves. This said animal obviously gained some advantage over the rest of his peers, because he could utilise a source of nutrition no other animal could. He had to spend less time finding fruits, more time resting, growing and reproducing. overtime, he had more offspring, who like him, were able to eat leaves. soon the balance shifted, the population make up of the animal community changed, there were much more dual (leaf and fruit) eaters as compared to just fruit eaters. this happens in nature and cannot be stopped. we can talk of what the optimal outcome is (just like the ideal world at the beginning whereby plants just produce some edible parts to sustain the food chain for the animals who happily eat only those edible plant parts so every one as a whole is happy) but the natural selection does not favour this, it favours only selfish gains.
lastly, why are some plants poisonous and some arent? (and conversly why are some fruits edible and some arent?)
by poisonous you mean poisonous to humans? be careful not to make this too human centric. what is food to you may be posion to another animal. the pong pong fruit is poisonous to humans but im sure some other animals do eat it, quite satisfyingly. no plant can be poisonous to every predator. being poisonous to some may by default make it edible to others. why? different species have different physiology, and are harmed / helped by different chemicals. secondly, to a plant, producing a poison incurs a cost. if you invest in being defensive using poison you lose out on being able to grow and reproduce. if your growth and reproduction were hindered by predators constantly eating your leaves / flowers / fruits etc, then perhaps it would make more sense for you to invest in being poisonous. yet if you invest in being more poisonous you may very well give up the opportunity of growth and reproduction. There is always a cost / benefit tug of war. evolution favours the more efficient outcome, which may not always involve becoming poisonous.
