Posted by admin | Posted in Plants | Posted on 28-07-2010
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.
You are correct that evolutionarily, it would be good for a plant to be inedible but produce edible fruit. However, evolution is still ongoing, and you must keep in mind that every animal eats other organisms to survive. Your question is somewhat similar to asking why pigs and cows are not poisonous meats. It is not that it makes evolutionary sense to be edible, but rather, these species have survived despite being edible.
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What a great question.
I’m not the best botanist, but I believe the answer is because not everything in nature is perfect. Look at the design of the human eye or back, for example. Both are "imperfect" and not the most efficient design. That is because natures modifies what already exists. It doesn’t start from a perfect design.
Some grass species, however, do benefit from grazing. Stimulates growth and grazing species leave waste behind as fertilizer.
as for other species, i would say that there has not been significant evolutionary pressure for them to evolve strong enough defenses against predation. remember, mutations leading to evolution are random and have to happen by accident and then be selected for. just because something would be beneficial to a species, it doesn’t mean it will just evolve because it is needed. that is a common misconception.
also, you note that some fruits are poisonous. you have to be careful, though, because you mean they are poisonous to humans. many berries that are poisonous to humans are not poisonous to birds – the birds eat them and disperse their seeds great distances.
i’m sorry i couldn’t be more specific, but that’s my best answer
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i’m a bio prof
This is a great question and I think there are many answers.
Carrots and turnips have huge edible root systems because they are domesticated. Of course their wild ancestors must have been somewhat edible as well, but there probably weren’t enough things eating them to exert enough evolutionary pressure on the plant to make it toxic. However, once we start to domesticate a plant (or an animal for that matter) becoming more edible is now it’s strongest trait, since that’s what we select for. Being tasty and nutritious is the best thing that ever happened to carrots in a Darwinian sense, since now we create crops thousands of times larger than any wild population. Of course, it’s still not very good for the individual carrot :p
For plants in general, it should be noted that the vast majority of plants -are- basically inedible to -us-. Almost every plant is going to be toxic to some animals and edible to others. I would be willing to wager that some animals can safely eat potato fruit. Remember that animals evolve too, and if their food source starts becoming toxic, they may well start evolving resistance. Even if the plant wins out, another animal with a different biology might come along and start happily eating the plant toxin and all. Basically it is a complex system in constant flux, and sometimes it will be to a plant’s advantage to be toxic much more so than other times.
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I am not an expert in botany but it seems that every living organism has a "best" growth environment. Some grow fine in a very restricted area, others appear to need a fresh influx of new nutrients and expulsion of wastes, specific weather and soil conditions, and some require other life forms to aid dispersal. If in your example, you allowed carrots to grow to maturity and allowed them to re-seed the area and did not interfere with them at all, would next years crop be as efficient, as healthy? How about over several years? In fact, might they become overgrown by a plant that favors the change caused by the carrots growth? In fact, a natural carrot might not look much like a grocery store carrot if allowed to grow on it’s own. Try planting carrots in hard soil and you’ll probably find a strange looking root system that you would not eat. If plants were not eaten by humans or domestic animals, humans would not as likely plant them and they would be part of survival of the fittest. If a plant kills the animal that eats it, the seeds may not get dispersed and again it becomes survival of the fittest. Also, while animals may eat some carrots from farmers fields, most carrots in nature probably do go to seed and get dispersed by wind or birds or rabbits which like the green part as well as the orange root.
Regarding the poisonous fruit that looks like an edible one, lets hope that animals use their power of observation and learn from seeing their peers poisoned. One possible observation, maybe those plants disperse adequately without the need for other helpers. If everything in life was crystal clear and we all sat around eating tasty fruits … never challenged … always satisfied … would we call that heaven? I suspect my kids would say it is "boring."
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plants didn’t evolve ‘edibility’
the organisms that feed on them evolved to be able to make food use of those plants
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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.
References :