Quizzing the Anonymous - Why are apples red?
Why are apples red?|
...In the big evolutionary picture of where the red pigment came from in the beginning, we are left to conjectural hypotheses. The seeds of the evolutionary ancestor were no doubt bird and mammal dispersed, so perhaps the red fruit was more visible and hence conferred an adaptive advantage.
Here is the problem with this popular way of reasoning: almost all mammals have dichroic vision, so they do not see a red apple as red, while the birds that do have color vision are not interested in fleshy fruit, but only seeds. There seems to be no adaptive advantage for an apple to be red.
The color itself derives from anthocyanin dyes that are responsible for red coloration in falling leaves, which is another mystery. From what little is known, the mechanism of coloration in apples is different from that in flowers. In the latter, the absorption of the anthocyanins is tuned by interaction with a flavonoid copigment. The apples tune their color by layering anthocyanin filled vacuoles with other pigmented organelles (such as plastids), so the approach is different from that in the flowers, it is more like in the leaves. Incidentally, one of the most popular theories about the leaves turning red in the fall is a warning signal (Hamilton's) hypothesis: the trees are warning aphids (which, like all other insects, have color vision) about their prowess to kill their eggs ("I am so healthy I can turn some of my sugars to pigment => I can also turn these sugars into poison for your progeny. Do not mess with me"). If apples are not red for mammals or birds, the logical conclusion is that either they must be red for humans (selectively breeding apples) or bugs. Since plenty of wild Malus fruit (such as crabapples) are red, humans are not the cause. It must be the bugs.
What can be the reason to attract insects? We do not enjoy maggots in our apples, but it is likely that mammals that fed on wild apples somewhere in the mountains of Central Asia not only didn't mind but, actually, valued the extra protein in their food. The plants can provide protein, of course, cereal grasses do, but protein synthesis takes energy and nitrogen, so the apples embarked on a better solution: providing cheap-to-make carbohydrates, attracting insects to infest the fruit, and then attracting their dispersing animals with a maggoty, protein-containing treat. Crabapples are the favorite food for many Asian moths
and crabapples would not be called crabapples for nothing: these are acidic, woody, and tart. The animals get interested in this kind of fallen fruit only when it begins to rot and the yamies get inside; most of these animals, like hedgehogs are omnivorous or preferentially insectivorous. I suspect that the apples are turning red to attract the insects infesting the ripe fruit, either before it falls or shortly thereafter.
There is certainly some connection between the redness and the insects, though its exact nature remains unknown. As recently as last week it was found that
...62.2% of Central Asian wild apple trees had leaves that turn red in the autumn, compared with just 2.8% for cultivated British apple trees. To test the idea that red leaves spell trouble for insects, in the fall of 2007, Archetti placed nesting aphids in red-leaved and green-leaved apple trees. The following spring, 60% of aphids nesting in green trees had survived, compared with only 29% of those in the red trees. The reason behind this disparity is unclear, but Archetti's and other studies suggest that the red leaves either have toxic chemical defenses or hold fewer nutrients for young aphids. http://sciencenow.sciencemag.org/cgi/content/full/2009/415/1
Why are the apples red?
Не знаю, но ёжик замечательный :)
|Date:||April 18th, 2009 07:36 am (UTC)|| |
Чтобы признак сохранялся и распространялся, он вовсе не обязан быть полезным - достаточно, чтоб он был безвредным.
It takes energy to make these anthocyanins; e.g., during the fall up to 15% of the energy stored through the summer in the sugars is spent on making the leaves red - and then waste these leaves. So it is hard to argue that it is "harmless". I think you are on a wrong track here. There is another possibility: some crabs have small fruit in bundles. These are of interest to birds. So the ancestral wild plant might've been red for this reason and the redness is vestigial. Note, however, that having maggots in a fruit would work well for the birds, too. It is not easy to be a tree. You have to figure out how to handle your animals without quite knowing about their existence. A bird is as abstract to an apple tree as G-d is to you. Theirs are subtle arts which we seldom appreciate.
|Date:||April 18th, 2009 08:50 am (UTC)|| |
While the majority of mammals are dichromats, could there be some particular apple-eating trichromat mammal in Central Asia (before humans)? For example, do any of Indian monkeys possess trichromatic vision and could their geographic range extend to Central Asia when the climate there was more humid?
Also, dichromats do not see the red color our way, but they may distinguish between red and yellow hues, albeit vaguely. What would be the prevalent apple color without this red pigment? Being red may still be more noticeable and so hold some level of advantage.
This article even suggests that dichromats may be better at spotting apples in a dense forest than trichromats:http://www3.interscience.wiley.com/journal/112192492/abstract?CRETRY=1&SRETRY=0
Some say that the genus originated in the Tertuary in southern China and spread through the belt of temperate forests, then the contact with the primates is possible. Others say it originated in the mountain ranges of Inner and Central Asia; then it is unlikely.
There are practically no animals picking ripe apples from a tree, because that is not needed, and so the skill in spotting the apples in the foliage is not needed, too. Apples drop down; all one has to do is to find a tree and enjoy oneself. The rotting apples smell for a mile around. Besides, many animals helping themselves to apples are nocturnal.
The color will be green, and many apples are green. Again, spotting an apple is not needed. Mammals do not go up the tree to eat apples - even primates! You know when it is ripe: when it drops down. Usually, color in fruit develops for berries that are plucked from a tree or consumed by birds.
|Date:||April 18th, 2009 09:38 am (UTC)|| |
This article suggests that the ability to distinguish color may not be that much worse in dichromats compared to trichromats:http://www.ncbi.nlm.nih.gov/pubmed/2336798
It is as I would expect based on understanding the physics of color vision.
Also, I do not understand your alternative explanation. All right, the red apples may be more attractive to insects and thus be more infested compared to the yellow or green ones. But if the mammals do not distinguish these hues, they just would not be able to find the more infested apples easily, even if you are right and they prefer their apples that way. Where is the advantage for the apples?
You should understand that the trichromatic vision in the New World monkeys are coupled to the X chromosome, and so it is present only in 2/3 of the females and in no males.
Scientific American just had a wonderful article on thathttp://www.sciam.com/article.cfm?id=evolution-of-primate-color-vision
However, the type of brain with advanced visual processing is present in all of them, as it has to serve a substantial subpopulation, hence the remarkable ability of the dichromates to bypass their limitations. There is no question that trichromate perceive colors better than the dichromates, especially the difference between red and green (ask anyone who is color blind).
The mammals prefer their apples to be infested and they do not care about the color, operating by smell. The color matters only for attracting the insects. Red apples attract more insects becoming more infested and attracting more animals. When apples are red, typically all of the apples on the tree are red; you do not have to look for one that is red. The trees might've developed this way of coloring their foliage red to combat pest infestations by warning insects. The genius of the apple was in turning this upside down: using exactly this approach for attracting the insects to infest its fruit in order to attract more Newtons.
|Date:||April 18th, 2009 07:36 pm (UTC)|| |
So, your argument is now that spotting first the best fruit does not hold a significant advantage for mammals. If true, it would strongly support your suggestion that the apples are red for the insects. However, it works both ways - it means that you challenge the accepted (?) view that the trichromatic vision in primates has evolved to help us spot the best fruit.
I am no expert on animal behavior, but I judge from my own experience. In an apple or peach orchard I would certainly try to spot the best ones, instinctively using my color vision to the fullest extent. Distinguishing red from green is only the crudest start, very subtle hue differences mean important differences in taste. The fruit is not all of one hue and the pattern is important too. There is the light and shade influence to take into account. My kids seem to do the same. You say that it is a late fashion, hungry forest animals would prefer ripe infested fruit. I find it hard to believe with no evidence.
I have talked to some color blind people, but the better way to understand it is to look at the sensitivity spectra of the cones and think in terms of color space. Of course, there are very different trichromatic hues that would look exactly the same for a particular dichromat. However, it does not automatically mean that dichromats are worse at pointing the right spot on the red-green linear hue scale. Their "green" versus "blue" cone sensitivity differential would be much weaker than our own "red" versus "green". But they may have a larger density of the color cones, enhancing it. It should probably depend on particular species. Of course, once a species develop trichromatic vision, it has a much larger potential for perfection.
I am not challenging this view, but, please, observe that apples do not grow for the primates. Primates forage in tropical forests, in the canopees, and so their skill is in plucking ripe fruit when it is still on a tree, so they do not have to go to the ground and expose themselves to the forest floor predators. For that you do need color perception -- and even that does not quite explain why the fruit is colored (unless one postulates coevolution, which is unlikely in the view that the primates seldom serve as seed dispersal animals).
You are describing your own behavior, but you just follow primate foraging strategy. Observing swine gorging on fallen apples in an orchard would be a better idea of animal behaviour that apples might use. In temperate forests where apples and crabs grow there is noone to climb up the tree, spot a red apple, and take it. You say, suppose it was not the case before? That is hard to believe, because wild apples (like wild citrus) are very tart, accumulating a lot of malate. The timing of the ripening and falling to the ground is matched. This suggests that the apple trees relied on dispersal by floor foragers.
BTW, there is another group of mammals other than primates which could've been involved: fruit bats. Despite being nocturnal, some Old World fruit bats have trichromatic vision and they would see red apples for what they are. If the ancestors of apples in southern China were targeting fruit bats, the red color can be explained without invoking the insect infestation.
I do not know the answer; these are my own speculations. I can not find anything shedding light on the question. Surprisingly there is very little, in general, on evolution of edible fruit or endozoochory.
|Date:||April 19th, 2009 07:33 am (UTC)|| |
All right, this is convincing, I have missed the distinction between the primates and the cute little hedgehogs or swine in your argument and needlessly rallied to defend my kin. I would not exclude "co-evolution" (apples reddening in a forest full of monkeys), but I have no support for this image, however charming.
Still, I do not understand your point in full. The apples are red to attract the insects and become infested, so that the hedgehogs or swine like them better when they fall down. But how do they know which ones are tastier and offer more of that extra protein snack inside? Not by the fruit color, right? Do the rotting infested red apples smell much differently from the not infested rotting green ones? I mean, for this strategy to be evolutionary advantageous, it would depend on the (smell?) advertising among the mammals no less crucially than on the color advertising among the insects you talk about. And the apple color evolution probably was incremental. I try to imagine a hedgehog smelling two apple trees, each of them a "mile" away. Which one will it go for? Can it smell which one has more maggots?
I am not sure that infested apples smell differently to these animals, though I cannot exclude that. What attracts animals is the smell of rotting apples. The apples are designed to produce it and our olfaction is designed to pick such smells. We do not pick the smell of malic acid particularly well, because appleas are not for us, but I can smell butyric acid from two rooms away (the smell of rancid butter). French cheeses smell of propionic acid. Rotting apples smell of malic acid and animals pick it in dense forests (wild apples do not grow in groves). The success of an apple tree in the woods is in attracting the animals by smell and them remembering the place and returning it, by association of this smell and food. If this association is strenghtened, so much the better. That is why apples start to smell only a few days after they drop. The fruit sugars begin to ferment and it becomes infested, if it is not already infested. It is not just maggots: there are slugs, etc. Mammals love that. So the selection is not in picking an infested apple but in picking a tree that provides the best quality meal and advertizes it most successfully. If red skin helps apple in either one of these tasks, that is enough. By attracting insects it can time the infestation. There is one more reason to desire it: when you eat an apple you do not it its core, because it is not palatable. But this is the whole game: to make you eat this part. The infestation may provide an additional incentive to consume this part of the fruit. A crabapple cannot afford depending on large animals only that eat the fruit whole. It is a very complex strategy taking every consideration into account. If you do not do that you will end up as an avocado: a fruit that no longer has anyone to disperse its huge seeds. Then there is indeed no other choice than attracting a certain kind of primates. You do not have to be red for that, though.
|Date:||April 19th, 2009 02:39 pm (UTC)|| |
So there may be no significant smell difference, but the hedgehog remembers that the apples under this tree were tastier the last time, perhaps last year, with more yummy maggots in it? And snails presumably do not care about the fruit color, so there should be no difference in snail count on red versus green apples.
Your additional argument is somehow easier for me to see. Does the apple seed require to be eaten first in order to grow into a tree? I mean, could it instead grow from the rotten core discarded by the animal after it has carried away the fruit and eaten its outer part?
It is very interesting about avocado (as is the rest of it).
Of course it can grow from the rotting core, but few animals carry apples around on their backs (hedgehogs do it only in children's books), so the apple seed needs to be eaten. That's why the seeds are so bitter, by the way: they should be mildly toxic, so animals do not get into the habit of eating the kernels, it also has laxatives in the skin. The approach is to be digested and ejested.
Avocado is not the only examplehttp://darwiniana.org/ghostsof.htm
there is a whole group of plants that interacted with the extinct megafauna.
|Date:||April 19th, 2009 04:01 pm (UTC)|| |
Sievers' apple of Tien Shan, the main ancestor of domestic apple, doesn't have red fruit - it turns pale yellow-green, almost white, upon ripening. Its major seed dispersers were probably wild ungulates and brown bears.
Birds (i. e. starlings and thrushes) eat Sievers' apple flesh a lot. Apparently among the wild species, only small-fruited ones turn red, probably because their fruit is small enough to be eaten whole by corvids, pheasants, etc.
Yellow is produced by the same mechanism as red, so that would be a small change; you do not need to develop new pigments, etc. There are plenty of red crabs, too. So you think that redness was to attract birds in a small-size ancestor. Possible, but that does not exclude the infestation idea.