I’m wondering about allergies here. Why would evolution make something as innocuous as a strawberry or peanut potentially fatal to human beings who have an allergy to them?
Evolution doesn’t make anything do anything. There are lots of things that are fatal to humans. Evolution has allowed us to build up defenses to longstanding widespread issues like disease because those who had better defenses survived longer and reproduced more. I’d WAG that fatal food allergies are not widespread enough to have selected out of the gene pool. Darwin’s Finch should be along shortly with the real answer.
I thought that allergies of that sort were caused by the body’s immune system overreacting. That is, usually this reaction helps the body fight something off that really is bad for it; but in this case, it’s reacting to something that isn’t bad by itself, so that the allergic reaction is the only bad thing. Perhaps having an aggressive allergy system is good for the survival of the organism most of the time, compared to the rare instances where it goes off and isn’t needed.
Whilst writing this I knew someone would not like the “Evolution Making Something” rhetoric…my bad. I am asking why certain humans have developed allergies to seemingly innocuous foods like strawberries, peanuts, wheat etc…etc…What role does selecting out of gene pools play in this scenerio?
If I may expand on the OP a bit; it’s not just the dramatically fatal allergic reactions that are of interest here but the more ordinary stuff like cedar fever that would be fatal to many under harsher living conditions.
Many mechanisms can influence the kind of severe reactions that can kill, but here’s a $0.02 tour of antibody-mediated immunity:
When you’re born, your immune system doesn’t know what it will be called upon to defend against. It can’t know, because ideally we’d want it to defend against germs/viruses that don’t even exist yet. Not only are the enemies constantly changing, but some of them -purely by luck- will ‘look’ similar to things you don’t want to react to (for example, rheumatic fever isn’t a terribly serious condition, if treated promptly, but the shape of one protein in the bacteria resembles a protein in your heart, so some rheumatic fever antibodies can slowly attack the host’s own heart tissues, causing heart and valve problems later)
Your immune system gears up to battle the infinite unknown by having each antibody (Ab) producing cell randomly reassort some of its antibody genes when you are a fetus (even identical twins don’t have identical antibodies), to produce hundreds of thousands of random antibodies. Each Ab-producing cell produces one kind of random Ab. It doesn’t know what its Ab will react to, and some mayl react to things in your own body. [This is called “generating antibody diversity”]
In the relative sanctity of the womb, nearly everything your antibodies will encounter is safe. There is a ‘delicate period’ in fetal development when immune cells aren’t “ready” to be stimulated. If any of your Abs happens to bind something in your body, it will ‘report’ the presence of ‘unknown enemies’ to the cell that produced it, and that cell will be stimulated to multiply and produce and army of antibody factories producing that particular activated antibody.
Alas, that ‘baby’ cell isn’t ready to for that kind of heavy lifting, so when it revs up its engines, it throws a piston rod and dies. By the time you are born, all the cells that produce Abs that react well to the myriad compounds in your body should be dead, and all remaining Abs will react to “foreign antigens”. This is called “self tolerance” meaning your body tolerates itself immunologically.
Some special organs, like the brain and testicles, are shielded from antibodies (and most other things) during development. Autoimmune neurological diseases can occur, if the Blood-Brain Barrier breaks down later in life A vasectomy may allow sperm ,which your immune system has never ‘seen’ before, to be exposed to Abs and cause a reaction (one reason vasectomies aren’t always reversible)
Since every person has different antibodies, we can be reasonably certain that a certain percentage of people will have antibodies against almost anything new that comes along. This is good for the species, but not necessarily ideal for each individual person. The same antibody that triggers you strawberry allergy might also react with some not-yet created protein found in the next Great Plague.
This is an incredible oversimplification of just one aspect of the immune system. Not all allergies or reactions use this mechanism. It’s just an illustration that may help explain how your body can react to things you wouldn’t want (or expect) it to react to, and why different people react differently.
As others have indicated, this is an accidental inappropriate reaction. The immune system is designed to protect you against disease organisms, but sometimes it can become sensitized to the wrong things. Overall, the immune system is so highly beneficial that it is selected for strongly, despite occasional malfunctions. Even though those with excessively sensitive immune systems may be selected against, there is even stronger selective pressure from disease organisms to to keep the immune system pretty sensitive.
KP has given a great overview of the immune system, which should give you some idea of why it’s so hard to select against allergies. One point I’ll add is that an overactive immune system or one that is active against inappropriate targets won’t be evolutionarily selected against unless it in some way interferes with reproductive fitness. People who develop severe food allergies are less likely to survive to adulthood and have children, so they are reporductively less fit, and the genes they carry will over time become a smaller percentage in the overall gene pool (they are being selected against). On the other hand, evolution can’t select against minor allergies which don’t interfere with reproductive fitness. Nature doesn’t care if you spend your entire life miserable, constantly sneezing, and with a runny nose, if the condition doesn’t prevent you from having lots of kids.
Humans have increased the diversity of their diets over thousands of years. As it gets more and more diverse, more and more people run into things that trigger allergies.
Think about the diversity of items available to you. Most animals have a bread ‘n’ butter diets.
Also, evolutionary processes are also such that bajillions of species go extinct. I’m taking this to an extreme, but the processes we’ve come to know as evolution are not processes that gaurantee endless success!
This relates to what I’ve been wondering about. Over the past few years, I’ve read numerous allergies about virulent peanut allergies. Schools are declaring “peanut-free” zones in their lunchrooms. Some kids are so allergic they’ll have a reaction if they just touch and item that has been touched by someone who has handled peanuts.
This wasn’t a problem when I was in school (20+ years ago). Are the cases of extreme allergies increasing? Or have we just gotten better at diagnosing these allergies? In other words, when I was a kid, were there the same numbers of highly allergic kids who just died really young?
I am sometimes suspicious that our over-reliance on antibiotics and obsession with disinfecting everything the children touch has prevented kids from developing the appropriate reation to allergens but I don’t’ have anything to back that up with…
Others have pretty much covered the “allergy” aspect (and probably better than I could have), but I do wish to point out that plants are not passive participants in the “struggle for existence”. They evolve defenses against being eaten, and it is the entire point, so to speak, that those who do eat them should get sick and/or die. At least, such is the ideal case, from the plant’s point of view.
Certainly, in many cases, some animal or another is necessary to help disperse a plant’s seeds, but often those same seeds can be hazardous to any animal which isn’t “designated” to be the disperser. Those who act to disperse the seeds evolve in tandem with those plants, evolving the necessary defenses against the plant’s defenses, and so on.
Of course, allergic reactions are not necessarily the fault, as it were, of the plant, and the above is not meant to explain why humans have allergic reactions to various foods in the first place. But it is a mistake to assume that plants are simply there to be eaten.
Note: that story is from 1993. I have no idea whether that idea has been supported or debunked since then, but on the face of it, it seems to back up the immune system gone amuck theory.
Some think there is a connection between food allergies and feeding children foods too young. (While their immune systems are still developing?) Perhaps in cultures where babies are fed only breastmilk for the first year or two there are fewer food allergies. I wonder what the rate of peanut allergies is in Thailand, land of delicious peanut sauce.
Yet another possibility is that we’ve gotten worse at diagnosing these allergies. I’m not saying that there are no people who are genuinely allergic to peanuts: There are. But many kids who think they’re allergic aren’t.
Imagine, for example, a kid who has a wheat allergy. He eats a peanut butter sandwich, and gets sick. His parents immediately conclude that he has a peanut allergy, so the next day they make him a bologna sandwich for lunch. He still gets sick. Now, the parents conclude that he’s so sensitive to peanuts that shaking hands with the kid sitting next to him (who had peanut butter) made him sick. So the next day, he gets bologna and needs to sit way off in the corner of the lunchroom by himself. When he still gets sick, it must be because peanut particles are escaping into the air, and those few airborne molecules are making him sick, and so on. Meanwhile, the poor kid is still getting bread every day, and nobody realizes that that’s the real problem.
Though my earlier post was a gross simplification of one type of immunity, it does explain why some allergies aren’t “selected against”.
You DON’T INHERIT antibodies from your parents. You inherit a ‘toolkit’ of parts. There is nothing wrong with the individual parts - in fact, each of them has been “selected for” from a vast pool of possibilities. Each individual makes his own random genes from this tinkertoy set. It is the complete randomly assembled antibodies, not the inherited segments that cause antibody-mediated allergies.
Recall that I noted that identical twins have very different (but compatible) Abs. Evolution can’t select for/against traits that aren’t passed to the next generation. (Even theories of social Darwinism, still rely on (cultural, not genetic) intergenerational transmission) If red sportscars are totalled sokmewhat more often more than other colors, they still won’t “die out” or be “selected against” because cars don’t pass their color on to their successors. Drivers will continue to order cars in various colors for their own reasons. In fact, they may feel the increased risk has a certain daredevil appeal.
A few common subunits are linked with an increased susceptibility to certain diseases (especially inflammatory or auto-immune conditions). It’s possible that they confer some benefits or possible combinations that are valuable enough to outweigh the risk. It’s also possible that these condition haven’t affected reproduction enough to cause a rapid elimination of the allelle (e.g. the disease may arise late in life, or may be infrequently triggered in certain populations, which serve as gene reservoirs: e.g. a pork allergy in Ashkenazi Jews)
Not all food or environmental reactions are antibody-mediated allergies. Plants and animals make protective irritants like urushiol (poison ivy and cashews plants), capsaicin (chiles) and bitter tasting substances. Other compounds short-circuit the immune response by (for example) directly triggering receptors on cells that release histamine (rather than binding antibodies, to cause histamine release or histocyte attack indirectly). Some immune reactions are hard-wired: chemicals found in/on Staph. aureus, a normal skin bacteria, trigger several direct blood responses, which can cause rapid shock after a large influx of S. aureus (e.g. Toxic Shock Syndrome associated with certain tampons and IUDs)
In the non-antibody cases above, a sensitivity may indeed be inherited (e.g. a gene for a specific receptor that triggers histamine release)
I’ve heard the proposed connections you cite, sugaree, but don’t know what additional data may be out there to support them. But this serves to bring up another important point about evolution - what qualifies as “fittest” depends on the specific enviroment the organism is living in. An individual who is “fittest” in an environment where parasitic infestations are endemic may be “unfit” and selected against when the environment changes and parasitism is no longer a significant problem. Likewise, a cultural shift away from prolonged breastfeeding (necessary in places where the typical diet is hard to chew) to earlier weaning (possible in a world of highly nutritious soft foods) may change which children are “fitter” than others. Traits which were beneficial to our ancestors for thousands of years may be deleterious today because we don’t live in the same environment they did - and conversely, traits which would have been lethal to them (such as type 1 diabetes) may not be selected against today. (Diabetes is much less problematic when injectable insulin and glucometers are part of the “environment”, and most people in the developed world with type 1 diabetes survive into middle age and bear several children, which is all evolution cares about).
I still think you are misunderstanding evolution. There is no “why”, other than that’s how DNA works. Mutations happen.
I can’t figure out what this means. Could you rephrase it more clearly? I get the feeling that this is at the root of what you are trying to figure out.
Potatoes are probably the salvation of the human race (at least as far as Europe goes). Since their introduction from Peru, the humble potato has fed the world’s poor -it is estimated that in pre-famine Ireland, the bulk of the population ate nothing but potatoes, milk and a little butter.
Anyway, the potato plant produces toxic alkaloids-these are concentrated under the skin (a “green” potato can contain enough toxic alkaloid tokill an infant). Why would a plant like this have developed such a level of toxicity? As far as I know, humans are the only species that eats potatoes…would the potatoe plant have evolved such a defense mechanism in the absence of human cultivation?
Fascinating stuff. So, is it theoretically possible for there to be a plague that wipes out everybody except people with severe peanut allergies?
Great, except we aren’t that stupid.
When we try to determine what someone’s allergic to, we give them pinpricks of various proteins. This allows the proteins to just barely enter the bloodstream, where the body can react or not as it will. Each protein has its own spot on the skin (or, rather, just beneath it), and we can tell what’s being reacted to by looking where the skin is being inflamed. An immune response is easy to spot if you know what you’re looking for.
So we’d prick the kid’s skin with peanut proteins, wheat proteins, and various other kinds of proteins (maybe it’s a mold in the air ducts, maybe it’s the new kid’s deodorant, etc.), and we’d wait and see.
I don’t know about peanut allergies (hence my questions) but sickle cell anemia is a mutation that protects against malaria. Here’s a shortarticle about it…