Generally our bodies are pretty good at letting us know what external, physical stimulus is good and bad for us. If you have a sprained ankle your body lets you know that you probably shouldn’t be walking on it. Got a bruise or a sunburn your body says not a good idea to touch these areas until they heal. In both cases some pain or discomfort is the deterrent your body uses. On the other hand our bodies also do a pretty good job of promoting positive physical stimulation, most (some? :dubious: ) of us should be aware of how our bodies respond/reward us with positive sexual stimulation. So why is it that our bodies developed such a positive response to scratching a rash/allergy? I’ve achieved near euphoric states of mind while scratching myself to the point of bleeding, not to mention whatever other negative side effects may result from the re-aggravating and further spreading of the rash. I understand that the rash is a bit different from the previously mentioned examples in that it has to do with your bodies dealing with some external element that it was exposed to and not a physical injury. But why didn’t we develop a negative response to something that if left alone would expedite the healing process.
My understanding is that scratching is nature way of removing irritant. Rash can be caused by toxic plant, for example. Scratching place that come in contact with it would remove juice or resin or that horrible tiny thorns or whatever it is. Ergo - scratching is good, unless you have antibacterial washes and concept of hygiene and running water and all that things that weren’t here when we evolved responses to stimuli.
That makes sense, so lets assume that for some reason i don’t have access to any creams or ointments that might help my poison oak (not the rash i currently have, mine is an allergy to neoprene.) go away. Would vigorously scratching the rash then make the rash go away quicker than if i just resisted temptation and left it alone.
Probably not, but it doesn’t have to. All it has to do is offer you a greater net genetic concentration in future offspring (through survival or reproduction) to be a feature that sticks around.
In other words, imagine two individuals – A does not scratch, B scratches when it itches. Chances of B getting an infection or a persistent skin problem from scratching poison ivy might be higher, but if there’s some deadly parasite or irritant that can be easily removed by scratching, B has a humongous evolutionary advantage. There’s just not enough selective pressure on humans to evolve specialization to scratch in general but not scratch things that are bad to scratch. In fact, even if most of things are bad to scratch there just has to be one thing present that you have to scratch, lest your balls fall off or you die – that’s enough to keep non-scratching individuals out of the gene pool.
Our bodies do what they do and if the results are sufficiently detrimental to our reproductive ability then the responsible traits are eventually evolved out. Otherwise, they stay.
It is also important to note that it is important that the total net effect on our reproductive ability was detrimental and the mechanism how has very little bearing on the results. For example if some gene mutation makes the female immune to a currently ongoing epidemic of viral reproductive cancer that makes them infertile, but as an unfortunate side effect makes them think that bleeding scabs are just damn sexy, then that will also produce selective pressure towards scratching in males. This selective pressure however will not do any good until at least somebody mutates some tendency towards scratching, and then he is in like Flynn 
.
Interesting that you chose poison oak for an example. Urushiol (the ‘active ingredient’ in poison oak) is an oddity in that only humans are affected by it. I’m not an evolutionary biologist, but from what I know it may have something to do with our advanced immune system. The reaction to urushiol is also different from some other allergies in the way it activates the immune system.
Another thing about urushiol is that there’s no way scratching will remove it - by the time the skin is irritated, usually a day or so after exposure, it’s bonded with the skin into a different compound. At that point it can only be waited out (there are drugs that reduce the reaction, though). Even before that, it’s notoriously difficult to clean off the skin, as water doesn’t always remove it.
Keep in mind that up until very recently, vermin like fleas and lice were a constant problem for people. Scratching where it itched probably evolved as a defense against these parasites.
The same could be said for scratching individuals staying in or out of the gene pool. If you happen to know that “one thing” that would keep the non-scratching individuals out then do tell, otherwise i see no reason (aside from the extra energy required) for us to not have specialized in scratching when we’ve managed to specialize in so many other situations in which we should or should not do something.
Interesting info, i’ve never actually gotten poison oak but i figured i’d use it as an example of something most of us have had and been told not to scratch.
True, but i’d say vermin still are a problem for us. Ticks (lyme disease), mosquitos (malaria), fleas (bubonic plague) not so much anymore but still recent in the grand scheme of things. Having been bitten by all of these not a single one itched at all (tick) or until it had already bitten me and would’ve been too late to prevent the potential transmission of a disease. Ticks which require a pretty long time to transmit (approx. 48 hrs. according to wiki ) are the ones that seem to go unnoticed the most. I do agree that the itch reflex can help prevent vermin problems but i think comparing it to an allergic reaction or infection may be apples and oranges. The tingling produced by a spider crawling on me seems unrelated to the tingling urge one gets to scratch the hell out of that chicken pock.
And if you want to talk about evolutionary advantages, the male/female that manages to scratch the chickenpox the least would seem to have the smaller chance of contracting a secondary bacterial infection because of fewer open sores. Which would increase their chances of living and reproducing in the future.
But they didn’t. That’s the thing about how human evolution that most people don’t get – none of it has a reason, it’s just random things that we get to keep because there isn’t a reason not to. Remember – any genetic property human beings have is a set of unrelated randomly occurring mutations that our ancestors had. In a way our ancestors evolved into us by doing nothing. You can go for an explanatory argument for why we kept a feature we have, or you can go for an explanatory argument for why we did not keep a feature we have evidence we had. An explanation for why we didn’t evolve something new is always the same combination of the two:
a) It hasn’t mutated in an individual.
b) If it did(or would) there wasn’t (wouldn’t be) a net reproductive benefit
The set of genes responsible for scratching (and whatever else they do, can’t forget that) must have a net survival/reproductive benefit because most if not all mammals scratch (so it’s an old set of genes) and we still have it (so it hasn’t been consistently selected against). Any further specialization, if it ever mutated didn’t prove to be all that better, or did not mutate to begin with. Since presumably some proto-mammals did not scratch, that was somehow detrimental to them. Hell, for all we know that same set of genes confers some immunity to some pathogen, or just makes mammals a lot sexier to other mammals.
To what extent are you saying that our evolution has no reason and is just random? In modern times i’d totally agree, our random mutations and genetic phenotype have much less influence on our evolution as a result of modern reproductive partners being chosen more on their aesthetic,financial,personal features/standings then their actual ability to successfully reproduce for the sake passing on genetic information to the next generation and maintaining a species. For example: Any hideous sickly female can not only make it to the age of reproduction but also become a desirable mate, with the assistance of money, modern medicine and plastic surgery. The idea of natural selection kind of fails in humans here because without all of the external influences she most likely would’ve never made it to a sexually reproductive age and even if she did she may have exhibited features that would’ve made males not want to mate with her. At one point however humans (or our ancestors) did abide by the classic rules of evolution in which a random mutation would occur and that mutation had the chance to be naturally selected for or against. Which brings us back to my question, what greater good was achieved by us scratching things that currently seem only detrimental to our overall ability to reproduce.
Why do people enjoy hot peppers? No animal will eat them-my dog will refuse them. Most humans react to hot peppers by spitting them out-yet, in time , many people develop a taste for them. Like bitter drinks 9hopped beers0-most children dislike them (they prefer sweet drinks like soda, lemonade, etc.). But adults like the bitter taste of these beers-why?
This itch did not suddenly develop after years of me having an allergic reaction and not the itch, nor do i see (although it may be possible) the urge to itch not coming around when the allergy pops up anytime in the future. Our tastes can evolve on a short term scale, one day i like sweet wine a few years later i like dry. Acquiring a taste for something seems a bit different from being genetically predisposed to reacting a certain way towards a stimulus.
Any feature not already present in some individual’s genotype can only occur through mutation (or gene splicing). This has always been the case. Evolution has always worked like this and it’s not going to get any more or less random until we start splicing genes into humans. At this point it will get less random.
But if aesthetics, finances and personal features determine choice of mate, then they ARE our actual ability to successfully reproduce unless you have proof that they are in no way influenced by genetics. They are not any different then factors that were selective pressures in the past – ability to chase down prey, nice boobs, upright posture, lungs or anything else that was selected for in the past. Just because it is happening to you and it is happening right now does not make it different. In a million years some animals might look at us in the fossil record and see the beginnings of the development of their amazing accounting abilities just like we look at the fossil records and go “Hey look, first placental mammal!”
The idea of natural selection cannot fail in any species that still exists and is capable of producing another fertile generation of any sort. There is no such thing as external influences. Everything human beings do is natural because, hey, we’re natural. We’re warm blooded placental mammals with our own quirky crap just like many other warm blooded placental mammals with THEIR own quirky crap. We seem to be dominating population-wise.
Evolution has no classic rules. Evolution is a descriptive concept. It describes what happens to a complex reproducing systems when errors or changes are introduced. It works equally well for animals as well as spreadsheets or rock erosion. Biological evolution is just the most complex and interesting example we have of the concept and the one we can relate to the most. Evolution is not a feature, but a term for natural tendency of reproducing things to adapt to each other and their environment. The reason this happens in the most general sense is 100% clear – it’s the same reason water fills whatever shape of a container you provide for it. There’s no hidden mechanism behind it – in fact there’s no mechanism of evolution other than imperfect reproduction and a changing environment.
A random mutation today will be naturally selected for or against (or ignored) just like it was a million years ago. Just because medicine, finances or cars get involved does not make it any less natural and it’s still the same damn evolution. We are able to specialize in society and become doctors and that’s just as much part of our environment as being chased down by giant cats is for an antelope.
There’s no greater good achieved by us scratching. Our concept of greater good is a concept that appeared much later than scratching did. Our concept of purpose appeared much later than scratching. Scratching, therefore, cannot have a purpose or meaning, only an explanation and a hypothesis of how it works and how it affects us and our ancestors. The only thing that resulted from us scratching so far that we can tell is that whatever mammals didn’t scratch in our line of ancestry seemed to have not fared as well as our direct ancestors did. Parasites is a good hypothesis as to what they succumbed to.
I am not sure your question is even consistent as it stands.
Are you asking why anything anywhere scratches?
Dogs scratch, do you think it would be more advantageous for them not to? Unless humans and dogs developed scratching independently in their ancestral lines it’s a reasonable guess that our common ancestor scratched as well.
At which point since or before then did you expect scratching to disappear, and due to what selective pressure?
Are you asking why scratching did not go away in humans after the advent of modern medicine? Because modern medicine is the adaptation we evolved that selected against people who scratched themselves raw but did not go to doctors 
As a side note, I made it all the way to post #7 before I had to scratch. Anyone else? 
All this talk of scratching aside for one moment, our bodies were not designed, they evolved. We have been trained over the years to think that there is a reason for everything, after listening to scientists say things like, “Giraffes evolved long necks to reach the tender leaves of the acacia tree,” or, “Pain is an evolved adaptation to cause us to move away from the source of an injury.” So now we think *everything * has an explanation, right from the Owner’s Manual of All Living Things.
Our bodies have many ways of reacting that don’t serve any particular purpose. Having a discoloration of the skin at the site of blunt trauma is just a consequence of an injury. You can’t find a rationalization for it. Not every single aspect of the human body has an explanation.