Eye color is a good example of a neutral trait. (Or at most sexually selected.) There is no extra energy spent by the person who has blue eyes. However, the person who has butt hair has spent some (very small) amount of resources (minerals, energy, whatever) to actually grow those hairs. So if the hair makes no difference at all (I’m not saying this is the case, it’s just hypothetical), then he would have a net disadvantage compared to people without butt hair. Thus it would be linked to fitness and reproductive success, just to a very small degree.
How do you know this though? Maybe there is a large fitness advantage to having butt hair. Then your explanation is, if not wrong, then very incomplete.
Perhaps, but it seems it’s a small enough degree that there is no effective selection against it.
It’s possible. But I sure don’t see it.
So you said. That’s why I asked before how small a fitness advantage has to be, for there to be no “effective selection” against it?
I would guess that the effectiveness of selection of some traits would surprise you. For instance, the variant conferring adult lactose tolerance. Here your arguments seem to me equally plausible, especially that the variant would be tangled up with other traits. But the fact is that this variant was propagated in the population in just a few thousand years. The trait of butt hair, on the other hand, has had millions of years, and thus require a far weaker increase in fitness.
So you have thought about all possible factors that butt hair could have an influence on, and evaluated than none of them can confer a fitness advantage higher than the limit from my question above?
Also, I urge you again to consider that Cecil finds the possible advantages of the hymen large enough to think about in the same way.
I just noticed that Knorf had the exact same discussion in another thread. Why do we have pubic hair?
In general I think that anytime there is an evolutionary biology thread, there are several people who say something like: “What you should understand is that evolution is not guided, it doesn’t make sense to discuss what the purpose of traits are, or what advantages they can cause.”
This bother me because GD is supposed to be fighting ignorance, and we can’t even fight this recurring ignorance here.
Yep.
Not only waxed but bleached. 
Sorry, I didn’t read this closely enough. Talking about the advantages in traits, sure, nothing wrong with that. I certainly never said there was. What I object to, is the assumption that all traits must create a reproductive advantage or else they would (or will) disappear. That isn’t necessarily true.
You say that it’s possible that traits can exist even if they don’t have a reproductive advantage. That’s fine. That doesn’t mean that any specific trait necessarily exists without having a reproductive advantage. In general, if there is a trait that’s universally present, we can mostly assume that it has some selection acting on it, unless there is a very good reason to think that there isn’t. It’s pretty well-established within population genetics. Your “that’s really not how evolution works” on my mainstream description of evolution doesn’t indicate that you have a deeper knowledge of the field, enabling you to disagree with the standard school of thought.
Another question: If there was no selection at all on the location of body hair, why is it always in similar places, instead of varying like eye color?
mr. jp, the source of my objection was your statement of how “costs” of traits operate in populations. The specific way you expressed it, to support an argument that butt hair must convey reproductive advantage or it wouldn’t be there because of its cost, is not “standard school of thought” in the literature on evolution I have studied.
Absolutely, traits have “costs.” But cost is not a zero-sum game that determines singularly whether traits are retained or not. Traits are retained if they are passed on from generation to generation. If the cost of a trait is small enough, there may be no significant selection pressure against it; no organism is usually ever completely optimized for its environment. “The rule probably is that most species most of the time are not fully adapted to their environments, but are just a little better than their competitors for the time being.” (Quoting P. J. Darlington, Jr.) I am arguing that of course “cost” exists, but it is not in itself the singular determining factor in whether a trait is retained or not.
There is a continuum between “selected for” and “selected against.” Evolution is not a binary system of good traits/bad traits. I never said there is no possibility whatsoever of arguing for some (probably) very small reproductive advantage provided by butt hair. I also never said that the location of body was not at some point selected for among our ancestors. The question posed by the OP was “What biological purpose does it serve?” The answer is, maybe nothing; meaningful discussion of the reasons for the trait to exist have little to do with “purpose” and everything to do with “it exists because our ancestors had it.” But here I am, repeating myself.
As I see it, your notion of “cost” as a singular element of selection only makes sense if the existence of a trait is selected against because it takes something (resources or whatever) away from a trait that would convey a clear selective advantage. I accept of course that there is some indistinct and highly variable threshold for “cost” above which this clearly does happen. But at or below that threshold, it seems to me it is not meaningful to discuss traits that way. I still see no evidence that butt hair is above whatever that threshold is for humans. In other words, the resources required for humans to grow butt hair are trivial enough that there may be no significant selection pressure (vis-à-vis its “cost”) against it at all, whether or not that trait provides a reproductive advantage any longer. If you have a citation for the contrary, or a persuasive argument to show I’m wrong, let’s have it. But here I am, repeating myself again.
After an admittedly very informal perusal, the only places I can find that make as big deal as you about a notion of the singular cost of genetic traits in selective pressure on an organism are from creationist sources. (By the way, I checked the legit sources for that article, and they do not use “cost” or any other resource-limited language in the way you did, as a necessarily limited and singular arbiter of the transmission of traits. Suffice it say I also disagree with the conclusions in that creationist article.)
If you continue to assert that, for a trait to be perpetuated in a population, it therefore must convey reproductive advantage, I can only accept that this statement is true if it includes the caveat that the “advantage” of some traits is at best very trivial. The resources available to an organism often (maybe usually) support lots of sub-optimal, useless, or trivial traits in addition to the very necessary traits required for reproduction.
But you do agree that our other body hair was removed due to selective pressures, right? Or in other words, that removal of body hair can be said to serve som kind of “purpose”, which gives it an evolutionary advantage. I presume we are in agreement this far?
So, how come butt hair (and pubic hair, armpit hair) wasn’t selected away due to those same selective pressures?
Well, the dingleberries give a hint. A LOT of bugs will avoid those. At least ones that could harm you.
As for the OTHER reason, I can only give personal experience. It goes, again, back to bugs.
Such as ticks. The hairless or largely hairless guys I knew needed me to remove them, all too frequently, as they were out of reach.
I’ve felt them EVERY time, save one, where my rolled sleeves abraded the hair off, near the underside of my biceps. Over a period of over 27 years in the woods, I’ll say that THAT is damned good. How many times have I FELT a tick on me? Tens of thousands of times. LONG before they tried to plant themselves.
Why? They walked across a hair.
Downside? The crap louse. One that I’ve thankfully avoided over my half century of life. Lice of ANY type USE hair to nest in and leave eggs upon. Said eggs are called nits. Hence, the term nitpick, as the eggs are extremely minute.
So, hair and be warned of ticks and other insects or no hair and no lice?
I’ll risk the lice. They’re easily killed, by the most primitive means, even water…
Or grease.