I read someplace - Pinker? Gould? - a term that is used to describe features of an individual that have come down through the ages as inadvertent side-effects of other adaptations. In and of themselves, they are thought to confer no particular reproductive or survival benefits, but they also don’t work against those imperitives. They just show up. A sense of humor is one that I think was referred to that way. Language ability clearly has significant benefits regarding the passing of genes along. The assertion was that a sense of humor, an inadvertent adjunct of our ability to use language, while a lovely feature of individuals, doesn’t in and of itself provide evolutionary advantage. While one might argue that point, the example stands as one that illustrates what I’m referring to - a side-effect of some adaptation. My question is, what is that term that refers to those side-effects? I seem to think that the word has another use, that it wasn’t made up for this purpose, or maybe it was borrowed from another use an applied to this concept. Dopers - where did I see it and what is that term? xo, C.
You’re thinking of “spandrel”. It’s originally an architecture term, describing a similarly accidental feature. The details escape me, though.
Spandrel or exaptation. Gould (with Lewontin) came up with the former at least. As I understand, the former are side effects, the latter are where something was adapted for one purpose but it’s function changed. Like how it is surmised that wings started as thermoregulation aids but came to be used for flight.
Spandrel. Yes. Thank you. I’ll sleep better tonight.
The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme
Gould, S. J. And Lewontin, R. C. 1979. Proceedings Of The Royal Society of London, Series B 205 (1161): 581-598.
Zowie! With page numbers, yet. Some place, this SDMB.
One of these days I’m going to get around to writing up a Staff Report on a question sent in to the Mailbag by a writer who wanted to know the evolutionary purpose of the human butt-crack. This is of course a prime example of a spandrel: the actual adaptive feature is the buttocks, which developed to hold the torso erect when we became bipedal. The butt-crack is just a non-adaptive consequence of this.
On a molecular level, we talk about hitchhiking genes, or “selective sweeps”. This is when a beneficial mutation arises and swiftly spreads throughout the population. It spreads faster than recombination can separate the good mutation from the surrounding DNA, so neutral, or sometimes even mildly harmful alleles will spread along with it, just from being located close together on the chromosome. This can be spotted in population genetic data as a decrease in variation around the beneficial mutation.
I disagree.
There are many places on the human body where two muscles are juxtaposed but which are not divided in the way the buttocks are.
The reason for the crack is the necessity for providing a path to vent solid waste. Of course there are other ways we could have evolved to deal with that problem - that’s just the one that happened to ‘catch’.
Sorry, this makes no sense at all. Unless you defecate in a very unusual manner, waste is not vented along the length of the butt-crack. The upper part of the crack has nothing to do with defecation, it’s simply a consequence of the hypertrophy of the gluteus maximus muscles, which is due to bipedalism. This kind of “explanation” is a good example of the adaptationist viewpoint than Gould and Lewontin were arguing against.
Ah, yes, the “Taco Bell adaptation.”
This needs to happen sooner rather than later. Please!
I think you should take a crack at it!
One of the more interesting aspects is that bipedalism, and hence the butt-crack, long preceded the evolution of large brain size in the human lineage. So the butt-crack is a much more fundamental (so to speak) feature of what makes us human than our large brains.
I think you need to do a little better than that. 
Looking at animals in general and mammals in particular I’m not aware of any case where two large groups of muscles press up against each other in a more or less fixed configuration and the organism has evolved two protruding lumps that have remained separate and bifurcated by a skin covered crease whilst spending the majority of the time in contact with each other.
Gould and Lewontin argue against some instances of adpationism. They don’t throw the baby out with the bath water.
Without the need to maintain a path for solid waste the crack seems to be an odd way for that area of the body to have evolved. It’s unnecessarily complicated when a smooth valley would be a more usual way for such a configuration to evolve.
Of course, I cannot say for certain why any particular such evolutionary path was taken. But then again, neither can you.
As I already pointed out, unless one is in the habit of defecating while standing on one’s head, the butt-crack does not provide a path for solid waste. And even if it did, because it is so narrow you would just end up with a butt-crack filled with waste, which hardly seems to me to be an evolutionary advantage (as well as being something of a social liability in most circles).
I can say with some confidence that, whatever the origin of the butt-crack, it is not as a conduit for solid waste. Your “explanation” makes no sense even in adaptationist terms.
Congratulations!
You have just failed the Turing test. 
As anyone who has actually defecated is well aware, the crack allows the buttocks to separate so that they do not get smeared with waste solids. The crack is probably the simplest evolutionary path that allowed the buttocks separate whilst we squatted thus aiding basic hygiene. Had we not needed that separation we would almost certainly have evolved a simpler shape for that area.
You are arguing Colibri’s point. If it wasn’t for the anatomical accident that the anus was located located right at the point where big leg muscles were required, there would be no danger of smearing feces on those muscles and there would be no crack. If the anus had been at, say, the small of the back, our cracks would probably be just a shallow dent between the butt muscles.
No, I’m arguing mine. 
You seem to be in complete agreement with me - at least according to what I’ve quoted below.
Colibri, on the other hand said: “Sorry, this makes no sense at all.”
Yes, that’s what I said.
Damn! Missed the edit window.
Edit/addition for above post: Hold on a second.
Would I be correct in thinking that the objection to what I said was because you/Colibri thought that I was suggesting that the crack appeared - was tunneled out of - something else?
If that’s the case I can well see why it was deemed senseless. That wasn’t what I was intending to say, though. When I said ‘necessity to provide a path’ I intended that to imply: ‘necessity not to block the path’.