- On the face
Beards are much more popular in safer times than during wild times. When the European homicide rate was two orders of magnitude greater than now, anywhere from the late 14th C. through the early 16th, the aristocracy -- who committed a disproportionate share of all violent crime -- shaved their faces. Fast-forward to the Victorian era, when crime had plummeted so much, and suddenly men look like the wolfman. That's despite the cheaper cost of shaving, since the industrial revolution had already begun. Nowadays lots of guys have extensive facial hair, unlike the '60s through the '80s when at most the average guy -- not a hippie on the periphery -- might have a moustache or sideburns.
Their evolutionary purpose is serve as signals in male-male competition, not courtship of females. Just look at all the boys and men who adorn the walls of females with the greatest reproductive value, roughly ages 15 to 24 -- zero percent have a beard, or even a prominent moustache or goatee. Sideburns at most. This does not reflect female preference for boyish looks -- square-jawed heavy metal singers, athletes, and guy's-guy actors are almost entirely clean-shaven too.
A clean-shaven face shows how healthy the skin is and how symmetrical the face is, whereas a beard obscures both of those features.
Girls will allow a week of stubble, but that's about it. I had a week's worth of hair and that didn't deter a tiny teenage cutie pie from dancing close at '80s night and telling me at the end, ...and now i have to kiss you on the cheekkkk. Women's objections to facial hair are all based on horniness -- they want it to feel better when they kiss. They're more tolerant of beards in a long-term relationship because those are less based on sex, and she is glad that the guy is preventing his sex appeal by growing a beard, lest other females try to poach him or lest he think too much of his looks and make an attempt at cheating.
- On the body
Hair is designed to grow universally in only two places in adults -- under the arms and in the pubic area. Therefore hair anywhere else cannot be strictly better from the point-of-view of natural selection, or everyone would have hairy bodies. Body hair is probably a side-effect of a behavioral strategy that is constrained by a trade-off. Again it seems like it serves as a signal in male-male competition, not courtship of females, for the exact same reasons above about facial hair.
So the current vogue for shaving or mowing down your pubic hair and armpit hair is about as unnatural as you can imagine. (Shaving the chest, etc., doesn't look so weird, since a good fraction of males look like that naturally.)
You'd think this high supply of shaved-down-there reflects a high demand for it, but lots of things that guys and girls do with their appearance is more to stay in fashion than to attract the opposite sex. For example, girls keep their hair way too short, and will often chop a good deal off even after asking a bunch of guys for their opinion, all of whom always say "don't cut it." I know that most younger guys are grossed out by a girl with even semi-natural hair. (By the way, one of greatest English slang words is "furburger" -- with all those repeated "er" syllables, it just sounds funny.)
But do girls really desire a trimmed or shaven lower ab region on guys? Hard to tell from the pictures of guys they like, since those are never fully nude. A couple weeks ago at '80s night, a group of honey bunnies passing by as I was dancing asked me to pull up my shirt and show my belly (I think they did use the word "belly"). Although they began cheering, I'm not sure why -- the mere fact that I was doing it, the look of my stomach, or what. But I don't mess around with the hair around my belly button, so perhaps they'd been longing to see a guy with natural hair there (not that it's copious either).
- On the scalp
There are three very different features that serve as signals for three very different traits, but people tend to ignore some or conflate others. They are:
1) Pigmentation. This is the signal of aging: only if you're into mid-life and beyond do you show decreased pigmentation.
2) Length, fullness, and luster -- how big and shiny it is. This is a signal of current health: if you're sick, your hair stops growing, becomes limp, and dries out. If you're in good shape, it grows longer, fuller, and more lustrous. Current health reflects both your current nutrition, disease burden, etc., but also how good your genes are at protecting against environmental insults. Big, long, lubricated hair is a handicap since all those proteins in the hair and the fatty acids used to oil it could be used for more productive purposes than to make you look purty.
That's why males who specialize more in courtship-of-females than male-male competition tend to have longer hair -- if it were short, the female might unconsciously think he wasn't healthy enough to grow a mane of hair. To the extent that healthy hair is a signal for good genes, females will be impressed by it, whereas male competitors in a guys-only contest will not (they aren't after your good genes like a female on the prowl is). And in physical male-male contests, long hair may be too great of a handicap -- it can be yanked, get in your eyes, etc.
3) Borders within which hair grows. This is a signal of male mating strategy: males whose hair grows more or less fully over their scalp (regardless of length or pigmentation) are more oriented to short-term mating, while those who show baldness are more oriented to long-term mating. The evolutionary function of baldness deserves a post of its own, but a brief review of key facts can't hurt here.
First, only some human populations have a greater-than-zero prevalence of baldness, and even in those groups that do have baldness, only some fraction of men will get it. In contrast, all males in all groups grow old and get sick. Thus, baldness is not a signal of aging (that would be pigmentation -- my grandfather had a full head of hair until he died in his late 80s, although it was white). And also, baldness is not a sign of poor health -- when you get sick, it's not as though your hairline recedes back six inches, and when you recover it shoots back to where it was. Health is indicated by big-and-shiny.
So again, baldness must reflect some kind of behavioral strategy that is subject to trade-offs, or else all males would grow bald, or none would.
Basically, a balding hairline is a guy's honest signal to his long-term mate that he won't be running around with anyone in the future. He's going to settle down and stick with her. How does this work? In the future, when he will be tempted to cheat with a young babe, he will be unable to find any willing partners because younger girls just get weirded out by a bald hairline, especially if he's middle-aged or older. Thus, the balding guy is saying, "Honey dear, don't take my word for it -- do you really think some pretty young thing is going to want to sleep with me if I'm bald?" Unlike cheap talk, this involuntary loss of his hairline is a credible commitment to be with only her. A less-balding or not-at-all-balding guy cannot be counted on so easily.
Notice that because it does not signal poor health, his wife will not interpret his balding as portending ill health later on. And because it is not related to male-male competition, she won't interpret it as a future inability to earn a living and provide for the family. He'll be perfectly healthy and able to hold down a job -- it's just that he won't ever get to sleep around. Baldness is an honest signal of self-domestication.
That's why the European aristocracy used to cover up any baldness by using wigs -- they wanted to stay on the mating market forever -- while the Victorian era and beyond gave us high-status males, bourgeois this time, who were OK with a bald head.
And that's why Australian aborigines don't go bald -- they have a gerontacracy where elder males monopolize women and are on the mating market into old age. Victorian England and the Australian aborigines are just two examples from the entire spectrum, but the rest fills in this way too. Look even at a smaller scale -- far fewer Scottish and Irish men go bald compared to the English. And sure enough, the Celtic groups are more in the rambunctious, fun-loving dirty-old-man direction than the English are.
I think there's even a decade-level change where guys are more likely to go bald during falling-crime times, when they switch to a more long-term and monogamous style. Looking through pictures of guys in their 20s or 30s from the 1960s through the '80s, you hardly see the same level of receding hairlines and baldness that you do among guys of the '90s and especially the 2000s. True nutrition went down the tubes in the past 20 to 30 years, but again that doesn't affect the hairline so much as how long, full, and lustrous the hair is.
Females want it all in a man, but because of trade-offs they must settle for going after this type of guy for this purpose and that type of guy for that purpose. For example, they want a gentle guy as the father of their children, but a more macho guy for a fling. They don't mind, and may even prefer, a balding guy as their long-term husband. But again look at who they want a brief adventure with -- the lead singer of a rock band, an athlete, a dreamy actor, a powerful executive or politician, all have less of a receding hairline than guys who are more suited to being good reliable dads. Just look at the faculty page of any academic department, men who carry their baby in one of those chest-pouches, and so on.