Sexual Selection, Stasis, Runaway Selection, Dimorphism, & Human Evolution

hehI saw that as I was signing off again.answers (a little short as my brain is already in bed)1 - vary significantly against what background? we now have clothes that hide this feature and the possibility that we have already reached the extreme possible. see article on male hair pattern (in women) cited above for some evidence of this still operating. and current mating patterns are not necessarily ones in practice when the issue was selectively involved.2 - a male could also be dominant in a group and choose who to mate with -- look at the difference in "age" of genetic "adam" versus genetic "eve" -- while restricting other males activities. his dominance gives his offspring advantage.more tommorrow. must. sleep.

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we are limited in our ability to understand
by our ability to understand RebelAAmerican.Zen[Deist
{{{Buddha walks off laughing with joy}}}

EZ:I am further answering your last post with an answer to this one, as the concept is linked: you quote the Globe and Mail article with: Let me augment that with a larger quote: Now the question arises, is female bareness similar. your last post asks And I would say that we have the same kind of evidence as provided for the mosquitofish, and it is readily available to anyone on the internet (well anyone without netnanny ...)I submit to you that internet porn demonstrates extreme and continued attraction to and selection for bare females, exactly like that displayed by the female mosquitofish to the fish porn movies.That even if you google "hirsute women pictures" that what you will get is not sites of women with hairy chests, beards, backs and the like, but just pictures of currently "normal" bare women that are not shaved.And most porn sites show unnaturally bare women, just as ads and magazines promote the extra baring of the female body with creams, surgical procedures and thousands of kinds of razors to remove "unwanted" hair. From this article in USA today: I also refer you (again) to the medical "condition" called Masculine Hair Distribution (in a female): I would say this selection is so prevalent that even women are actively engaged in it (as well as men) in order to attract mates, and that they seek to {remove\suppress} their body hair growth to an unnatural level (and get depressed if they cannot).Remember that one of the criteria is that features selected for have reached a point where they can not occur normally: even longer tail feathers, even bigger gonopodium, even more babyfaced-ness, etc.And I would close by saying that the variance of hairiness in the population is not from hirsute male type hairiness, but from almost bare to nearly almost completely bare appearance, so the female population is already highly skewed.What say?

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we are limited in our ability to understand
by our ability to understand RebelAAmerican.Zen[Deist
{{{Buddha walks off laughing with joy}}}

I would say that I agree with pretty well all your observations and the correlations you have drawn among them.However, I might explain them a little differently.No doubt that hairlessness is a sexually desirable trait in the human female and that it is pretty much at the extreme level possible. But I would argue that the use of clothing has diminished what would otherwise be significant dis-advantages (in terms of thermal insulation) under natural selection.
This wopuld not constitue selection for hairlessness, but it would relax selection against it in both sexes equally.To invoke runaway SS as a cause for the high level of the trait, it is not sufficient to demonstrate the the trait is a desirable feature of the opposite sex.
One must show that it is directly linked to increased reproductive success in the sex that is the subject of mate choice.
In other words, you would need to show that hairy women *leave fewer offspring* than hairless women, and that hairless women achieve higher fitness through the increased fecundity of their hairless daughters.This might be difficult for two reasons.
First, almost all women get mated and leave some offspring, no matter how hairy or how ugly, unlike males for whom variation of reproductive success can be considerable (in polygynous systems, at least, some get no matings, others get large numbers).
Secondly, there is not a lot of quantitative variation in RS among human females, at least not compared to fish or insects. Each usually leaves only 5-10 progeny max, so its not a lot of variation to work with.
So it is MUCH harder to get the model to work for male choice than for female choice.But I won't argue that it can't work.
Rather I will hypothesize a scenario in which it could.Your best evidence is the dimorphism between male and female in this regard.
It can only be explained by some selective force acting in a sex-specific manner.Since we agree the trait has been decoupled from survival criteria, we are inclined to exclude NS as a cause for the dimorphism.
The problem becomes how to formulate the reproductive advantages when there is very little *quantitative* variation in female Rep. Success.I propose that the answer is to infer *qualitative* differences in reproductive success that are conceivable in a monogamous mating system where both parents share parental repsonsibility for ensuring offspring survival. This type of mating system has been prevalent in many human societies for a long time, I suspect.So here is how it could work.
In a monogamous mating system, there is opportunity for both sexes to be choosy, since both make an almost equivalent investment in reproduction.
Hairlessness in females attracts males of 'high quality' (females are free to select males on the basis of other criteria such as size strength etc. - doesn't matter for the females trait) so they get paired up with better quality mates than hairy females and this then improves the survival probability of their offspring, rather than their number, because they have better quality fathers contributing to their care.
The hairless females then experience improved RS in terms of *fertility* (offspring quality) rather than fecundity (offspring number) and their choosey mates benefit via the increased RS of their hairless daughters. THEN you have a runaway effect, so that very soon all males are choosing females based on hairlessness and all females are virtually hairless in the extreme.So I am now convinced that it can work, provided the following conditions are met.1. A monogamous mating system.
(a pre-requiste for any male choice model, I would contend)2. Quality of male parental care is important for offspring survival.3. Female fertility varies significantly as a function of the quality of parental care contributed by her mate.Lots of fun, but now I have to do some real work

I'm not convinced that strict monogamy is (1) all that traditional and (b) required. Polygamy with more successful males being able to choose mates and more of them will also work, allowing more of his genes to be dispersed into the {more select} females thus spreading both the (xx) bareness genes and the (xy) preference for them. This also leaves the {less select} females to be divided between the remaining males with a more random result. Trickle down theory?Certainly before 'marriage' was {codified}, behavior along these lines could reasonably be expected: we see similar behavior in chimpanzees (Pan troglodyte, Bonobos behavior is ... special ...), and it is rampant in Gorillas: but where the dominant male mates with (virtually) all females there is not much benefit to more attractiveness.There is probably a small range of group dynamics that would allow such sexual selection, but one that includes known human behavior (consider successful men and attractive women and the propensity for human "affairs").(2,3) I also think all the male needs to have done is provided a protected environment for his offspring and mate(s) and a ready supply quality food stuffs, to ensure their health and survival. But I think you are "right on" with the quality over quantity argument, especially in a species with limited offspring compared to the more fecund species (ie - the "choice" for quality is already selected).Thanks for your insights.

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we are limited in our ability to understand
by our ability to understand RebelAAmerican.Zen[Deist
{{{Buddha walks off laughing with joy}}}

only works for female choice in the Fisherian model You lost me. I won't say no because I don't understand yet,
(you already caught me that way)
but if so, it wouldn't follow the Fisherian preference-choice linkage model. Not hard to imagine significant variation in this behavior This message has been edited by EZscience, 05-14-2005 08:01 PM

ps - did you see this article?http://biology.plosjournals.org/perlserv/?request=get-doc...some evidence of sexual selection in males.

Let me digest this and respond over the weekend.

good. then you can help me understand why they say and only talk about male fixing ... when they are also talking about the X chromosome (not the Y)? (it wouldn't have anything to do with 11 male and 2 female authors ...)

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we are limited in our ability to understand
by our ability to understand RebelAAmerican.Zen[Deist
{{{Buddha walks off laughing with joy}}}

from
EvC Forum: Trait changes in a species. Not necessarily, hair thinning could have occured earlier. One thing to note is that the eccrine glands are distributed over the body with the same pattern and density in Gorillas, Chimpanzees and Humans (in these two apes they are (possibly) linked with sexual pheromones). Only in Humans have these glands developed into sweat glands (necessary for overheat regulation). The eccrine glands on other apes are limited to the pads of {hand\feet}.

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we are limited in our ability to understand
by our ability to understand RebelAAmerican.Zen[Deist
{{{Buddha walks off laughing with joy}}}

It has to do with hemizygosity of males.
Since males have only one 'X', a faulty allele with important function will be quickly eliminated, recessive or not.The X chromasome carries many important functional genes important for both sexes (the Y has very few), but because males have no 'backup' X chromasome, the genes undergo very strong selection when they occur in the male genotype, which is normally 50% of the time.It is interesting that in the haplo-diploid sex determination of the parasitic hymenoptera (called 'arrhenotoky'), deleterious recessive alleles are extremely rare on ALL chormasomes. This is because males have only a single copy of ALL genes, not just one chromasome. Deleterious recessives cannot survive if they are non-functional because they are immediately eliminated whenever they occur in the male form. This makes problems with 'inbreeding depression' virtually non-existent in these animals - there is no genetic load of deleterious recessive alleles to cause these problems. As a result, you have gregarious parasitoids where sib-mating is the norm, rather than the exception, and Hamiltonian sex ratios can evolve (highly female biased) when sib-mating becomes the norm.More later...This message has been edited by EZscience, 05-27-2005 10:49 AM

So, the sweat glands evolved as the hair became thinner? Do you suppose that they are connected? Perhaps some transitional apes existed that had thinner hair and eccrine glands that produced more than just pheromones. Maybe it was a very gradual thinning of the hair rather than a mutational leap to hairlessness. When I saw that hairless chimp at the zoo it made me think it was possible that a genetic mutaion could jump right from hairy to hairless, and possibly change that trait of the species if the mutation stuck.

I'll get into this in greater detail later (I hope), but I think it (reduced hair) had to be spread over several generations as a minimum.The problem with the hairless chimp is that it would likely have a very low survival rate in the wild (as opposed to a {temperature\climate} controlled enclosure) because the sudden loss of hair presents two simultaneous problems:Too much exertion on a windless hot sunny day: in the absence of any mechanism to {shield\protect\divest} a body of excess heat, hyperthermia becomes a foregone conclusion, and likely to be fatal if no counteraction is taken. This happens to humans who lose the ability to sweat (or have maxed out their sweat-ability). See Hyperthermia - Wikipedia: On the opposite side is the problem of surviving cold windy rainy nights, with activity at a minimum, for then hypothermia becomes a foregone conclusion. This happens to humans that get wet when it is cool and windy. See Hypothermia - Wikipedia: If you've ever had your skin turn blue with goose-bumps and your teeth chatter when swimming on a summer day, you've experienced hypothermia (and "goose-bumps" are the retained muscle reaction to cold which made the body hairs stand on end to increase insulation value).From 90 F to 104F is a pretty narrow window for acceptable body temperatures, and to expect any natural (uncontrolled) environment to always remain within those parameters for a body unable to adjust to changing conditions is very shortsighted. And this true for humans that evolved to have finer hair than all other apes at some distant pre-historic, pre-human art recorded time (if not a time millions of years ago).The Serengeti Plain in Tanzania, home to Olduvai Gorge, is thought to be one of the ranges of early {hominids\humans}, and from Serengeti: That's 60F to 75 (F59F to 77F), and the "rainy season" lasts for months.There are some things you can do to stave off the effects of {overheating\underheating}, but it would be virtually impossible to maintain them for months on end.This makes a sudden change (by mutation or {medical\environmental} aberration) to a {thin\short} hair condition highly unlikely to survive long enough to breed a second generation without intervention of some kind.I would not be surprised to find that the medical condition of the hairless chimpanzee is not any more uncommon than the condition is in humans, where it occurs in: But that it has not been observed before due to the poor individual survival prospects.Enjoy.

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we are limited in our ability to understand
by our ability to understand RebelAAmerican.Zen[Deist
{{{Buddha walks off laughing with joy}}}

I'm sure I get what your saying. A jump from hairy to hariless would most probably not work, because it isn't likely to be survived through the environmental changes that would be expected. And, it seems that the amount of hair a species has isn't portrayed through their fossilized record, so, perhaps we will never know.One thing I would like to point out is: bold added for emphasisbut, a lot of the evolutionary advances that I've learned about seem highly unlikely, so perhaps we can't rule this possibility out all toghether. I understand what you are saying though, and it seems that a gradual change is much more probable.

No highly unlikely or improbable event can be ruled out, especially where the evidence is thin. Like you say, {hair\fur} does not usually fossilize: but occasionally it does and we may get a surprise when that happens.But in the case of hairless (which we aren't) apes evolving from a single individual, that individual has to survive from birth the breeding: one day of {fatal\extreme} enough weather in 10-11 years is within the experience of most people (check the coldest day on record, the hottest day on record, in the last 10 years).Not to belabor the point, but those temps before were the means. From Page not found - Kabiza Wilderness Safaris and those are still not record 10 year extremes.The essential difference between humans and the hairless chimpanzee however, is that we are not bare, and that in fact we have virtually the same numbers of hairs as chimpanzees. As noted in the Message 41The difference is not in the numbers of hairs but in the {types\length\diameter} of hairs. And we are not the only apes with thinly haired areas:We just have taken the {apparent bareness} to the greatest extreme (see reference to run-away sexual selection in the original post)EnjoyEdited by RAZD, : update picture links and sig

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Thanks, that makes sense. The other positively selected mutations on the X gene though -- they don't talk to much about them, would they likely be {ones\areas} more related to females?I also found this interesting (same article): It would seem possible from this kind of specific selection to diverge a species with very little difference in overall genetics. One would expect a high correlation in just diverged species on all but genes like this one.

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we are limited in our ability to understand
by our ability to understand RebelAAmerican.Zen[Deist
{{{Buddha walks off laughing with joy}}}