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Author Topic:   Sexual Selection, Stasis, Runaway Selection, Dimorphism, & Human Evolution
EZscience
Member (Idle past 5154 days)
Posts: 961
From: A wheatfield in Kansas
Joined: 04-14-2005


Message 55 of 131 (211785)
05-27-2005 11:47 AM
Reply to: Message 53 by RAZD
05-27-2005 7:27 AM


Re: Human female hairlessness - a function of male choice ?
RazD writes:
only talk about male fixing ... when they are also talking about the X chromosome (not the Y)?
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

This message is a reply to:
 Message 53 by RAZD, posted 05-27-2005 7:27 AM RAZD has replied

Replies to this message:
 Message 60 by RAZD, posted 05-28-2005 10:31 AM EZscience has replied

  
EZscience
Member (Idle past 5154 days)
Posts: 961
From: A wheatfield in Kansas
Joined: 04-14-2005


Message 61 of 131 (212647)
05-30-2005 1:28 PM
Reply to: Message 60 by RAZD
05-28-2005 10:31 AM


Selected genes and polymorphisms.
RazD writes:
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?
Not necessarily. Remember, many genes on the 'X' chromasome serve important functions in both sexes, and many genes for 'female' traits are located on other chromasomes - there are just not expressed to the same degree in males. I like to think of it this way. ALL genes have to survive in BOTH sexes (in an obligately sexual population), and most serve some function in both. The differences in 'gender' that seem so pronounced to us in the final phenotype are mostly just the result of subtle changes in degree of expression of the same genes at particular times in development. I expect Wounded King could elaborate further on this if he were around.
With regard to the apparent deficiency of polymorphisms observed at some loci, the observation is significant because of the number of apparent changes (base substitutions) that have occurred in these loci. Lack of polymorphism implies that, of many possible versions of the gene that did occur, only one was 'tolerable' for that lineage. Strong evidence for selection driving the fixation, otherwise where are all the other variants of the gene that had to come into existence at some point?

This message is a reply to:
 Message 60 by RAZD, posted 05-28-2005 10:31 AM RAZD has replied

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EZscience
Member (Idle past 5154 days)
Posts: 961
From: A wheatfield in Kansas
Joined: 04-14-2005


Message 64 of 131 (212781)
05-31-2005 7:16 AM
Reply to: Message 63 by sfs
05-30-2005 10:40 PM


Re: Selected genes and polymorphisms.
I stand corrected.
I was unfamiliar with the term 'selective sweep'.

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EZscience
Member (Idle past 5154 days)
Posts: 961
From: A wheatfield in Kansas
Joined: 04-14-2005


Message 67 of 131 (213690)
06-02-2005 10:59 PM
Reply to: Message 65 by RAZD
06-02-2005 7:47 AM


Re: ADDENDUM II
RazD writes:
The likelihood is that certain areas (breast and buttocks?) were selected for {apparent bareness}, with the rest remaining as before.
So maybe there were advantages under SS for bare breasts and buttocks to be used in sexual signaling and communication of receptivity in females ?
RazD writes:
The next question is how did the mechanism of sweating evolve, and especially whether any mutation is required to enable it.
I follow your logic so far. Interesting that pigs are virtually bare and yet do not sweat. They are forced to compensate behaviorally by finding a mud hole to wallow in when it is hot.
So here's my shot regarding conditions for evolution of sweat glands in humans:
1. low relative humidity - no evaporative cooling takes place if the air is saturated with water (I've lived in Florida)
2. good average airflow over the body (more likely in a plains environment than in a forest)
3. lack of beahvioral compensatory mechanisms (like the pig). Once again points to an arid environment with little shade and water holes few and far between.
4. potential advantages of sweat glands as sexual signalling devices via olfaction (Ummmmm.... baby you smell gooooood).
...just off the top of my head.

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 Message 65 by RAZD, posted 06-02-2005 7:47 AM RAZD has replied

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EZscience
Member (Idle past 5154 days)
Posts: 961
From: A wheatfield in Kansas
Joined: 04-14-2005


Message 73 of 131 (221272)
07-01-2005 9:39 PM
Reply to: Message 72 by RAZD
06-30-2005 9:49 PM


Re: last addendum
Yes, that would confirm my own (limited) experiences with female preference. They don't like back hair or unibrows. But what is the basis for this choice criterium? Arbitrary, or some link to a natural selection advantage? Looking across the range of traits apparently selected by female choice, most are very arbitrary (e.g. tail length, bright coloration) with little adaptive value. On the other hand, a man choosing a women based on hairlessness seems plausibly linked to natural selection. Hairlessness correlates with youth and hence an extended period of residual female fertility. And yet in males, hairiness is a sign of physical maturity that is generally deemed desirable by females, older males being of proven genotypic fitness. Is the female choice component of SS generally indicative of a tendency among females to choose traits that run counter to what is adaptive under natural selection? Could this explain many of our (men's) frustrating experiences trying to impress women with behaviors we would logically assume them to construe as adapative and, therefore desirable, only to be passed over for Joe Hot-rod with his souped-up trans-am and shoe-size IQ?

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EZscience
Member (Idle past 5154 days)
Posts: 961
From: A wheatfield in Kansas
Joined: 04-14-2005


Message 80 of 131 (311009)
05-11-2006 10:47 AM
Reply to: Message 79 by Brad McFall
05-11-2006 10:37 AM


Re: the difference between male combat and female choice
Good find Brad.
I am going to have to spend some time on this and put together a cogent analysis once I get the time to read everything you linked - hopefully this weekend.
I once did an independent studies project on Sexual Selection back in the 70's and it still tickles my interest.
EZ

This message is a reply to:
 Message 79 by Brad McFall, posted 05-11-2006 10:37 AM Brad McFall has replied

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EZscience
Member (Idle past 5154 days)
Posts: 961
From: A wheatfield in Kansas
Joined: 04-14-2005


Message 83 of 131 (311386)
05-12-2006 11:00 AM
Reply to: Message 79 by Brad McFall
05-11-2006 10:37 AM


A strongly biased attack on Sexual Selection theory
Regarding this book by Joan Roughgarden, Evolution's Rainbow.
I think it falls very short of a credible attack on sexual selection theory.
Does sex role reversal occur in the animal kingdom? Of course, but it has specific ecological correlates that result in a reversal of the asymmetries between male and female. Females become larger than males and take on the aggressive behaviors. Males become specialized in parental care. We see this in the polyandrous jacanas and pharalopes. This does not mean that the ”normal’ asymmetries reflected in conventional sex roles are not important forces in shaping male and female behaviors.
Is the term ”gender’ useful or merely anthropomorphic as the author claims? I say the former. It is valuable to distinguish between ”sex’, meaning ”gender alignment’ and ”sex’ meaning meiosis plus recombination followed by fertilization. Gender has consistent morphological and phsyiological implications across taxa. It is not a useless anthropocentric term as the author wishes to contend.
Are same-gender sexual interactions ”normal’ in many animals? Yes. Does this type of interaction have social functions and value in reducing aggressive interactions? Yes. Does that in any way negate the validity of sexual selection as a powerful force shaping behavior and physiology? I think not.
The asymmetries between the sexes, starting with the differential investment in gametes (anisogamy), and ending with differential investment in parental care, create many incontrovertable conflicts between the sexes that are resolved in different ways in different species. The author attempts to dismiss the significance of these differences in a very unconvincing manner.
Why did internal fertilization evolve? To give the female greater control over the fertilization process.
Why are >90% birds monogamous?
Because of the energetic constraints of bird parental care that reduce brood survival dramatically if both parents do not invest in rearing.
Whay are >90% of mammals polygynous?
Because the female’s mammary glands represent a specialization for rearing altricial young that facilitates her early abandonment by the male.
This author clearly has a political axe to grind because of her own gender reversal. She has provided no credible argument against the importance of sexual assymetries and sexual selection in evolution, only decided quite subjectively to devalue them in favor of vague ecological alternatives.
Sperm ARE cheaper than eggs - if they weren’t, polyandry would be just as frequently observed a mating strategy as polygyny - but it is very, very rare indeed.
Females ARE more choosy about mates than males, because of their greater investment in oogamous reproduction.
Males ONLY start becoming choosy in *monogamous* mating systems when they are required to make comparable or greater investment in parental care to ensure offspring survival.
Males ARE larger and more aggressive in polygynous systems because of having to compete for females and defend them or their resources.
All these trends are undeniable to anyone who has studied mating systems across a broad number of taxa.
Sexual selection remains without parallel as the most convincing and consistently effective mechanism for explaining sexual dimorphism and gender-sepecific behavioral differences in nature.
Finally, the press articles you linked popularizing this book are among the worst lolly-gagging, insipid, non-critical, ”awestruck by ignorance’ tripe that can pass for coverage of science in the press.
Just my (not so humble) opinion - you asked for it
This message has been edited by EZscience, 05-12-2006 10:05 AM

This message is a reply to:
 Message 79 by Brad McFall, posted 05-11-2006 10:37 AM Brad McFall has replied

Replies to this message:
 Message 84 by Brad McFall, posted 05-12-2006 3:03 PM EZscience has replied

  
EZscience
Member (Idle past 5154 days)
Posts: 961
From: A wheatfield in Kansas
Joined: 04-14-2005


Message 85 of 131 (311460)
05-12-2006 3:58 PM
Reply to: Message 84 by Brad McFall
05-12-2006 3:03 PM


Re: A strongly biased attack on Sexual Selection theory
I am not sure I can respond adequately to all your points here, but I will start with what I feel I understand.
Brad writes:
Is there a credible difference of social selection and sexual selection within a nexus of artifical and natural selection?
I think these dichotomies are worthy of some consideration.
I see social selection as a force imposed by advantages/disadvantages mediated by the social structure specifically. I am in favor of a narrow definition, but I see social selection as acting on behavior that is both dependent and independent of gender. In contrast, all sexual selection is gender dependent and it can occur independent of any social structure at all.
The next dichotomy is a little more ambiguous, simply because by ”artificial’ we imply a ”human-mediated’ effect. Since our social environment is mediated ONLY by human interactions, it is hard to classify ”social selection’ as anything but a form of (unintentional?) artificial selection.
Brad writes:
. because the conflict in my own reproductive effort happened AFTER not before agreeable sex.
That’s pretty typical, from my experience
Brad writes:
It seemed to me that a priori one might consider male combat and female choice as not only intra sexual selection..
I am not sure I follow this, but let me clarify.
By ”intrasexual selection’ I am refering specifically to conflicts within one gender that influence their access to, or mating success with, the other gender. So its pretty much always based on competition or conflict, direct or indirect, between members of the same sex.
By ”intersexual selection’, I mean active choice of mates, normally the female being the choosy one, but not always. One sex selectively influencing mating opportunities, and hence genetic fitness, in the other. In monogamous systems it becomes reciprocal once repoductive investment is equalized by male parental care. The landmark experiments by Anderson on the widow birds in Africa showed beautifully how this type of sexual selection can produce to the runaway effect hypothesized by Fisher and lead to exaggeration of a ”favored’ trait in the selected sex, in this case tail length.
I think these are valuable concepts with great explanatory power that are consistent with a lot of observation and evidence and they certainly haven’t been debunked by M(r/s) Roughgarden who wants to replace them with her concept of ”social selection’.
Brad writes:
. is the "ecology" meant in the old fashioned of Roughgarden sense?
The traditional sense, as in the Orians-Vernier polygyny threshold model. For example, polyandry is strongly correlated with inhabiting very poor habitats. A female needs to enlist the help of many males to successfully rear a brood, or must defend such a large territory to have sufficient resources that she doesn’t have time to brood the eggs. Thus, in jacanas large aggressive females defend large territories against other females that contain many smaller, inconspicuous males that each brood a nest of their own. Females are always trying to steal males from each other so they can start an another nest. In this situation, males become choosy about the female they hook up with, and females welcome all newcomers.
It seems from reading the first page of this article that she and her colleagues are really only challenging the ”notion’ that female choice is necessarily based on ”genetic quality’. I don’t think this is a very deep challenge to the theory. There have been plenty of models showing how females can potentially choose on the basis of traits that are negatively correlated with fitness. I am sure you are familiar with Zahavi’s ”handicap principle’ and its recent resurrection. Indivdual females may also simply make the wrong choice as a function of their own sub-optimal genetics. And when there is reciprocal intersexual selection, both sexes have to ”settle’ for a mate that is commensurate with their own level of fitness, at least when the situation reaches equilibrium. So I don’t see that conventional sexual selection theory faces any difficulty in “sustaining a stable genetic hierarchy within a gene pool in the face of continued directional selection for high-fitness genotypes”. There are many constraints on directional sexual selection and SS-traits are ultimately limited by their survivability. And from whence the assumption that this perceived hierarchy is necessarily stable? This is a straw man argument and it wouldn’t have made it into science if it hadn’t had some novel application of Maynard Smith’s game theory.
I hope this clarifies my take on the issue, but I am not sure I understand how you see this as relating to “favored mutation directions”, or in any way contradicting Gould. I’ll have to pull out my copy of “Structure..” and look through it for his passages on SS.

This message is a reply to:
 Message 84 by Brad McFall, posted 05-12-2006 3:03 PM Brad McFall has replied

Replies to this message:
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