How can evolution explain body symmetry?

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But there should be at least some, and in most cases numerous external asymmetries in all or just about all land creatures.

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There are, though. That's been the point of about ten messages to you, so far.

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Sorry, I should have mentioned "obvious" external asymmetries.

How else would a blind creature judge a mate? And again, they're not looking for total, perfect symmetry - just a healthy amount of it. Um, no bats are blind. (Well, maybe by mishap or disease, but certainly not by nature.) I don't think any snakes are, either. Right, because you don't understand it. Once again, sexual selection doesn't lead to perfect symmetry. Only gross symmetry. Here's an example of Drosophilia (the fruit fly):

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Sexual selection for size and symmetry in a diversifying secondary sexual character in Drosophila bipectinata Duda (Diptera: Drosophilidae).

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Polak M, Starmer WT, Wolf LL.

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Department of Biological Sciences, University of Cincinnati, Cincinnati, Ohio 45221-0006, USA. polakm@email.uc.edu

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Results of intrapopulation studies of sexual selection and genetic variation and covariation underlying elements of the sex comb of Drosophila bipectinata are presented. The magnitude of the sex comb, a sexual ornament, varies significantly among Australasian populations, motivating research into the evolutionary mechanisms responsible for its incipient diversification. The comb is composed of stout black teeth on the front legs of males arranged in three distinct segments: C1, C2, and C3. Significant sexual selection in field populations in northeastern Queensland, Australia, was detected for increasing C2 and body size, and simultaneously for reducing comb positional fluctuating asymmetry. In contrast, sexual selection was not detected for other comb segments, nor for sternopleural bristle number or symmetry. Selection intensities for C2 and comb positional fluctuating asymmetry were similar in magnitude, and although they were opposite in sign, values across twelve sampling dates, or selection episodes, were uncorrelated. Heritability estimates for C2 were high and significant across years, whereas heritability estimates for comb positional asymmetry were small, and generally nonsignificant. The major sex comb segments (C1 and C2) were significantly and positively correlated genetically, indicating the potential for correlated evolution of these components of the comb under sexual selection. The original finding of a significant positive genetic correlation between the magnitude of this sex trait and its positional asymmetry indicates that the counteracting and independent selection pressures detected could contribute to the maintenance of genetic variation sustaining sexual selection. The study documents the simultaneous presence of sexual selection in nature and of heritable genetic variation underlying expression of the sex comb, fundamental conditions necessary for its adaptive diversification. Drosophila bipectinata may be a valuable model for studies of adaptive diversification and incipient speciation by sexual selection.

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from Pubmed.org. Took me about 5 seconds to find; you probably could have done it yourself if you weren't busy with these little potshots. Oh, well. I'm happy to do it for you, though. Right, because this ability is a "hardware" function of the brain:

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Evidence that both area V1 and extrastriate visual cortex contribute to symmetry perception.

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van der Zwan R, Leo E, Joung W, Latimer C, Wenderoth P.

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Department of Psychology, University of Sydney, New South Wales, Australia. rickv@psych.usyd.edu.au

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Bilateral symmetry is common in nature and most animals seem able to perceive it. Many species use judgements of symmetry in various behaviours, including mate selection [1-3]. Originally, however, symmetry perception may have developed as a tool for generating object-centered, rather than viewer-centered, descriptions of objects, facilitating recognition irrespective of position or orientation [4]. There is evidence that the visual system treats the orientation of axes-of-symmetry in the same way it treats in orientation of luminance-defined contours [5], suggesting that axes-of-symmetry act as 'processing tokens' [6]. We have investigated the characteristics of neural mechanisms giving rise to the perceived orientation of axes-of-symmetry. We induced tilt aftereffects with symmetrical dot patterns, eliciting perceived angle expansion and contraction effects like those usually observed with luminance-defined contours [7,8]. Induction of aftereffects during binocular rivalry resulted in a reduction of the magnitude of these effects, consistent with the aftereffects being mediated in extrastriate visual cortex, probably between visual areas V2 and MT [9]. In a second experiment in which the aftereffects were induced monocularly, their magnitudes were measured in the unadapted eye. Contraction effects transferred completely, suggesting that they are mediated by binocular cells. Expansion effects did not transfer completely, consistent with their having a monocular component. These data suggest that information about the orientation of axes-of-symmetry may be available as early as area V1, but that processing continues in extrastriate cortex.

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Again from Pubmed.org. Yeah, like the height of your ears. The difference can be as great as a half-inch or so. If you look at people, I mean really look, you begin to notice that most people have one eye higher than the other. This one girl I knew had such a strange facial asymmetry that I never even noticed it until I saw her through a mirror - and was suddenly struck by how much higher one of her eyes was than the other. I had always known there was something different about her face, but I simply couldn't put my finger on it until the familiar context of her face was removed. Tests have been conducted, I've presented one of them, and you can surely search for others yourself if you're curious. They confirm what I've been saying - in most species, there's sexual pressure to be symmetric. Those are adaptations to environment, or that pesky sexual selection again. Just because evolution choses a certain shortcut sometimes - gene mirroring, for instance - doesn't mean it has to use it every time.

People had about a 60% chance of living long enough to breed, and then died at about the age of 40-50, at the end of a significant period of failing health, accumulating injury, and debilitating disease. Survives just well enough to breed, yes. That's all we'd expect from evolution.Look, if you're so convinced that the human body is such a great design, can you explain the upside-down retina? Nobody else has been able to, except the evolutionists.

Shit, dude, this study will blow your mind - women can actually smell symmetry.

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Menstrual cycle variation in women's preferences for the scent of symmetrical men.

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Gangestad SW, Thornhill R.

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Department of Psychology, University of New Mexico, Albuquerque 87131, USA. sgangest@unm.edu

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Evidence suggests that female sexual preferences change across the menstrual cycle. Women's extra-pair copulations tend to occur in their most fertile period, whereas their intra-pair copulations tend to be more evenly spread out across the cycle. This pattern is consistent with women preferentially seeking men who evidence phenotypic markers of genetic benefits just before and during ovulation. This study examined whether women's olfactory preferences for men's scent would tend to favour the scent of more symmetrical men, most notably during the women's fertile period. College women sniffed and rated the attractiveness of the scent of 41 T-shirts worn over a period of two nights by different men. Results indicated that normally cycling (non-pill using) women near the peak fertility of their cycle tended to prefer the scent of shirts worn by symmetrical men. Normally ovulating women at low fertility within their cycle, and women using a contraceptive pill, showed no significant preference for either symmetrical or asymmetrical men's scent. A separate analysis revealed that, within the set of normally cycling women, individual women's preference for symmetry correlated with their probability of conception, given the actuarial value associated with the day of the cycle they reported at the time they smelled the shirts. Potential sexual selection processes and proximate mechanisms accounting for these findings are discussed.

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Insane!

There is a sane explanation for this, Crash.Symmetry is expensive to produce, slight asymmetries are significantly easier to make - which is why symmetry is so universally sexually attractive (it's not only humans, it's also birds, cats, dogs, mice, rats, dolphins and monkeys and probably a whole load more animals they haven't tested it in). Thus more symmetrical individuals are, in general, more gentically fit as well as having more enviromentally preferable lives (childhood illness, and malnutrition both produce asymmetry). Thus what the girls are smelling is not symmetry but instead fitness that itself produces symmetry.

I know.

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Um, no bats are blind. (Well, maybe by mishap or disease, but certainly not by nature.) I don't think any snakes are, either.

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Ok, you got me there. I really blew that one. But there are blind land animals, including some snakes such as the Texas Blind Snake.
http://entowww.tamu.edu/extension/bulletins/uc/uc-007.html

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Right, because you don't understand it. Once again, sexual selection doesn't lead to perfect symmetry. Only gross symmetry.

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I get it! I understand that humans and animals are not perfectly symmetrical. But I don't agree it is due to sexual selection because I've still not seen the proof of it.

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Here's an example of Drosophilia (the fruit fly):

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The females are selecting based on the sex comb, a sex ornament. So naturally differences, including asymmetry, are going to be significant. We were not talking about sex ornaments, but asymmetric mutations in general and the ability of animals to detect and deselect for it. You're going to need to find an example where animals can detect general asymmetric mutations at any random point, and not one at some specific point that it being used for sexual selection. Nice try though.

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Bilateral symmetry is common in nature and most animals seem able to perceive it.

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Saying "seem able" seems to say they are not sure that it is absolutely true.

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Many species use judgements of symmetry in various behaviours, including mate selection [1-3].

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Well, if they're not even sure that the animals are capable of perceiving symmetry, then this statement cannot be made with any certainty. Plus no reference is supplied for the reference number listed for that quote so I can't check it out any further.

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Those are adaptations to environment, or that pesky sexual selection again. Just because evolution choses a certain shortcut sometimes - gene mirroring, for instance - doesn't mean it has to use it every time.

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Even if evolution did use a shortcut sometimes as you say, it doesn't always. So everytime it doesn't use the shortcut there should be a chance for asymmetries.

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this study will blow your mind - women can actually smell symmetry

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Do you really believe they are smelling it? Of course not. The first flaw is that they are using humans, into which all sorts of external causes can be generated to give those results. For example, those symmetrical males are probably more highly desireable in real life and know it and are very much into "the game" and have a lot of concern for how they smell. Less symmetrical males may not be so much into the game and not care as much about how good they smell. The symmetrical males may also be more concerned with their health and fitness, trying to maintain what they consider to be a perfect image, and this health and fiitness may affect they way they smell. So actually there is a very good chance there is absolutely no direct correlation between symmetry and smell. Only an indirect one.But hey, good job on finding that. You're certainly better than me at finding stuff on the net.

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People had about a 60% chance of living long enough to breed, and then died at about the age of 40-50, at the end of a significant period of failing health, accumulating injury, and debilitating disease.

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But that is still much better than any human built machine, and no comparison to something a junior highschool student could build. And the fact that the death rate is so high or that diseases and old age play a part says nothing about the designer's abilities or intentions. If you believe the Bible, it says that sin came into the world, and by sin death. So actually it could be that the designer built the body to last much longer but external causes introduced have caused the design to deteriorate much faster than under the original environment. Of course if you don't believe the Bible, (which is obviously the case with you), then that probably means nothing. But for someone who does believe in an intelligent designer, it is one logical explanation for such early death. But actually, I think this is all getting way off the original topic.and has nothing to do with symmetry of biological creatures.

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Look, if you're so convinced that the human body is such a great design, can you explain the upside-down retina? Nobody else has been able to, except the evolutionists.

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From a scientific/engineering prospective, I don't really have much to say on this. I have read both sides of the argument. And I could just quote something some creationist said, but I have no idea of how valid their claims are, so I won't. Not to mention it is also getting off topic. What I will say is that you are judging the designer's design based on your own logic and ideas of what the design should look like. You have no idea of why it was designed that way, so regardless of what you or anyone may think it really doesn't say anything about the designer because you don't know his thoughts, intentions, or reasons behind that design. There are certainly cases in real life where a human designs something that looks backwards to another, but the design is not backwards but actually the way it was intented for reasons unknown to the observer. Bottom line is it works.

The test is reasonably well known, however MHC's are thought to be one of the main differences selected for. These are part of the immune response, so more variation means you should be able to have a broader range of resistance.And yes they do smell it, the test i read of had 30 men wear a tshirt, do 30 mins of exercise, no deoderants etc. the tshirts were numbered or sometihng, then given to the women to smell. In this test close relations didnt smell as good as people who werent as closly related.Scientists arent daft enough to claim that its smell if theres another more sensible reason. Everyone smells if not wearing deoderant etc, the test is fine. Once again leaving a good correlation between symmetry and smell.Unseul

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Give a man a fire and he's warm for a day. Set a man on fire and he's warm for the rest of his life....Do unto others before they do unto you.

of symmetry, it can easily be done.Unfortunately, you are wandering down the wrong path to do so.If you will step back from living things for a moment, you will find that symmetry is a very basic result of some of the more fundamental laws. It is inherent in the basic facts of the three dimensional world we live in.Stop for a moment and consider crystals. They are all symmetrical because there is no way that the basic building blocks can combine without producing symmetry. The human body, in fact almost NO organisim beyond simple cellular forms, is symmetrical. Yet at the earliest stages we still find symmetry. And that is where your analogy totally fails.Look at any animal. You may find some symmetry from right to left, but then you will find the analogy fails when examined fore and aft or top to bottom. Look though at a crystal and you will find such symmetry.The only way that you can explain living creatures, and the fact that they, unlike crystals are not symmetrical, is that living creatures are not subject to the restrictions imposed at the lowest level. Living things do not follow the same set of basic rules laid down for crystals. They have evolved and differentiated based on random changes that allowed one set to reproduce while another failed.

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Aslan is not a Tame Lion

I am not making a case of total or perfect symmetry. I am making a case out of the general observed bilateral external symmetry of animals. I am saying nothing about non-life forms such as crystals that do not follow the same proposed rules of evolution as do living organisms. I fail to see the significance that symmetry rarely exists fore to aft or top to bottom with respect to my argument, and I am not looking to try to prove it should exist because I don't think it should either from an evolutional or from an intelligent designer perspective. The symmetrical life forms we do see are consistent with what an intelligent designer could produce, and are not consistent with what I would expect to see given claimed rules by which evolution must operate. That is my one issue with symmetry. Prove that there is some evolutionary mechanism that adequately explains the symmetry that we see, while also explaining the asymmetry we see, or else expect that I and others will doubt evolution could have ever occurred due to this glaring inconsistency in the theory.So far many evolutionists have responded with the claim that sexual selection can account for the observed symmetry, but everytime I ask for proof, nobody can offer any of non-human sexual selection except one case which doesn't apply to general body symmetry but rather extremely limited symmetry of only a sex ornament. Their silence with respect to this point is now making me think it would be a very good test for creationists to use in proving evolution to be false, as such a test should be easy to conduct on certain animals. For instance, the referenced study of flies, and their selection based on the symmetry of the sex comb would be a good start. Being that some people already think they are selecting for symmetry, take a group of similar male flies and remove a section of one wing (alternating between left and right wings on alternate flies), while removing a section of the wings on both sides of others so that the modification is symmetrical. On yet others, make modifications to both wings, but such that they are asymmetrical. Thus you would have some symmetrical and some asymmetrical in various configurations. Assuming the asymmetrical modification was not significant enough to adversely affect flight over the symmetrical ones, or vice versa, you could see if the females preferred symmetrically shaped mates, or didn't care. Similar tests could be done for many classes of animals, and if no selection for symmetry occurred outside of areas that might be considered a sex ornament, then I think a realistic conclusion could be drawn to say whether or not symmetry is sexually selected or not.The problem with using naturally occurring asymmetries is that it is not known if the asymmetry is the result of some greater defect or other effect that can be detected by the mate doing the selecting. If the specimen has some major defect that results in some asymmetry, then obviously it has a greater chance of being deselected. You have to be sure the symmetry is the reason for the selection and not a secondary effect due to some other unaccounted for difference in specimens.Now that is what I would call real science. Using a real test to either prove or disprove one of the claimed mechanisms of evolution. Both creationists and evolutionists are very good at promoting pseudo science where they gather data and interpret it in a way that matches their believe system, but failing to realize or accept that other interpretations might be equally valid. Very little can be said scientifically about our past with a high degree of certainty, because there is no way to go back in time and do a scientific observation of it first hand. I know I'm going to catch a lot of flak from that statement by the scientists on this board, but it really is the truth. If it wasn't, there wouldn't be all the debates there are between evolution and creationism. The speculation and interpreting of data on both sides is interesting, but I don't think will ever really do much to convince people to change sides.Anyone up to the challenge of doing such a test? Or if you know of any such test already being done, please say so.Sorry to get up on my soap box and get so off the subject.

hi CrackerJack. Tall order. I do not think there is anything in the way of "proof" as this is a function of mathmatics. But observation and applying what works can shed some light on the "why" there is symmetry and asymetry in nature. I believe that in nature shape dictates function. Have you ever seen a one legged bipedal organism try to walk? Or how about a centipede with legs on only one side? How fast do you think it would be? How far could a bird fly with one wing? Fine binocular vision anyone? (requires 2 eyes.) It seems to me that symmetry is a by product of function. As far a Asemmetry in nature goes the same principal applies, if it works
exploit it. Intellegent design or not nature is beautiful. IMO Mother nature or God does it really matter who did the handywork? *edit typo.This message has been edited by 1.61803, 06-01-2004 05:52 PM

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"One is punished most for ones virtues" Fredrick Neitzche

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When you look at the animal world as a whole, you see external symmetry as the norm, not as the exception.

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Right.This is because we evolved from aquatic creatures in which symmetry is very advantageous. Symmetry continues to be advantageous in MOST cases.

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If internal and external structures evolve in exactly the same way, then there should be lots of asymmetry in both.

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Why?You can't see the inside of other creatures, so sexual selection in favor of internal symmetry cannot happen.

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Being there isn't, evolution should be discounted because as of yet nobody has given a good enough explanation to account for such a high degree of external symmetry. And the sexual selection argument doesn't cut the mustard as I've already shown.

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You have shown it? Where?

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And there is no need in the environment for such a high degree of external symmetry in land creatures,

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Really?Are you typically sexually attracted to people with significant facial deformities?That makes you pretty rare.

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so no need to deselect asymmetry unless it negatively impacts locomotion due to an imbalance or some other such effect. Proof has already been provided of limited asymmetry, so why is it so selective and not widespread?

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When your basic body plan is externally symmetrical from embryonic development, and external symmetry is sexually selected for, why do you expect asymmetry to become widespread at all?

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Ok, I have done some research today on symmetrical vs. asymmetrical mutations and yes there are a fair number of symmetrical mutations as you have pointed out.

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At last, you are doing some research.Now, perhaps you might consider doing all of your research before you make exterme claims instead of after.

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But polydactylism in cats is not always symmetrical.

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So?The ToE doesn't say that it has to be.

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In general, some mutations are symmetrical, and some are not. So unless someone can provide some actual scientific data on what is the ratio between the two, we can't really say anything more than both types are well known to occur.

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That's what we've been saying all along, you know.

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No, I don't limit it to just humans. If evolution were true, there should be some probability that some percent of all mutations that are retained will be clearly asymmetrical.

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Why?

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Statistically, most asymmetries (not including those that negatively impact the species) should have stopped being deselected once fish became land animals.

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Why?

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Being man is claimed to be a descendant of the original land animal, that means that some very early ancestor should have some noticeable asymmetric feature and all descendants from that creature, including man, should exhibit that feature.

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Why?Where is the selection pressure for asymmetry?

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Along with it, many other asymmetries should have been picked up along the way, and the end result should be a wide range of asymmetries across the entire spectrum of the animal kingdom.

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Why?

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If evolution was true, I would expect just about every land creature to exhibit some clearly visible external asymmetry.

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Why?Where is the selection pressure for asymmetry?

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I'm just saying that if evolution is creating internal asymmetry, it should also be creating external asymmetry. There is no reason for it to be so selective.

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SEXUAL SELECTION and LOCOMOTION selects in favor of external symmetry, as well as our basic body plans being derived from sea-dwelling creatures in which external symmetry was also highly advantageous.

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Agreed that in humans, a new asymmetry would be a disadvantage.

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Why?It might not be a disadvantage at all.In fact, for it to become fixed in a population, it would have to tend to be advantageous in some way, to some population.

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But if all humans had the same asymmetrical feature, it would be considered normal and not deselected.

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Since we already sexually select FOR external symmetry, an obvious external asymmetry is not likely to become widespread in the first place.

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Lower animals are not capable of the same sort of reasoning and inspection that a human is.

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Actually, our ability to reason is why people with facial deformities and other external asymmetries are able to get mates.Unlike animals, we are able to be attracted to a personality and overlook the physical.Anyway, why do you think that animals don't reject deformed offspring or potential mates? My friend's cat just had kittens, and there was a malformed one in the litter. The mother ate it.

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So some distant ancestor of man should have had an external asymmetrical mutation that was retained and considered normal and thus not sexually deselected.

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Why?Why would it be selected for in the first place?

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What do you mean by "most"?

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The majority of selective pressures favor external symmetry. A minority do not.

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And by what means (other than what has already been mentioned) are they culled out?

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They are less likely to reproduce, thus they do not pass on the mutation.

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Many mutations, symmetrical or asymmetrical, will create a big disadvantage and should obviously be culled.

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They are, in the form of failed implantation, reabsorbed, miscarried, and stillborn offspring, etc.

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Asymmetrical mutations that create one longer leg than the other are obviously going to be culled. But would a long ear lober be culled?

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No, that's why we see them today.

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What about all the positive mutations that have occurred. Take opposable thumbs for instance. Everyone agrees that they are a huge advantage for humans. What if an early ancestor of man evolved an opposable thumb on just one hand and not the other, would that be culled because it was a disadvantage? Of course not.

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The opposable thumb isn't human in origin. We got that from our primate ancestors.Anyhow, haven't you been reading what we have been telling you about how embryonic development occurs?

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It would be retained. Somewhere along the line, some of the advantageous mutations should have been asymmetrical and retained.

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Like what?

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Why does evolution select them out?

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Because they are actively selected against due to our preference for external symmetry in mates, mostly.If they conferred an advantage in some way, though, we'd still have them.

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The sexual deselection argument doesn't cut it as I've already shown.

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I haven't seen this. Where have you done this?Did you know that there are many studies which show that symmetry of facial features is the most important factor in someone finding a face attractive?

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Please provide a reason that applies to ALL externally symmetrical animals. Why is it being culled out in blind animals? How can lower level animals count and determine an uneven number of digits between the left and right?

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Don't be silly. You don't have to see and you don't have to be able to count to tell if another of your species is symmetrical or not.And, symmetry is not as important in other creature's sexual habits as ours, anyway.

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If life as we know it didn't come about by evolution, and it didn't come about by an intelligent designer, then by what means was it created?

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If you reject all evidence for evolution (of which there is a VERY GREAT DEAL more that the tiny bit we have been discussing here), then you have to explain what we see with another scientific theory.That theory must have positive evidence to support it, make testable predictions, and be falsifiable.

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Those are the only two possibilities I can think of, and being evolution is ruled out in my mind due to the reasons I've explained, I assume it was an intelligent designer. If you can show another way by which we were created I'd be glad to entertain your idea.

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You cannot make a scientific claim based upon a lack of evidence.Nor can you make a scientific claim based upon another theory's faults or shortcomings.Nor can you make an Argument from Personal Incredulity.Nor can you make a God of the Gaps argument.OK, you CAN make all of these claims and arguments, but none of them are valid arguments.Here is a question for you...How can we tell the difference between an Intelligently Designed system and a natural one that we;1) don't currently understand but may in the future, and/or2) may not ever understand?