How can evolution explain body symmetry?

Just to add, ask any barber and you'll find out that everybody has asymmetrical ears - one is always a little higher than the other.Most people have a facial asymmetry to one degree or another. Not to mention one longer arm, one longer leg, one larger foot.And of course there's great asymmetry inside the human body.This message has been edited by crashfrog, 05-31-2004 01:43 AM

No, I understand.It's actually you who doesn't understand your own position, or at least, the very obvious consequence - if we're the product of intelligent design, and that designer is a perfect God, then we shouldn't have just some overall, large-scale, exterior symmetry.We should be perfectly symmetric, inside and out. We're not, so we're obviously not the product of a perfect designer. Evolve seperately? Of course not. My left side doesn't reproduce separately from my right, so my two sides obviously won't evolve separately.Look, your question has been answered twice over. Why are we mostly symmetrical?1) Because we're the decendants of organisms for whom symmetry was an adaptation to environment.2) Because symmetry is strongly sexually selected for (humans are most attracted to mostly symmetric humans.) Now it's you who doesn't understand my point. Of course something is "forcing" symmetry in humans - sexual selection. But it only forces symmetry with one big restriction: it only enforces symmetry you can see. That has two consequences - one, that the symmetry will only be true enough to fool the eye, and not the mirometer; and two, that the symmetry will only be on the outside.Look, it's pretty simple. The source of symmetry goes back to our invertebrate ancestor, who was adapted to an aquatic environment, where bilateral symmetry works better than asymmetry. It persists in our species and others because it's very strongly selected for, sexually. Also, it's a requirement for walking around on any number of legs on a flat surface.Now what about that explanation do you feel falls short?

Well, who says even a perfect designer would have any concern about symmetry at all? If anybody's making assumptions about what designers are like, isn't it you in your opening post? I had thought my point was pretty obvious - your opening post is based on a false premise. You ask why such perfect symmetry exists, but the problem is, it doesn't. There's only as much symmetry as we would expect from evolution. Uh, right, that was the purpose of Point 2. Duh. That hardly means they have no ability to sense symmetry. Plus, most of the species you're talking about are aquatic. No, of course not. But they do tend to prefer mates that appear symmetrical. Perfect symmetry doesn't exist.Look, it's proven fact that humans prefer symmetrical faces. Even infants prefer them. Why on Earth would that require counting? How stupid. All you have to do is look at the organism. If one side is different from the other side, it's not symmetrical.Everybody does this, and it doesn't even take thought. For instance, this famous picture:Ah, a masterpiece. And that smile - so enigmatic, so mysterious. The smile that has captivated poets and lovers for centuries. What is it, exactly, about this famous gaze that defies us to look away?As it turns out, it's a clever asymmetry in the picture that your brain notices immediately, but your consious mind fails to see. If you look closely, you can see that the horizon on the left is lower than on the right. It's a visual impossibility that immediately triggers "wrongness" in your brain but your conscious self wouldn't have noticed if I hadn't pointed it out.Like all vertebrates, your brain has hardware that detects symmetry. The reason you detect it is because often, asymmetry represents poor health or injury, which are not things you want in a mate. Of course they don't observe all minor asymmetries. That's why minor asymmetries persist, like your ears and stuff.You don't seem to be getting it yet. We're not perfectly symmetrical. We're only asymmetrical up to the point that a potential mate might notice, because asymmetry is selected against.We're not perfectly symmetrical. We're only as symmetrical as evolution predicts.There's a third potential explanation for the persistence of symmetry - it's genetically simpler. It takes a lot more genes to code for two separate sides than than to provide the genes for one side and simply duplicate for the other. That's the "evo-devo" explanation, I guess.But ultimately the answer is pretty simple - why does symmetry persist? Because in most cases it's better than asymmetry. And again, your inital premise is false, anyway - nothing's perfectly symmetric. We're only as symmetric as evolution predicts. Get it, yet?

If that's supposed to be true, what do you think all those doctors do? What do you think all those medicines are for? I imagine that your average junior high school student wouldn't have designed the human retina backwards.

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

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!

I know.

I've shown you several studies where individuals - of a number of species - prefer more symmetric mates. What else do you need, exactly? Why? If they can see the symmetry in a sex comb, you don't think they can see it anywhere else? So, you'd prefer the explanation involving the designer we can never see, never find, and never ever know anything about, just because I can't honestly tell you that I know the scientific explanation is "absolutely true"? If you think scientific tentativity is the place you're going to be able to squeeze in God, I'm not even sure you're capable of understanding scientific conclusions. And there is. The fiddler crab, and others. Again, showing you that symmetry isn't universal has been the subject of some ten posts to you so far.Christ it's like you forget our posts the second you're finished reading them. You do know that you're supposed to remember our arguments, right?

There's a clock in Winnsboro SC that's been running continuously for 160 years. I'm not sure how much replacing of parts you could do on a running clock, so I imagine that the movement, at least, is all original parts. I disagree. It's a well-understood principle of engineering that engineers must take into account potential points of failure.Engineers don't rely on elevator cables to never break. They design elevators to stop themselves in the event of a fall. You're telling me that your hypothetical designer doesn't know as much as an Otis engineer? Both sides of what argument? The human retina is upside-down. The light sensitive cells are at the bottommost layer, under/behind the structural cells and circulatory capillaries.If it was right-side up, you'd be able to see in the dark a lot better. There's no advantage to an upside-down retina. I don't understand what "other side" you think you've heard, because there is no other side. There's just the fact that human retinas are upside-down, and they either evolved that way, or were designed that way.Evolution makes sense, because the retina still works well enough the way it is that it's not maladaptive. The design hypothesis is just incoherent. It offers no explanation except "obviously, it's supposed to be that way for reasons we don't understand."Take my word for it. We don't reject your ideas about a designer because we hate God and all that is good and holy. We reject them for the simple reason that your explanations don't make sense. They don't allow for useful predictions. They just don't take us anywhere, or help us make sense of the universe.Evolution does all that. That's why creationists in the 1800's came up with it - they were better scientists and more honest than any of you creationists today.

Look around at human vehicles and tell me the answer to that question isn't obvious. How many asymmetric airplanes have you ever seen?

Evolution isn't random. There's an obvious selective benefit to those organisms that can move with the greatest degree of efficiency and freedom; its not surprising in the least therefore that we find them adapted to move efficiently.And, in fact, we did design them randomly; prior to supercomputer drag modelling developing aerodynamics was largely a process of trial and error. These are all examples of vehicles for which drag is not an issue, i.e. they don't go fast enough. As I asked, how many asymmmetric airplanes have you ever seen? How many asymmetric boats or submarines? Among these vehicles, for whom fluidic drag is the major limit to speed/efficiency, symmetry is the norm. So what? Also you seem to overlook the water. The earliest organisms were marine, not terrestrial. Symmetry has a huge benefit for even slow-moving marine organisms. No need to repeat your question, it was already answered. There is no "law of symmetry", only the laws of physics that dictate survival benefit for symmetric organisms and their ancestors.

Did you, as you wrote it? Apparently not. By the way it would be helpful if you would employ, in your future replies to me (or any other person), the reply button with the red arrow located directly below the post, instead of the reply button located at the very bottom of the page. This will preserve thread continuity, autoinsert subject lines, and notify me that I have a reply to my post. Thanks! Yes, you did, quite directly, in the passage that I quoted:

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Because we, the humans, have intelligently, not randomly, designed them to be efficient.

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Now why would you need to have specified "not randomly" unless you were implying that evolution is random? Let's not be disingenuous, ok? Because many organisms are faced with the same design problem that airplanes and submarines are - the mediums through which they move necessitate certain morphological characteristics for efficient travel. Symmetry is part of that morphology. And once symmetry evolves, it persists and is selected for in its ancestors. Nonsense - the ancestors of insects were marine organisms, and even slow-moving marine organisms have bodies shaped by fluid dynamic issues, because water is so denser than air. That's "evolutionist", and I answered it. Twice now I've answered it. Have you ever seen a tree get up and move? Plenty of plants are mobile. Sunflowers turn their flowers to track the sun. Most flowers open and close their petals during the day.And of course, every plant has some kind of seed dispersal mechanism. But plants don't have the cellular biochemistry to permit the sort of active movement and travel associated with the animal world. They simply don't have the energy for it.This message has been edited by crashfrog, 03-14-2005 03:24 PM

It's the same argument. That's not what "random" means, however. Random means "the outcome is not determined."Look, let's not be disingenuous, ok? You meant that unlike evolution, intelligence doesn't design randomly. Otherwise there would have been no need for you to specify that intelligence was "not random." Now that you've been called on that claim, you're backpedalling away from it. You can post-hoc redefine all the words that you like but it doesn't change the fact that you clearly and mistakenly defined evolution as "random".Lets not play games, ok? That statement couldn't be farther from the truth. For instance, there are many, many aqautic insects, in every insect order from Coleoptera to Ephemeroptera (I love that name; that's the order of mayflies and it perfectly captures their short, ephemeral life cycle), Hemioptera to Trichoptera. Many more insects are aquatic during one or more of their life stages (moquitoes, etc.)Insects are of course arthropods, and outside of Class Insecta the vast majority of arthropods are marine organisms. As I said, few plants are mobile on their own initiative, as they don't have the biochemistry to have access to that much energy. Since they don't have to solve fluid dymatic issues they lack such definite symmetry. I'm not sure why you keep bringing up plants when its obvious that the shape of plants proves my point.