How can evolution explain body symmetry?

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.

Edited by Sumer, : No reason given.

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Neither the ancestors of insects required any aerodynamics (if such ancestors ever existed), high speed and flying could only come from a prior slow speed and non-flying creature.

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If a daddy longlegs had all its legs on, say, the left side of its body, surely it would not be able to move at all.Oh no! Here our daddy needs symmetry despite neither flying nor swimming, but merely walking!Sumer, please engage your brain. The argument to hydrodynamics or whatever is pretty clearly an argument to an environmental pressue. You could use that sort of logic yourself, if you chose.

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Have you ever seen a symmetric tree?

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Sort of - many plants are RADIALLY symmetrical:

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radial symmetry

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a form of body symmetry in which the parts of an animal are arranged concentrically around a central oral/aboral axis and more than one imaginary plane through this axis results in halves that are mirror-images of each other. Examples are cnidarians (Phylum Cnidaria, jellyfish, anemones, and corals).

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Actually, as I remember it, you wandered off into jumping plants and such and touched on the issue of flight and mobility as well.Frankly, I didn't bother responding to you because it was pretty obvious that you had no idea of what you were talking about and were just prattling.You took what might have been a good question and turned it into what may well be the most absurd paragraph I've seen yet at EvC in question #3 We often get the drive by shooter here who comes sweeping in with the killer insight. They generally last a day or so, a few even as long as a week. Then they pass away into the fog bank they came from.For now, I'll continue to glance over your posts. When you finally explain why question #3 is totally absurd, I'll begin to read your posts in depth. When you start asking pertinent questions I'll try to respond.

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Aslan is not a Tame Lion

Sumer, you ask "Why did they specialize their bodies according to some law of symmetry?".If you read up on development of body axes in drosophila, you will find that symmetry arises quite naturally, in fact in quite a "lawlike" manner through simple physical processes. So you may be right to talk about "some law of symmetry". But it's not a mysterious law and it's not a challenge to evolution.In insects, for example, body axes result in the embryo (in part) from the process of diffusion. Imagine you have a random bunch of cells (the embryo). One cell within the embryo starts to produce a soluble protein. It's just a random cell, we don't have any axes yet so we can't say that this cell is on the "left" or "right" or "front" or "back". But becuase the embryo is basically made of liquid, and because the protein is soluble, we end up with a radial gradient of the protein's concentration throughout the cells of the developing embryo. Cells that are near the protein-producer have a high concentration of the protein, and cells far away have a low concentration.Imagine that our hypothetical soluble protein is responsbile for switching on various genes within the nucleus. Cells close to the protein-producer will have these genes activated because they contain a sufficient concentration of the activation protein. but cells far from the protein-producer will not have these genes activated because their concentration of the "switch" protein is too low.All it takes is for ANY protein produced by the developing embryo to be hooked up to some developmental gene(s) (i.e to take on a regulatory role) and we get linear body axes quite naturally and quite straightforwardly. Imagine that the protein turns on genes that produce phenotypes we call "the front of the animal". Cells near the protein-producing cell will become "the front" and cells far from it will become "the back". The beautiful thing is that the "protein switch" doesn't have any complex chemical properties or complex biological function. ANY protein will do, as long as it is soluble.Given that living organisms are largely made of water, and given that diffusion occurs, why do you find it surprising that symmetry should evolve? Once we have linear concentrations of regulatory gene products within a simple organism, you naturally get symmetrical axes of the body where similar genes are switched on or off. I really don't understand why you consider this problematic. Or am i missing the point?Best wishes,mick

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

And just to add: Plants are very symetrical and grow in a very ordered sequence known as the fibonacchi sequence. Or Phi.
I do not think he has thought out his argument before he posted it.

Edited by Sumer, : No reason given.

But the aero/hydro dynamics argument is environmental pressure. The environment selects for shapes that move through the air/water better. As CF pointed out with his plane/submarine it is an advantage to be symetrical and therefore it is selected for in living things.I think it's worth noting that you are actually quite asymetrical. The outer organs (legs arms, face etc) are, more or less, the same on both sides, but your inner organs are not in the least bit symetrical. Imagine the problems if we were to have a gut that ran straight down the middle of our body, or a heart with two identical sides. For relatively simple things like fruit flies, the inner workings of the body can be set out symetrically but for vertebrates like us it is a distinct advantage to to have things on one side or the other, and it is therefore selected for. This is a real pity, as the basis for symmetry/asymmetry is set out in the development of the embryo and is therefore vitally important for this discussion. I don't mean to be mean, or snipe at you in any way but I would really suggest you start to get at least a passing knowledge of such processes. If there are any questions you have about what Mick has said then feel free to ask them here. I'm sure Mick (or a number of others for that matter) will be only to happy to answer them.This message has been edited by Ooook!, 15-03-2005 09:58 AM

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Of course daddy longlegs needs the legs on both sides. The question is why both sides are symmetrical (including the legs, eyes, body, etc.)?

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For the same reason - it would be no good to see only one side, or be able to walk on only one side, or whatever.

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Can you tell me which plant, as the entire organism, is always symmetric? (hints: roots are also parts of the plants!, bent and twisted stalks, split trunks, north-south sides, etc.)

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As Ooook cogently points out, NO creature is perfectly symmetrical. But then again, symmetry is not magic or divine princiepl, just adaptation to environmental pressue, so we would only expect to see it where it is needed, or useful.Rosses, IIRC are symmetrical in 5 planes. You can see how efficient this is for the rose - it just has to have one petal design and apply it five times. But I would doubt the roots are symmetrical, becuase they have a very different role to play, in which cymmtery would not be useful.Symmetry is not astoundingly weird. "make one and copy as needed" is a sound principle of efficiency.

I always wondered why high school kids get better looking each year. My parents were kinda plain and ugly and it seems that the human form is getting more symetrical over the years. (My Mom was pretty in college, though! }

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.

The "hardly" present refers to the "sea". I take this as a marine environment. There are some marine insects but not, I think, many. There are, of course, many fresh water examples. However, I don't see why the lack of them in the current "seas" is a big problem.

I presumed he meant "aquatic", because why would oceans have anything to do with it? Water is water, from a hydrodynamic standpoint. About 300 species, in fact. (Compared to the 2 million described insect species.)

Edited by Sumer, : No reason given.