How can evolution explain body symmetry?

Well, ok. Let's start over. I'll summarize the main points of my argument and you can address them, and we'll both try to be very careful about our word use. Ok?The issue seems pretty simple to me. Why do active motile organisims display, in the vast majority of species, body symmetry of some kind?1) All animal organisms either live in environemnts/have lifestyles for which fluid dynamics pose mobility issues, or they are the decendants of organisms that do.
2) In all studied cases body symmetry seems to be positively selected for either sexually or by environment.I don't see the weakness of either of these points; it would not be correct to say that (1) is based on an "assumption" of evolution, but rather, on the conclusion of evolution from other lines of evidence. Insects largely co-evolved along with flowering plants; there aren't too many flowering plants at sea. Also they would largely be outcompeted by their crustacean cousins, who are already there.

I apologize... I posted examples of symmetry, but not bilateral symmetry which is what is being discussed. my mistake.This message has been edited by 1.61803, 03-15-2005 05:51 PM

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Hypothetically, if we remove the gas/liquid medium, and the insects were to evolve on out planet in the vacuum, would they need the symmetry?

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They would need symmetry of propulsion, just like all spacecraft. A daddy long legs with 1 foot on one side and 10 on the other would just move around in circles. Not very effective. A cheetah with 3 feet on one side and one on the other would do the same. Not only do we have to factor in fluid dynamics but we also have to look at propulsion and weight balance. Even the basic design of our automobiles are symmetric due to this factor. Four contact points make for a very stable and even propulsion, allowing for strong manuevering and balance.Also, the ancestors of all terrestrial insects are aquatic archopods. You know, like shrimp, lobsters, crabs, etc. So there is a basis for fluid dynamic adaptations.

I think it was Hamilton who suggested that one not think in adaptationst terms if the process can be reduced to a physical principle and Maynard Smith who suggested that we do not need to refer to gravity as to explaining leaf fall but you see as long as biologists continue to only look at the relation to gravity as a RESPONSE during development they will never think to ask if the symmetry is rather not an expression of falling from the plant along a geodesic in terms of Newton's straight lines that the plaent itself tracks in time. The reason for this failure to make such a down to earth connection seems to be in the need to renounce commutativity and we just dont get taught that that much in school.So is it evolution or constraints on the morphospace due to invariants that results in the symmetry observed? I wish that could be explained without the question mark. I have not been able to advance beyond the word "apex" in that regard.My comments work for bilateral symmetry in plants but for the reasons I gave we dont see how this loss of commutativity in pedagogy only might be the same as animals' bilateral symmetry genetically- oh well- they were nice pics while they lasted!This message has been edited by Brad McFall, 03-15-2005 06:02 PM

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Right, no creature is perfectly symmetrical. But there is this symmetry principle for animals--I don't know if it is "magic or divine," I'd call it design (or construction) principle. Hypothetically, why don't our friend daddy longlegs develop one side flat and furry for sleeping on, and the other side with some protection function to defend it when asleep?

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Well, I have already given you my answer - because it is simpler to have one design you copy twice than two different designs. Yes I could imagine a badly asymmetric animal - and there are some crabs that are very unbalanced. But these exceptions seem to prove the rule - there is no magic need for symmetry, and it can be explained like everything else as a useful adaptation.I'd like to propose a something similar for you consideration. All animals work on an energy budget; if you are going to pick food up to eat it, it is pointless to lift it against gravity again. Thus, our mouths are higher than our anuses; we let gravity help us rather than wasting energy fighting it. This is efficent.From that observation I propose a rule from first principles: there will be no organisms found with anuses higher than their mouths.There might be some exceptions - I'm thinking of fish mostly, because the energy cost of lifting mass in water is discounted. But I think the general case is probably pretty solid. There might also be certain aspects of mechanical ebngineering that counteract this in specific instances. But as I hope you can see, such a rule, if true, would also apply to any alien animals we encounter - becuase regardless of the specifics of their local ecosystem, the universe always works the same way (as far as we know).

Hi Sumer,Just to clarify the issue regarding symmetry in plants - plants are somewhat different to animals in that they are constructed of numerous repeated units (leaves, flowers, roots, etc.) which are structurally homologous. Now you do find this kind of structure in some animals (millipedes, centipedes, starfish) but it isn't common in higher animals. Symmetry in plants is a very active research area. For flowers, see "Evolution of floral symmetry" by Endress (Curr Opin Plant Biol. 2001 Feb;4(1):86-91), for leaves, see "Patterns and symmetries in leaf development" by Pozzi et al (Semin Cell Dev Biol. 2001 Oct;12(5):363-72.). The latter article is particularly relevant - here's a quote from the abstract:"The subdivision of the meristem into molecularly defined domains is regulated by the interactions of a number of gene products and by receptor kinase-mediated signals. The acquisition of symmetry axes in the emerging leaf is a process coordinated by hormones (such as auxin and cytokinins) and the expression of classes of genes (such as the knox and the ARP, as1/rs2/phan, genes). As with simple leaves, the architecture of compound leaves is defined by spatial/temporal gradients of regulatory gene functions: complexity results from the interplay between leaf differentiation processes and genes maintaining a partial level of indeterminacy in the developing primordium."The differences you describe in the symmetry of animals and plants are largely due to the modular nature of plant tissues which results in symmetry being expressed in low-level structures (leaves, flowers) rather than in the high-level body-plan. But the quote from Pozzi et al clearly shows that the genetic processes underlying symmetry in plants and animals are broadly similar. Interestingly, the macroevolutionary processes that result in these patterns of symmetry in plants and animals are also similar. Others in this thread have mentioned the importance of sexual selection in maintaining symmetry in animals. According to Moller (EXS. 1997;83:255-68.) this also applies in plants:"Sexual selection may give rise to increases in the general level of stress experienced by individuals, either because intense directional selection reduces the ability of individuals to control the stable development of their phenotype, or because extravagant secondary sexual characters on their own impose stress on their bearers. Sexual selection often acts against individuals with asymmetric or otherwise deviant phenotypes, particularly if such phenotypic deviance occurs in secondary sexual characters. A small number of studies suggests that such characters also are more susceptible to the disruptive effects of deviant environmental conditions than are ordinary morphological characters. Plants often show extensive phenotypic asymmetry, and pollinators avoid asymmetric flowers, either because they are generally less attractive or provide fewer pollinator rewards. Floral symmetry may give rise to sexual selection with direct or indirect fitness benefits, as in animals. "Hope this is of interest!Mick

Interesting idea, but in need of major revision, I think.First, it would seem to be that the position of the stomach relative to the mouth would be more important, since we aren't significantly assisted by gravity for passage of (ex)food from our stomach to our anus - in fact a significant portion of our GI tract is orientated so that the fecal matter is moving against gravity (hence the "ascending colon"). Also, we retain the ability to pass food from mouth-to-stomach standing on our heads.Also, this rule assumes that organisms maintain a standard orientation relative to gravity. There are plenty of organisms that have no such orientation - worms, many insects and aquatic creatures.Additionally, there are many organisms that spend feeding time with their stomachs or anuses higher than their mouths - many bats feed while hanging upside down.Perhaps a more testable (and less general) idea is that organisms' potential esophageal peristaltic force will correlate in a meaningful way with their standard mouth-stomach orientation during feeding. In other words, a bat should have greater peristaltic force than a rat.I think this sort of analysis would test your essential concept of evolution of gastrointestinal characters relative to orientation to gravity - definitely much better than counting the heights of mouths and anuses...

I think this is a good point. I hope I'm not deviating from your point by saying, ...We can learn a lot about symmetric design by looking at things designed by humans. So much of what we design is symmetric. Cars, spoons, even milk cartons (I'm eating breakfast). Why are these things symmetric?A lot of the principles are the same between evolved body symmetry and designed symmetry. And often times designed symmetry is easier to understand for the layperson. Maybe it's useful to embark on this path a bit within this thread.

I found that interesting. Thanks for the info. I think Brad does a lot of studying on (judging from the photos he uploads), and I think this was one of the most believable things presented within Wolfram's texts (yes I realize it's most likely not his original idea). I found this quote interesting. I wonder if, for the visual system, visual symmetry of a scene / object affords a computational advantage in object identification and classification. It seems like a reasonable suggestion.Either way, the question of "why are symmetric things 'attractive' (and not just symmetric, but also 'balanced,' such as the judgement of the writing of Chinese characters) seems like an important one in understanding "sexual selection" and some human design principles. It screams 'computational principles' to me, and 'computational principles' translate to 'macroscopic evolutionary pressures'--since computational principles are the very foundations of behavior, and macroscopic behavior is one driving force of evolutionary pressure.

I have six natural children. Only my last three had extra fingers. The fourth and sixth had one on each hand, and the fifth had only one (on the left hand, I think...he's not here for me to check).My dad had it on both hands, and I believe my brother had it on only one, the only one of us four children to have it. His oldest son had at least one extra finger, but I don't know whether he had two.The extra finger is small and attached by only a sliver of skin, though it does have a nail on it. They've all had it removed, because otherwise it would surely be torn off.

see mick's above
I am not sure, that

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in the developing primordium."

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is the correct nexus binding the discussion of plant and animal similarities even though Gould seems to have sanctioned the thought. I have instead thought something different.The automata is from Wolfram's book and the Croizat pics from PRINCIPIA BOTANICAOf course if Mick and I were to debate this point I would loose in the same sense that I do with Mammuthus because my own idea vs "literature" is hard ever to 'win.' But BEN!!!!!!!!!!!! thanks, for REALIZING what my different perspectives' might indeed be. That is all a scholar can ever hope for.The term in dispute might be meristic variation. The strength of my position is that regardles of what the "debate" is about, i can see plants and animals, under the same set of transformations and that is simpler than all the teachings that FIRST set up linguistic differences that are THEN substantiated by research. So if the Chinese might have seen the symmetry that Pascal later saw and I see some symmetry that Croizat might have already purposed to Da Vinci et al, these provisions are not complex and thus more easily accepted even if there still exists a showing of less paradigmatic strength than the stardard forbears as re-presented by MICK for their backing. I have no qualms with trying to use words to understand form but the forms themselves and not the researchers' subjective viewing seuquence in educational time( or academic history for that matter) is what determines the truth of the comparative approach to the matter. Maybe my symmetric comparasion of plants and animals doesnt work. It might!
I can see plants and animals with the same brain without having to think through cells to organism evolution proposals. Those only come after, not before.I would love to work up the second suggestion in terms of cRoiZat's contributions and my own rather arbitrary combination of plant and animal geography but these thumbs are still a bit too yellow. This message has been edited by Brad McFall, 03-16-2005 08:07 PM

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Additionally, there are many organisms that spend feeding time with their stomachs or anuses higher than their mouths - many bats feed while hanging upside down.

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I did think about that, but decided that bats and similar might well excrete only when upright, and so not need unusually strong peristaltic muscles. This would also save on socially embarrassing "shit fur", but I'm no expert on bats.Also, in terms of the rising colon, the bolus I would think is in receipt of significant suspension by the body tissues, even without actual peristalsis, and so this may not impose a significant cost. Hence I addressed only the input and output "ports".But yes, thanks for the input on a totally off-the-cuff idea. I subsequently though of another thing - the vertical, i.e. gravitational, axis is pretty much a given. It seems to me that most/all symmetry in animals is perpendicular to this vertical axis.

Sorry - I thought you were arguing that gravity aided the passage of food from mouth to anus; now I'm not sure what you mean. Perhaps that gravity aids food falling into our mouths and feces falling out of our asses? Then all that should matter is that most mouths/inputs face upwards, and most anuses/outputs face downwards, which is not the case. This is quite off - without peristalsis the GI tract fills and expands until it ruptures, killing the organism (as in Hirschsprung disease).Gravity has a significant effect on internal organs other objects/fluids within our bodies. It seems like you are stating that the opposite is true - "suspension by the body tissues", whatever that means, doesn't negate gravity.

OK, let’s try and take a step back towards the embryo again .I’m going to stick my neck out here and say that it isn’t symmetry that is needed to be explained in multicellular organisms, it’s asymmetry. If you think about dividing cells in an embyro, symmetry is produced naturally without need for magic or ‘design’ — just by the process of cell division.When one cell divides into two identical cells, symmetry is produced. Another plane of symmetry is formed when two cells become four and so on. Something different has to happen in order for asymmetry to occur: different signals have to be received on one side and not the other. I think this was what Mick was getting at in his first post.If the cells in the bottom half of an embryo start to develop differently to those at the top then you’ve lost symmetry. Likewise, if those at the front receive different signals to those at the back then you’re left with only bilateral symmetry. There are even examples of the last plane of symmetry being lost (like the internal organs) that involve different proteins being produced solely on the left or the right side.So, it’s no longer a question of:"Why did animals evolve symmetry?"It is more like:"Why did animals retain a certain amount of symmetry?""and why do plants not display this particular type of symmetry?"The answer is of course because the selective pressures made it an advantage. Animals and plants have different selective pressure (including things like mobility) and therefore have evolved different types of body pattern.This message has been edited by Ooook!, 17-03-2005 04:44 PM

I followed my own train of thought to this point but I still havent reached selection but instead that metazoans are being often improperly compared in terms of water balance and excretion to protos. Thus the contractile vacuole, laced with ER, is thought to be an adaptation in the metazoan sense for hypotonic environments as it is not found in the protozoans that live in the isotonic environs but instead I speculate that the intake of water is compared with the force of movement of the cell sorting out supramolecular entities. Thus to the extent that metazoans have static endoplasmic reticulum compared with the that in the proto's contracts they retain whatever symmetry they can that way.Of course carring the argument this far without saying how natural selection operates is a bit far but there is still room for more evolutionary thinking in my train trailing words.Nice thought Ooook! Thus in my position the key to the different kinds of symmetry of life lay in the endoplasmic reticulum.This message has been edited by Brad McFall, 03-17-2005 12:01 PM