How can evolution explain body symmetry?

I found an interesting article with lots of examples of asymmetry and the mechanisms by which it is produced: http://www.drmichaellevin.org/Chiralityconf1.pdfHope it is of interest

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

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

Hi Oook,Good points. I came across an interesting editorial in Nature regarding the development of assymetry in vertebrates. Apparently the basic problem is that the early developing embryo has no blood circulatory system, so it uses cilia which waft fluids around the "body". I guess it's more efficient to waft fluids in a circulatory manner, so the cilia all move in the same direction and circulate body fluids in an anticlockwise direction. This circulatory movement of fluids in the early embryo is enough to move particulate matter (I guess this refers to cells?) and results in the initial origin of asymmetry in vertebrate embryos.I don't understand this stuff completely, not having read much on embryology either, but the editorial is available at shorter linkmick Long link shortened by AdminJar. Use peek mode to see how you can avoid long links that expand beyond the page edgeThis message has been edited by AdminJar, 04-04-2005 04:47 PM

Don't forget the starfish anus. Having an anus is an ancestral state for starfish but there is at least one clade in which it has been lost, and excreta is regurgitated directly from the mouth. Of course this is completely off-topic. You might even accuse me of talking out of my anus.mick

Hi Sumer, great question. Apparently the fin whale is always asymmetric in its coloration (the right side of the head is always pale, the left side always dark). It seems that when they feed, they always keep the pale side of their head pointing downward into the water, so there may be some sort of adaptive explanation for it.There is a picture of a fin whale at New England/Mid-Atlantic | NOAA Fisheries where you can see the difference in coloration.mick

Sumer,phallostethid fishes have an asymmetric priapum, which is a unique clasping structure used by males to grasp onto females during mating. Exactly as you predict, females are also asymmetric, and "left handed" males mate more successfully with "right handed" females. See Just a moment...Mick

Hi Brad,Thanks for your last message, it will take a bit of digesting so I'm just letting you know I skimmed over it and will look at it properly over the next few days.Anyway, i immediately wanted to say I agree that teleology is an interesting and problematic issue in evolutionary biology. At the moment I haven't really made up my mind about it. I don't see any immediate reason why "random" mutations shouldn't be teleological sensu stricto the creationist viewpoint. i'm just not sure of any examples from nature that would change me from being ambivalent to convinced one way or the other. at the moment, I'm keeping an open mind.More later,Cheersmick

brad, have you ever considered submitting some of this stuff to a major art competition? I strongly suspect you would win!added in edit:your personal image, below your name, is a bit like the artwork of dave mckean, for example.Best wishes,MickThis message has been edited by mick, 04-30-2005 08:55 PMThis message has been edited by mick, 04-30-2005 08:57 PM

nice idea for a research project though, peter. You need to find a bunch of closely-related species, some of whom are blind and some of whom can see. You might be able to find these in insectivora or rodentia.next you measure morphological asymmetry, fluctuating asymmetry or what have you, in each species. Complete your analysis with a phylogenetic test of the prediction that fluctuating assymmetry tends tends to increase when a lineage loses the ability to see.publish if you get a phylogenetic correlation, add to the filing cabinet if you don't.

can I ask what it is?Mick

Thanks!Is that its real colour, or is it under UV light?mick

Presumably you would agree that this also applies to people who disbelieve in evolution. By your own argument, doesn't your admission invalidate any claim you make about evolution, including the claim that it is spread solely or laregly by indoctrination?Mick

Er... what are these things? Are they similar to F4, TK and R*?This message has been edited by mick, 08-16-2005 07:11 PM