Humans are (almost) symmetrical. We have two legs at same length and two arms at the same length. We have two eyes at the same height and the same distance from the nose. In the same way, animals are symmetrical and even flowers and leaves. Does evolution say anything about it? How come we don't have three legs or one eye lower than the other? Why does a flower have the same length and shape for each petal?
Evolution definitely has something to say about it: http://en.wikipedia.org/wiki/Symmetry_in_biology#Bilateral_symmetry
Coding efficiency is one factor - symmetry is the default state of development, with consistent and functional (non-pathological) asymmetry requiring much more information, extra genetic material and many more coherent and viable evolutionary events.
The wiki article merely acknowledges symmetry in different organisms. It doesn't answer as to WHY there is symmetry and how it occurs. An animal runs probably faster if both legs have the same length thus the slower runners die and the symmetric animal makes its symmetrical offspings. But what about flowers? What is the advantage of having 5 equal petals? A flower can't run. Sexual selection can't work with flowers either as their reproduction depends on insects. So logically you shouldn't expect the majority of flowers to have symmetry.
Could you elaborate on that? And preferably in a more layman's language. State of development of what? Are you talking about the DNA helix?
The Wikipedia article references: The Origin, Evolution and Development of Bilateral Symmetry in Multicellular Organisms (pdf) which you might find more informative. As for floral symmetry in particular, perhaps you could take a look at: Floral symmetry genes and the origin and maintenance of zygomorphy in a plant-pollinator mutualism (pdf) Ontogenetic origins of floral bilateral symmetry in Moringaceae (Brassicales) Developmental genetics of floral symmetry evolution (pdf) Evolution of floral symmetry: a state of the art (pdf) There's a hell of a lot more out there on the topic as well.
Oh, and here's a focus on a selective pressure, not that that is all there is to it: Bumblebee preference for symmetrical flowers (pdf)
Symmetry implies minimal entropy compared to random. Picture a gas with molecules moving and colliding in a random fashion. All of sudden, the gas molecules continue exerting pressure, but now in symmetrical way. This is an analogous example of minimizing the entropy via movement into symmetry. The paradox is, the entropy of the universe is supposed to increase, not decrease. Moving into symmetry would be like taking six dice and each time throwing six-sixes. You need to load the dice; sense of direction. If we use the selective pressure argument, flower symmetry leads to loss of entropy. The easiest way to explain this, is with the entropic force, which is the fifth force of nature. The entropic force is based on entropy. This force can be demonstrated in the lab with osmosis and water. The osmotic pressure equals force/area, with this force the entropic force since it generated by water entropy. If we apply pressure (selective) to the output of an osmotic device, we can create reverse osmosis. The reverse osmosis will cause the entropy of the water to decrease back into pure water; symmetry. Water hydrogen bonds into symmetrical clusters. The value of this is loss of entropy, is entropy contains energy, while decreasing entropy into symmetry releases energy for other things. This can show up as higher efficiency. Based on statistical models, proteins should have an average fold due to weak binding within proteins and the thermal fluctuations, yet experimental evidence shows protein show unique folds. The cell gets rid of the expected entropy within the predicted protein randomness, and in favor of a specific fold. This is another example of induction toward lowered entropy. Symmetry is also connected to lowered entropy and makes use of the same driving force. Both are due to water and the entropic force, which are generated within life due to membranes, selective diffusion and pressure.
Symmetry existed in chemical forms, long before there were biological entities to evolve. Consider that all organic matter is made up of molecules that were symmetrical, and it becomes obvious why these molecules form symmetrical structures. It's all about stability. Asymmetrical things fall over.
Evolution might explain certain aspects of symmetry for biological stuff, but symmetry is also common in a lot of non biological stuff. I suspect it has something to do with chemistry and the laws of physics. Please Register or Log in to view the hidden image! Please Register or Log in to view the hidden image!
This is a difficult subject because it involves the very complex and ill understood interaction between biochemistry and geometry.
Embryonic development - the growth of the form - is managed by evolved genetic code in feedback with a given environment. Given an environment, simple code produces symmetry - the same rule followed the same way at all the growth locations will produce matching structures at the equivalent times and locations: symmetry, from the observer's point of view. To produce an asymmetrical structure there must be more information supplied; usually, if not supplied by accident and mishap and incoherent feedback from the embryonic environment, that would mean auxiliary or additional or modified genetic code that acts to identify the locations separately and manages their development differently as the embryo grows (note that every cell must carry and properly employ all this code). Symmetry is "easier", is all. Less code, less complexity, fewer possible problems in development, fewer successful mutations and evolutionary selection events to begin with, etc. Given that basic situation, actual evolutionary selection for symmetry (sexual or otherwise) can arise, because most asymmetry would be accidental or pathological or otherwise evidence of weaker, less robust, more vulnerable genetic management - it would pay (in an evolutionary sense) to be able to recognize and (usually) avoid asymmetry in any situation in which benefit was to be had from well-functioning and robust genetic management. In addition, sensory organs built as efficiently as possible (i.e. mostly symmetrical) are almost certain to register symmetrical input as reinforcing and clarifying- as a stronger signal or one more easily registered at weaker levels, and one more easily filtered for noise (the noise is unlikely to match).
It is easy to understand when you think of the fact that all of your body was formed from one cell that divided into two parts one that formed you left and one that formed your right, then it split again into two top cells and two bottom cells and those wen off to create the different parts of your body so instead of having two foot genes you have one foot gene that can be simply used as a left foot on one side and a right on the other side depending on if the left or right proteins are present during the formation of the feet. To deviate from this simple structure would be extremely difficult for a species to survive because it would involve significant genetic complexity and it would lead to many mistakes that could be fatal. This is fairly simplified version but you get my point. The truth is DNA in and of itself is not that complex, but once you put it into context it changes small things that turn into big things down the line. SO the same sequence of DNA can be used for the production of many traits or alleles
The easiest explanation is connected to entropy. Say we build a machine, the waste heat due to inefficiencies will result in an increase in entropy. To increase the efficiency we need to lower the entropy. Symmetry is not connected to randomization or higher entropy. It implies a path toward lower entropy. Lower entropy, in turn, means higher efficiency. This is not too far from natural selection, with the highest efficiency of design, having selective advantage. If we started life from scratch, with low efficiency, we would still end up with two arms and two legs since this is most efficient; minimizes entropy.
