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This question already has an answer here:

Is symmetry in animals due to DNA or natural selection? By symmetry, I mean the left side is an almost (not perfect) reflection of the right half. Is this due to DNA trying to balance the two half or is it due to natural selection where unbalanced organisms die off more frequently and thus reproduces less. Or is it due to some other factor I had not considered?

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marked as duplicate by MattDMo, Remi.b, kmm, James, rg255 Jul 20 '16 at 7:13

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    $\begingroup$ Your question presents an artificial dichotomy - "Is symmetry in animals due to DNA or natural selection?" Everything about how an organism appears is ultimately programmed in its DNA, (sometimes heavily) influenced by environmental factors as well. Natural selection creates selective pressures that positively or negatively affect the propagation of different DNA sequences. To learn more about natural selection and evolution, I strongly recommend Understanding Evolution from the University of California, Berkeley - it's really great. $\endgroup$ – MattDMo Jul 17 '16 at 17:43
  • $\begingroup$ Welcome to Biology.SE. As MattDMo said, the question presents a very obvious dichotomy that is wrong and make the impossible to answer. DNA is a molecule while Natural Selection is a process. You should make sure to acquire some very basic understanding of evolutionary biology. I am voting to close as "unclear" as the question contains issues of semantic. $\endgroup$ – Remi.b Jul 18 '16 at 10:38
  • $\begingroup$ @Remi.b I have a basic understanding of the two concepts. You are saying it is a dichotomy but this is my question and it is not entirely unrelated. DNA is a molecule, but it is the one that holds the blue print to our bodies. To say it's just a molecule is ridiculous. I want to know if there is a trait in DNA that causes symmetry or if symmetry in complex organisms is do to the process of natural selection. Lastly, you say that I should learn about basic evolutionary biology, but how do I do this if people criticized me for making an effort to ask questions. Isn't that the point of this site? $\endgroup$ – MakPo Jul 18 '16 at 15:19
  • $\begingroup$ @MattDMo it is not a duplicate of that post. His post is asking why some organs in the body is not symmetrical. I'm wondering what makes us symmetrical at all. $\endgroup$ – MakPo Jul 18 '16 at 15:29
  • $\begingroup$ @MakPo you are not being criticized for making an effort to ask questions, and I was simply pointing out an external resource that could help clear up some of your misconceptions about evolution and natural selection. We are just pointing out that you're framing the question the wrong way - it's not DNA vs. natural selection, symmetry (and all other aspects of life) are a result of the processes of natural selection acting on DNA. While DNA is a very important molecule, it is "just a molecule". I would suggest you edit your question to focus on the basics - how did symmetry evolve? $\endgroup$ – MattDMo Jul 18 '16 at 19:29
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Before you consider the development of bilatteral symmetry (which is what you are talking about), you need to look at the overall changes relating to animal body plans.

To begin with, sponges (Porifera) completely lack symmetry and came very early in the evolution of animals. Basically, it's a large cluster of growing cells with some differentiation (ie: choanocytes, amoebocytes, etc.). So the lack of of symmetry is simply related to the lack of complexity.

Then, radial symmetry, which has a "top" and "bottom" but no "left/right" came into play with phyla like Cnidaria. An evolutionary jump from assymetry to bilateral, would be astronomical, so we can think of radial symmetry as an intermediate relating to the increasing complexity of body plan development.

On Bilatteral Symmetry, one key thing is the cephalization. Cephalization is where a majority of the nervous tissue and sensory tissue are clustered in one area (a head). In the evolution of animals, Cephalization came with the evolution of Flatworms (Ptalyhelminthes) where sensory tissue such as ocelli (eye spots) were placed in the head. With the head developed, a "sense of direction" can develop where the right/left/dorsal/ventral sides of the body naturally become distinguished.

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    $\begingroup$ Some of the information contained in this post requires additional references. Please edit to add citations to reliable sources that support the assertions made here. Unsourced material may be disputed or deleted. $\endgroup$ – MattDMo Jul 18 '16 at 19:30

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