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In a prebiotic environment smaller chains of RNA and smaller molecules have the competitive advantage over longer chains and molecules.  The elaborate defenses are not yet in place.  Any "improvement" in the small chains is readily lost.

   If you want to argue over my description of the source of genetic variation then you should have one different from mine, no?

   The source of genetic variation has been assumed to be a failure to copy.  Genes have to be copied to be passed to other generations.  The mechanism of duplication has requirements.  An unhealthy animal might be missing certain required nutrients or have conditions not suitable to the duplication process.  That is often described for lack of better terms as "random."  The failure to copy an existing pattern might in unusual cases cause a pattern with various advantages the old pattern did not have.

   That is the basic foundation of evolutionary theory, which no one here is arguing.  It is however irrelevant to the discussion of the origin of life.  As explained here it has been observed in laboratories for decades that short RNA chains do not develop long ones because the shorter chains have the competitive advantage.  The longer chains have no protective phospholipid bilayer or cell walls to prevent the onslaught.  The short chains are more difficult to catch hold.