Cladistic language concepts

[Michael Ghiselin]

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          Dear Dr. Dyen:
               Yes, we can separate different parts of the genome
          according to how rapidly they change.  The DNA that codes
          for RNA changes relatively slowly though it depends on the
          part of the RNA molecule.  It is thought that the most
          conservative parts of the RNA molecule are areas where a
          change would abolish function: and since the function is
          making proteins that would be a lethal.  Mitochondrial RNA
          evolves faster than nuclear RNA and the reason seems to be
          that there is less of that kind of constraint.
               I am not sure what you mean by rates of branching.  If
          there is rapid speciation the probability that there will be
          evidence of this in the data is greater if the molecule in
          question is evolving rapidly.  But remember that branching
          rate is a different thing from the rate of substitution.  It
          would be wonderful if all substitutions took place at a
          constant rate, for then we would have a "molecular clock"
          but we don't
          Sincerely,
          Michael Ghiselin