In all of the synthetic evolution systems reviewed here, the genetic system is ultimately represented by a bit string. In some systems, the composition of this string is fixed. Such systems have a blank in the ``Evolving Genetic Structure'' column of the table. In others, the composition of the string is able to evolve, so that the form of the ``solution'' represented by the string is not fixed, but changes through evolution. Such systems have an X in the ``Evolving Genetic Structure'' column of the table.