In most systems, selection is determined by an external ``fitness function'' which is fixed and does not change through the course of the simulation. These systems have a blank in the ``Natural Selection'' column. In the Genetic Images system, the selection is determined by the aesthetic whim of the user, indicated by an ``A'' in the table. In the system involving Sims' virtual creatures competing for possession of the block, the external fitness function is fixed: ability to possess the block. However, at the same time, this ability depends in complex ways on the interaction with the other creatures with whom you compete. Therefore we have partial natural selection in this system, indicated by a ``P'' in the table. In a few systems, the selection is completely natural, based only on the ability to survive and reproduce, and is indicated by an X in the Natural Selection column.
Table 1: Comparison of Synthetic Evolution Systems