If there were no mutations at the outset of the run, there would be no evolution. However, the bits flipped as a result of copy errors or background mutations result in creatures whose list of 80 instructions (genotype) differs from the ancestor, usually by a single bit difference in a single instruction.
Mutations in and of themselves, can not result in a change in the size of a creature, they can only alter the instructions in its genome. However, by altering the genotype, mutations may affect the process whereby the creature examines itself and calculates its size, potentially causing it to produce an offspring that differs in size from itself.
Four out of the five possible mutations in a no-operation instruction convert it into another kind of instruction, while one out of five converts it into the complementary no-operation. Therefore 80% of mutations in templates destroy or change the size of the template, while one in five alters the template pattern. An altered template may cause the creature to make mistakes in self examination, procedure calls, or looping or jumps of the instruction pointer, all of which use templates for addressing.