Replication Period

The replication period (defined for programs that replicated faithfully at least twice during their lifetime as the number of time slices between the first and second faithful replication) is a more direct measure of a program's fecundity than is program length. From Figure 5.8 we can see that, if the length of the copy loop is increasing in proportion to the length of the program as a whole, we would expect trends in replication period during a run to roughly track trends in program length (because, generally, longer programs take a longer time to replicate in this case). However, if the length of the copy loop remains fairly constant as the program length as a whole increases, we would not expect such an association.

The observed change in replication period over the run is shown in Figure 5.4. As can be seen, the general pattern is very similar to that of change in program length (Figure 5.2), which suggests that the length of the copy loop is increasing as the programs as a whole get longer. This, together with the observation that the replication period increases during the run, suggests that there is some other factor playing an important role in determining the fitness of the programs. As suggested earlier, this factor could be a program's ability to collect energy. We will look into this in Section 5.2.7.