One important property to keep an eye on is population diversity. Two particular measures that can be useful sources of information are:
Frequency of primitives Recognising when a primitive has been completely lost from the population (or its frequency has fallen to a low level, consistent with the mutation rate) may help to diagnose problems.
Population variety If the variety --the number of distinct individuals in the population-- falls below 90% of the population size, this may indicate that there is a problem. However, a high variety does not mean the reverse. GP populations often contain introns (Section 11.3 ), and so programs which are not identical may behave identically (Gustafson, 2004; McPhee and Hopper, 1999). Being different, these individuals contribute to a high variety. So, a high variety need not indicate all is well. Measuring phenotypic variation (i.e., diversity of behaviour) may also be useful (McPhee, Ohs, and Hutchison, 2008).
Insufficient diversity may cause significant problems. Panmictic6 steady-state populations with tournament selection, reproduction and crossover, for example, are prone to premature convergence. If you find this to be an issue, measures should be taken to encourage population diversity such as: