Individuals in GE don't necessarily use all their genes. For example, to generate the expression X * X + X only five genes are required, but an individual that generates this expression could well be made up of many more. The remaining genes are introns and serve to protect the crucial genes from being broken up in crossover. While this certainly serves the selfish nature of genes in that it increases the longevity of a particular combination, it is of no advantage to us. In fact, it would make sense for an individual to be as long as possible, to prevent its combination from being disrupted. The fact that such disruption could be crucial to the evolution of the population as a whole is of consequence to the individual. Figure 2 illustrates the effect of too many introns.
To reduce the number of introns, and thus increase the likelihood of beneficial crossovers, we introduce the prune operator. Prune is applied with a probability to any individuals that don't express all of their genes.. Any genes not used in the genotype to phenotype mapping process are discarded.
The effects of pruning are dramatic; FASTER; BETTER CROSSOVER.