In this section:
According to both fitness and luck of the roulette, individuals are selected to reproduce with modification, creating the necessary genetic diversity allowing for adaptation in the long run.
In nature, several modifications are introduced during the replication of the genomes (e.g., mutations, small deletions/insertions, etc.); others, like homologous recombination and mutations, occur after replication. Thus, in nature, it is not always possible to know when a modification occurred. In GEP, though, we know exactly when modifications occur. On the one hand, all of them occur after the replication of the genome. On the other, the genetic operators perform in an orderly fashion, starting with replication and continuing with mutation, transposition, and recombination. However, the order in which the latter modify the genome is not important to the final outcome.
Except for replication, which copies exactly the genomes of all the selected individuals, all the remaining operators randomly pick up the chromosomes to be subjected to a certain modification. However, except for mutation, each operator is not allowed to modify a chromosome more than once. For instance, in a population of 10 individuals, a crossover rate of 0.8 means that eight different chromosomes are randomly chosen to recombine.
Thus, in GEP, a chromosome might be randomly chosen to be modified by more than one genetic operator at a time. Therefore, during reproduction, the modifications carried out by the different genetic operators accumulate in the chromosomes, to the extent that the new population is usually very different from its predecessors.
Presented below are some of the genetic operators used in GEP, in the order in which they are usually applied, obviously starting with replication and selection.