The process begins with the random generation of the chromosomes of a certain number of individuals or programs (the initial population). Then these chromosomes are expressed and the fitness of each program is evaluated against a set of fitness cases (also called selection environment or training set). The programs are then selected according to their fitnesses (their performance in that particular environment) to reproduce with modification, leaving progeny with new traits. These new programs are, in their turn, subjected to the same developmental process: expression of the chromosomes, confrontation of the selection environment, selection, and reproduction with modification. The process is repeated for a certain number of generations or until a good solution has been found.