Gene expression programming was invented by myself in 1999 (Ferreira 2001), and is the natural development of GAs and GP.

GEP uses the same kind of diagram representation of GP, but the entities produced by GEP (expression trees) are the expression of a genome. Therefore, with GEP, the second evolutionary threshold - the Phenotype Threshold - was crossed, providing new and efficient solutions to evolutionary computation.

So, the great insight of GEP consisted in the invention of chromosomes capable of representing any expression tree. For that I created a new language (Karva) to read and express the information of GEP chromosomes. Furthermore, the structure of chromosomes was designed to allow the creation of multiple genes, each encoding a sub-expression tree. The genes are structurally organized in a head and a tail, and it is this structural and functional organization of GEP genes that always guarantees the production of valid programs, no matter how much or how profoundly we modify the chromosomes.

In the next section I describe the structural and functional organization of GEP chromosomes; how the chromosomes are translated into expression trees; how the chromosomes function as genotype and the expression trees as phenotype; and how an individual program is created, matured, and reproduced, leaving offspring with new properties, thus, capable of adaptation.