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mutation is extremely important for GEP evolvability and several new traits
are introduced in this manner. However, less drastic mutations can also be
found in gene expression programming. As a matter of fact, some mutations
change expression trees very smoothly and they might slightly or significantly
increase the efficiency of the expression tree. Furthermore, in gene expression
programming, some mutations also have a clear neutral effect. For instance,
mutations in the noncoding regions of genes have no effect whatsoever in
the structure of expression trees. Other neutral mutations are not so easy to
spot because they result in structurally different expression trees. In this case,
the new expression tree is equivalent (in mathematical terms) to the parental
expression tree. We will see that all kinds of mutation, from the most con-
servative to the most radical, are important to the evolution of good compu-
ter programs.
Recombination
Proteins gradually evolve by accumulating different kinds of mutations over
eons of time. But mutation is not the only source of genetic diversity. In the
remainder of this section other important genetic operators are presented.
One such operator is recombination. In nature there are varied kinds of
recombinational processes, involved in different processes and playing dif-
ferent functions. However, during all recombinational processes some frag-
ments of genetic material are exchanged between two distinct donor mol-
ecules, as such that genetic information from each donor is present in the
offspring (Figure 1.4). For example, during sexual reproduction two paired
homologous chromosomes exchange DNA fragments.
The simple image of homologous recombination where homologous chro-
mosomes (chromosomes with extensive sequence homology) are paired, can
be useful to help understand the recombinational processes used to create
genetic diversity in GEP populations, although in the latter case no sequence
homology is required. This simple image is nonetheless very handy, because,
in GEP, recombining chromosomes share a structural homology and also
because two new daughter chromosomes are created in the process. Thus,
during recombination, two chromosomes (not necessarily homologous) are
paired and exchange some material between them, forming two new daugh-
ter chromosomes. Note, however, that due to the structural homology, a frag-
ment of a particular gene occupying a particular position in the chromosome
is never exchanged for a fragment of a gene in a different position; or a
fragment of the gene tail is never exchanged for a fragment of the head. In
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