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2.3 Cells and the Evolution of Linking Functions
The linking of sub-ETs by a particular linking function is simple and highly
efficient in evolutionary terms. Indeed, from unigenic to multigenic systems,
efficiency increases considerably (see, for instance, section 5 of chapter 12
for a discussion of The Higher Organization of Multigenic Systems). De-
spite this artificial increase in complexity (“artificial” in the sense that it was
not evolved by the system itself), evolution in multigenic systems still oc-
curs efficiently and therefore they can be efficiently used to evolve good
solutions to virtually all kinds of problems.
In principle, it is possible to impose from outside higher levels of com-
plexity, but this is no guarantee that an increase in performance will be
achieved. Natural evolutionary systems are not created this way: higher lev-
els of complexity are created above lower levels and the evolution of com-
plexity occurs more or less continuously.
Notwithstanding, the linking of sub-ETs in gene expression programming
can be implemented by using a higher level of complexity. For that purpose
a special class of genes was created - homeotic genes - that control the
development of the individual. The expression of such genes results in dif-
ferent main programs or cells, that is, they determine which genes are ex-
pressed in each cell and how the sub-ETs of each cell interact with one an-
other. Or stated differently, homeotic genes determine which sub-ETs are
called upon (and how often) in which main program or cell and what kind of
connections they establish with one another. How this is done is explained in
the next section.
2.3.1 Homeotic Genes and the Cellular System
Homeotic genes have exactly the same kind of structural organization as
conventional genes and they are built using an identical process. This means
that they also contain a head and tail domain, with the heads containing, in
this case, linking functions (so called because they are in fact used to link the
different sub-ETs encoded in the conventional genes) and a special class of
terminals - genic terminals - representing conventional genes, which, in the
cellular system, encode different sub-ETs or sub-programs; the tails contain
obviously only genic terminals.
