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fitness. Then the roulette is spun as many times as there are individuals in the
population so that the population size is maintained from generation to
generation. And, obviously, the bigger the slice the higher the probability of
being selected.
This kind of selection, together with the simple elitism presented in sec-
tion 3.1.2, was used in all the problems of this topic. It is true that with this
selection scheme, sometimes some of the best individuals might be lost, while
the worse or mediocre ones are passed on to the next generation. But this is
not necessarily bad: for one thing, these good individuals might be created
again and be more fortunate in the next round and, for another, the descend-
ants of the mediocre individuals won't necessarily be mediocre and adapta-
tion just goes on, perhaps through an unexpected path that might lead to the
global optimum. Nonetheless, due to the cloning of the best individual of
each generation, the survival and reproduction (without modification) of the
best is guaranteed, what, in practical terms, means that at least the best trait
is never lost and a continuous improvement is also accomplished.
Other selection schemes can be found in the literature but the most popu-
lar - roulette-wheel, deterministic, and tournament selection - will be com-
pared in chapter 12 (section 7, Analysis of Different Selection Schemes).
And we will see that, if elitism is present, no appreciable difference exists
between all these methods. Indeed, evolutionarily speaking, all of them are
very good if the rest is also good. And the “rest” is the essence of each artifi-
cial evolutionary algorithm. And, at least for GEP, this essence does not re-
side in the kind of selection mechanism used but in the power of the genetic
operators operating on a fully functional genotype/phenotype system. As we
will see in the next section, the genetic operators are the real “eagles” of the
fitness landscape.
3.3 Reproduction with Modification
According to both the fitness and the luck of the draw, individuals are se-
lected to reproduce with modification. And this creates the fundamental ge-
netic diversity that will allow 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 yet more mutations, occur after replication.
Thus, in nature, it is not always possible to know when a modification took
