Information Technology Reference
In-Depth Information
The comparison of the sequence of this perfect solution with the sequences
of all its ancestors shows that it is a direct descendant of chromosome 5 of
generation 12. The event of IS transposition that led to the creation of this
perfect solution is shown in Figure 3.19.
As you can see in Figure 3.19, during IS transposition, IS elements are
copied into the head of the target gene. In this case, the sequence “cab” (posi-
tions 14-16 in gene 1) is activated and jumps to the insertion site, namely, bond
1 in gene 1 (between positions 0 and 1). As a result, a copy of the transposon
appears at the site of insertion. Note also that a sequence with as many sym-
bols as the IS element is deleted at the end of the head (in this case, the se-
quence “caa” is deleted). Thus, despite this insertion, the structural organiza-
tion of chromosomes is maintained and, therefore, all the new individuals cre-
ated by IS transposition are syntactically correct programs.
Note that IS transposition can also be a macromutator, causing very pro-
found modifications in the expression trees. Obviously, the more upstream
the insertion site the more drastic the change. For example, the sub-ET
1
shown
in Figure 3.19 was shortened by two nodes due to transposition. But obvi-
ously IS transposition has also the power to make small adjustments in the
sub-ETs, making IS transposition a very well balanced genetic operator.
Also interesting is that neutral sequences on the noncoding regions of
genes might be activated by this operator and jump to another place in the
genome where they will get expressed. This was indeed what happened in
the example of Figure 3.19, where a sequence in a noncoding region jumped
to the middle of a coding region, changing considerably the sub-ET.
Root Transposition
All RIS elements start with a function and, therefore, must be chosen among
the sequences of the heads. Thus, a point is randomly chosen in the head and,
from this point onwards, the gene is scanned until a function is found. This
function becomes the first position of the RIS element. If no functions are
found, the operator does nothing. So, the transposition operator randomly
chooses the chromosome, the gene to be modified, and the start and termina-
tion points of the RIS element. Typically, a root transposition rate
p
ris
of 0.1
is used as this operator is seldom used as the only source of genetic variation.
In the example shown in Figure 3.20, a much higher RIS transposition rate
of 1.0 was chosen as only this operator was used to create genetic modifica-
tion. By the results shown in Figure 3.20, you can see that this operator on its
own is also capable of making populations evolve efficiently. In this case, by
