For the sake of simplicity, we are going to illustrate the mechanisms of gene transposition using the compact, linear representation of chromosomes used to describe the
structural organization of
chromosomes in the previous chapter. In this representation, each element (function or terminal) is represented by a single character so that each element can be easily identified by its position in the chromosome.
In gene transposition an entire gene works as a transposon and transposes itself to the beginning of the chromosome. In contrast to the other forms of transposition, in gene transposition, the transposon (the gene) is deleted at the place of origin.
The default value for the gene transposition rate in APS 3.0 is 0.1, as this operator is usually used together with other, more powerful operators such as
mutation. But if you want to introduce genetic modification by using this operator alone, the better results are obtained with gene transposition rates of 1.0.
The gene transposition operator randomly chooses the chromosome to be modified and then randomly chooses one of its genes (except the first, obviously) to transpose.
Consider the following chromosome composed of three genes, each with a
head length of 7:
012345678901234012345678901234012345678901234
/cQQ*bQbccadbdc-cbd+Qdbaabdacd+*dbbcbccaabcad
Suppose gene 2 was chosen to undergo gene transposition. In this case the following chromosome is obtained:
012345678901234012345678901234012345678901234
-cbd+Qdbaabdacd/cQQ*bQbccadbdc+*dbbcbccaabcad
Apparently, gene transposition is only capable of shuffling genes and, for
sub-expression trees
(sub-ETs) linked by commutative functions such as addition or multiplication, this contributes nothing to adaptation in the short run. Note, however, that when the sub-ETs are linked by a non-commutative function such as subtraction or division, the order of the genes matters and, in this case, gene transposition becomes a macromutator. However, gene transposition becomes particularly interesting when it is used in conjunction with
recombination, for it allows not only the duplication of genes but also a more generalized shuffling of genes or smaller building blocks.
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