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C. FERREIRA Complex Systems, 13 (2): 87-129, 2001

Gene Expression Programming: A New Adaptive Algorithm for Solving Problems

Root Transposition

All RIS elements start with a function, and thus are chosen among the sequences of the heads. For that, a point is randomly chosen in the head and the gene is scanned downstream until a function is found. This function becomes the start position of the RIS element. If no functions are found, it does nothing.

Typically a root transposition rate (pris) of 0.1 and a set of three RIS elements of different sizes are used. This operator randomly chooses the chromosome, the gene to be modified, the start of the RIS element, and its length. Consider the following 2-genic chromosome:


Suppose that the sequence “+bb” in gene 2 was chosen to be an RIS element. Then, a copy of the transposon is made into the root of the gene, obtaining:


During root transposition, the whole head shifts to accommodate the RIS element, losing, at the same time, the last symbols of the head (as many as the transposon length). As with IS elements, the tail of the gene subjected to transposition and all nearby genes stay unchanged. Note, again, that the newly created programs are syntactically correct because the structural organization of the chromosome is maintained.

The modifications caused by root transposition are extremely radical, because the root itself is modified. In nature, if a transposable element is inserted at the beginning of the coding sequence of a gene, causing a frameshift mutation, it radically changes the encoded protein. Like mutation and IS transposition, root insertion has a tremendous transforming power and is excellent for creating genetic variation.

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