ABSTRACT
In biological systems (see section 5.4), crossing-over is a complex process that occurs between pairs of chromosomes. Two chromosomes are physically aligned, breakage occurs at one or more corresponding locations on each chromosome, and homologous chromosome fragments are exchanged before the breaks are repaired. This results in a recombination of genetic material that contributes to variability in the population. In evolutionary algorithms, this process has been abstracted into syntactic crossing-over (or crossover) operators that exchange substrings between chromosomes represented as linear strings of symbols. In this section we describe various approaches to implementing these computational recombination techniques. Note that, while binary strings (Chapter 15) are the canonical representation of chromosomes most often associated with evolutionary algorithms, crossover operators work the same way on all linear strings regardless of the cardinality of the symbol alphabet. Accordingly, the discussion in this section applies to both binary and nonbinary string representations. The obvious caveat is that the syntactic manipulations by crossover must yield semantically valid results. When this becomes a problemfor example, when the chromosomes represent permutations (see Chapter 17)— then other syntactic operations must be used.
