Page 31 - 49A Field Guide to Genetic Programming

2.4 Recombination and Mutation 17

Parents Offspring

Mutation Mutation
+ Point + Point

+ 3 + ∗

x y x y y /
Randomly Generated
Sub-tree x 2
∗

y /
x 2

Figure 2.6: Example of subtree mutation.

per-node basis. That is, each node is considered in turn and, with a certain
probability, it is altered as explained above. This allows multiple nodes to
be mutated independently in one application of point mutation.
The choice of which of the operators described above should be used
to create an offspring is probabilistic. Operators in GP are normally mu-
tually exclusive (unlike other evolutionary algorithms where offspring are
sometimes obtained via a composition of operators). Their probability of
application are called operator rates. Typically, crossover is applied with the
highest probability, the crossover rate often being 90% or higher. On the
contrary, the mutation rate is much smaller, typically being in the region of
1%.
When the rates of crossover and mutation add up to a value p which is
less than 100%, an operator called reproduction is also used, with a rate of
1 − p. Reproduction simply involves the selection of an individual based on
fitness and the insertion of a copy of it in the next generation.