A variety of recombination operators have been used in ESs. Some, like the GA crossover operator, combine components from two randomly selected parents, while others allow components to be taken from any of the solutions in the parent population. Recombination is applied not only to the control variables but also the strategy parameters. Indeed, in some ES implementations different recombination operators are applied to different components of the solution representation. The most commonly used recombination operators are described in the following sections.