Selection Methods

1. Selection Methods • Fitness Proportionate Selection • Tournament Selection • Rank Selection • Linear Selection • Exponential Selection • Truncation Selection • Selection methods are invoked each genetic operator.

2. Fitness Proportionate Selection • Calculate the adjusted fitness for each individual. • Create a mating pool. • Randomly select an individual from the mating pool.

3. Fitness Proportionate Selection: Example Tree SF AF NF No. of Occs. (NF x Pop. Size) 0.3765 0  0 Tree1 15 0.0625 0.0753 Tree2 2 0.3333 0.4018 2.009  2 Tree3 4 0.2 0.2411 1.205  1 0.1429 Tree4 6 0.1723 0.8615  1 Tree5 10 0.0909 0.1096 0.548  1 Total 0.8296 Mating pool: Tree2 , Tree2, Tree3, Tree4, Tree5

4. Tournament Selection • A tournament size is set as a GP parameter. • A group of individuals, equal to the tournament size is randomly selected from the population. • The raw fitness value is calculated for each individual. • The individual with the best fitness is chosen. • Selection is performed with replacement. • Selection pressure.

5. Tournament Selection: Example Tournament size = 3 Tournament: Population: Chosen individual Tree2 2 Tree1 15 Tree5 10 Tree2 Tree2 2 Tree2 2 Tree2 2 Tree2 2 Tree3 4 Tree4 6 Potential Problems Tree5 10 Tree5 10

6. Other Selection Methods • Rank Selection • Each individual is assigned a rank based on their fitness measure. • Two types of rank selection: linear ranking, exponential ranking. • Linear Selection and Exponential Selection • Similar to fitness proportionate selection. • Fitness of an individual is calculated relative to the entire population. • Truncation Selection • More than one offspring is created. • A subset of best individuals is chosen as parents for the next generation.