1 Does high selection pressure cause growth?

Tackett suggested that the rate of growth is proportionate to the amount of selection pressure [Tackett, 1994] and Langdon and Poli (1998a) showed that removing selection pressure stops code growth. To see the effect of varied selection pressure on code growth, one can look at different selection schemes and objective functions used in the literature. Smith and Harries (1998) investigated selection schemes and variable tournament sizes to show the same trend: less selection pressure generally leads to reduced code growth. Poli (2003) studied a method for reducing code growth by periodically worsening the fitness of above average sized individuals. In the control experiment with no code growth control, the results (Fig. 1, page 211) show trends of smaller tournament sizes producing less growth.

Considering the theories of code growth, particularly that individuals with similar size and shape are less likely to produce children with worse fitness than two dissimilar individuals, an increased rate of growth is expected with higher selection pressure. In this case, higher selection pressure becomes more likely to pick the same individual repeatedly. Given that the algorithm contains some bias for producing bigger individuals (as bigger children are more likely to have a higher chance of survival), the repeated selection of these individuals will consistently produce offspring which in turn have a better chance of survival.