Like rank selection, *Sigma Scaling* attempts to moderate selection pressure over time
so that it is not too strong in early generations and not too weak once the population has stabilised and
fitness differences are smaller. The Greek letter Sigma is used in statistics to denote standard deviation
and that's what it means here too. The standard deviation of the population fitness is used to scale the
fitness scores so that selection pressure is relatively constant over the lifetime of the evolutionary
program.