mutationRate

The probability that an offspring individual is mutated — i.e. that the mutation operator is applied to it at all. Must be in 0,1.

This per-individual gate compounds with any per-gene gating the operator itself does. The default GaussianMutation mutates each coordinate independently with its own perGeneRate, so the effective per-gene, per-generation mutation probability is about mutationRate * perGeneRate — roughly 1% at the defaults (0.1 × 0.1), not the 10% either value suggests on its own. To change how much mutation actually happens, adjust the operator's per-gene rate; mutationRate only controls how often the operator runs, not how far it perturbs when it does.