The Zerg have developed a unique trait which allows them to enhance their warriors per individual strains. This means that the Zerg can be selective in their choice of units. Improving units will require the process to be done a few times per Zerg as they will benefit from different boons with each step. For instance, a Zergling may obtain enhanced speed at first, but then will have gained more ferocity in its attacks and at the same time a stouter hide - both on the second evolution. This would seem to apply to every Zerg strain.