The evolution pit underwent considerable change during the development of StarCraft II: Heart of the Swarm. Formerly called the evolution chamber, the structure enabled changes to units. Each unit featured three upgrades and a final, irreversible choice. For instance, the zergling can get the Metabolic Boost, Posthumous Mitosis, and Rapid Genesis upgrades. Afterward it could become a raptor (which gains combat bonuses) or a swarmling (which spawn 3:1).