The following section contains information from a previous version of StarCraft II: Heart of the Swarm which is no longer valid.
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).
In Co-op Missions, earlier in development Abathur was to have the evolution pit as a unique structure, similar to Karax and the solar forge. The evolution pit would have contained researchable upgrades to improve Abathur's toxic nests, locusts, and the symbiote mechanic that was removed later in development.