Evolution mission

Evolution missions are a gameplay mechanic in StarCraft II: Heart of the Swarm. They are optional missions the player may complete to upgrade specific zerg strains. They are accessed through the evolution pit.

Story
During the Second Great War, Abathur attempted to improve the evolutions of the zerg swarm, and sought out various opportunities for this, including unique adaptations of feral zerg, wild animals with useful biological traits, and abandoned zerg breeds that could be reclaimed for the swarm. However, due to conflicts between the potential mutations he discovered, only one could be permanently assimilated into the swarm, while the other would be discarded. Sarah Kerrigan ordered Abathur to make limited groups of either mutation for her to field test them in battle, then and made the personal decision of which to keep for the swarm.

Gameplay
Individual evolution missions are unlocked by completing specific missions in the main campaign. They focus prominently on a specific breed of zerg, and each mission is divided in two parts, each part allowing the player to utilize a specialty strain of the breed to accomplish their objective. Upon completing the mission the player will be given the choice to permanently upgrade the base strain to one of the two featured strains for the rest of the campaign.

Zergling evolution
Zergling: after either Fire in the Sky or Shoot the Messenger
 * Raptor strain: Gains the ability to jump up and down cliffs, leap over obstacles and onto targets from range, and gains +2 damage (40% increase).
 * Swarmling strain: Spawn 3 zerglings per egg, and their spawn speed near instant (2 seconds).

Baneling evolution
Baneling: after Old Soldiers
 * Splitter strain: Banelings split into smaller baneling spawns upon detonation, each baneling spawn does 5 damage.
 * Hunter strain: Banelings gain the ability to jump up and down cliffs as well as leap toward opponents.

Roach evolution
Roach: after Enemy Within
 * Corpser strain: Units attacked by roach will spawn roachlings if killed.
 * Vile strain: Roach attacks slow enemy movement and attack speed substantially.

Hydralisk evolution
Hydralisk: after Supreme
 * Impaler strain: Hydralisks can morph into impalers. Impalers have a long ranged anti-armor attack. Impalers must burrow to attack.
 * Lurker strain: Hydralisks can morph into lurkers. Lurkers have an anti-light linear area of effect attack. Lurkers must burrow to attack.

Mutalisk evolution
Mutalisk: after Hand of Darkness
 * Brood lord strain: Can morph into a brood lord, which besieges ground targets from long range with spawned broodlings.
 * Viper strain: Can morph into a viper, which can pull distant units close, slow enemy ground units and prevent them from attacking for several seconds, or drain life from allied units to restore its own energy. Also enables air-to-air combat.

Swarm host evolution
Swarm host: after Conviction
 * Carrion strain: Spawns flying locusts that can attack ground targets. Flying locusts are faster and deal 50% more damage than regular locusts, but have 25% less life.
 * Creeper strain: Swarm host generates creep while burrowed or rooted. Enables Deep Tunnel ability to quickly travel to any location with creep.

Ultralisk evolution
Ultralisk: after Planetfall
 * Noxious strain: Emits Noxious Cloud that deals 5 damage per second to nearby enemy ground units. Toxic Blast ability deals 20 damage to enemy ground units and structures.
 * Torrasque strain: When killed, enters a chrysalis for a short period before resurrecting with full health. Rebirth cooldown is 60 seconds.