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Spore Mobs being able to Evolve is what progresses the mod. It allows lower tier mobs like Infected to become much stronger, like Evolved, followed by Hyper Evolved. It also allows the creation of some nasty monsters like the Proto Hivemind. When Evolving, Mobs will keep any Kill Points, Evolution Points, Search Position, if it is Linked, or a custom name will all be kept. This applies for failed Evolution and Hyper Evolution too.
Instant Evolution can be caused by Proto World Modifier (30% chance for an Evolvable Mob to Evolve as soon as it spawns) and Mutagenic Reactant (10% chance when an Evolvable Mob is hit with a Weapon that has Mutagenic Reactant to Evolve).
Evolution
If a Mob from the Infected tier has gotten a kill or consumed Human Remains within the five minute Starvation period, another five minute timer will begin. After the five minute Evolution timer is up, the Infected will Evolve into one of its available Evolved forms. Note Spore cannot gain kills when infecting a mob, only when actually killing it.
Evolution will be slowed if the Mob has Weakness, in which the Evolution timer stops and won't resume until the target no longer has Weakness.
Infected Vindicators and Evokers are considered Evolved due to how strong they are. Each Infected has a few Evolved they can become. The list goes as follows:
Infected Human - Knight, Griefer, Braiomil, and Phayres
Infected Husk - Verdathorn, Jagdhund, and Scavenger
Infected Drowned - Bloater and Naiad
Infected Villager/Infected Diseased Villager/Infected Wandering Trader - Leaper, Slasher, and Spitter
Infected Witch - Volatile and Mephitic
Infected Pillager - Howler, Stalker, and Brute
Infected Scientist - Conductor, Chemist, and Inebriater
Failed Evolution
When an Infected tries to Evolve, there is a 10% chance Evolution will fail, and it will become a Scamper instead. This is an important part of the mod's progression.
Hyper Evolution
If an Evolved mob has a Hyper Evolved form, after reaching seven kills another five minute Evolution timer begins. Once the time is up, the Evolved will transform into their Hyper Evolved form. Note that if an Evolved mob has one or more variants, it will not have a Hyper Evolved form. Phayrie will not have a Hyper Evolved form and Knights do. Each Evolved that has a Hyper Evolved form only has one. The transformations are as follows:
Special Evolution
Some Mobs have unique forms of Evolution that function differently than other Mobs.
When having 1 Evolution Point and waiting for the 10 minute age timer to finish, a Scamper will Evolve into a Gastgeber. A Scamper also has a 30% chance to Evolve into a Gastgeber if it is Linked when the age timer finishes, regardless of how many Evolution Points it has.
Infected Diseased Villager - Reaper
When a Linked Infected Diseased Villager has 1 Evolution Point, after waiting for the 5 minute Evolution timer to finish, they will become a Reaper.
Infected Wandering Trader - Specter
When a Linked Infected Wandering Trader has 1 Evolution Point, after waiting for the 5 minute Evolution timer to finish, they will become a Specter.
Raid Captain Infected Pillager variant - Vanguard
When a Linked Infected Pillager Raid Captain variant has 1 Evolution Point, after waiting for the 5 minute Evolution timer to finish, they will become a Vanguard.
After 360 minutes (6 hours) have passed, Hivetumors will Evolve into a Proto Hivemind.
If there is a Proto Hivemind within 300 blocks, it will instead despawn and give 1000 Biomass Points to the nearest Proto Hivemind.
Gallery
An Infected Villager Evolving into a Leaper.
A Knight Evolving into an Inquisitor.
A simple Evolution tree of Spore.