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, their search position, if it is Linked, and any custom name. Evolution will be paused if the Mob has Frostbite, in which the Evolution timer stops and won't resume until the target no longer has Frostbite. This all 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 Syringes and the /spore:evolve command allow players to manually Evolve Mobs.
Basic Evolution
If a Mob from the Infected tier has obtained at least 1 Evolution Point or consumed Human Remains within the 5 minute Starvation period, another 5 minute timer will begin. After the 5 minute Evolution timer is up, the Infected will Evolve into one of its available Evolved forms. 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, Mephitic, and Gorgon
Infected Pillager → Howler, Stalker, and Brute
Infected Scientist → Conductor, Chemist, and Inebriater
Hyper Evolution
If an Evolved mob has a Hyper Evolved form, after obtaining a total of 7 Evolution Points, another 5 minute Evolution timer begins. Once the time is up, the Evolved will transform into their Hyper Evolved form. This is the final form of Evolution for basic humanoid Mobs.
Note that if an Evolved mob has one or more variants, it will not have a Hyper Evolved form (e.g. Phayrie will not have a Hyper Evolved form, but 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 had.
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 1200 seconds (20 minutes) have passed, a Hivetumor 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.