When a Spore Mob kills another mob, they will receive 1 Kill Point and 1 Evolution Point. All Mobs (excluding Organoids) have an internal counting system for both and can use them in different ways depending on the Mob. Organoids don't get Kill or Evolution Points because they are only temporary and have natural passive regeneration. Upon a Mob Evolving, they will keep any Kill and Evolution Points they had.
Kill Points
Kill Points can be obtained by different Mobs in multiple different ways.
1 Kill Point is gained by Spore Mobs (excluding Organoids) whenever they kill a mob. This does not count when they infect a mob.
4 - 6 Kill Points are granted to Spore Mobs (Infected by default) spawned by Overcharged Scents.
0 - 3 Kill Points are granted to Infected, Evolved, Hyper Evolved, Experiments, Bairns, Scampers, and Gastgebers within a 16 block range of any Calamities that performs a Spore Burst.
Bairns, Collector Protector variants, and Specters can receive Kill Points by consuming Food in Containers.
Reapers can receive a Kill Point per 25 hunger points obtained by breaking various blocks.
Reapers will, if having more than 10 Kill Points, scan in a 32x32 block area every 10 seconds for Infected that lack enough points to Evolve. If finding any, the Reaper will give that Infected enough Kill Points that it can Evolve (this is able to adapt to Config changes).
1 Kill Point is granted every minute to Calamities that are Rooted.
1 Kill Point is granted to Infected and Bairns when consuming a Biomass Bulb or Drowned Lump due to Starvation.
1 Kill Point is granted to Infected, Evolved, Hyper Evolved, Bairns, Scampers, Gastgebers, Calamities and Structure Blocks by Infested Spawners every 18 seconds in a 32 block radius.
This information is currently unconfirmed.
Upon a random chosen tick, if the check for a certain Effect is made successfully, there is a ~5% chance the Mob will receive that Effect. The Effect can only be granted if the Mob doesn't already have it. Granting a buff will consume a Kill Point, and multiple buffs can be given at once if all the criteria are met. Spore Mobs can get the following Effects under the following circumstances:
If the Mob is below 100% health they will receive Regeneration I for 10 seconds.
For Specters, Reapers, and Vanguards, it'll be 20 seconds of Regeneration I.
For Specters and Reapers below 50% health, it'll be Regeneration II for 20 seconds.
For Calamities, it'll be 30 seconds of Regeneration I.
For Calamities below 50% health, it'll be Regeneration II for 30 seconds.
For Calamities with the Rejuvenation Attribute, the amplifier of Regeneration will be increased by 1 for every additional level of the Attribute the Calamity has, capping out at level 64.
If the last damage the Mob took was from drowning they will receive Water Breathing I for 5 seconds.
If the Mob has more than two Kill Points and has a target, there is a 2.5% chance the following two checks will be performed:
If the target is farther than ~14.14 blocks, they will receive Speed II for 10 seconds.
If the target is within ~7.75 blocks, they will receive Strength I for 5 seconds.
A Kill Point is also given to Structure Blocks by Spore Mobs when they are called over and touch the Structure Block.
Some specific Mobs can use Kill Points in unique ways.
Depending on the Mob's Kill Points when being fed to a Womb will depend on the amount of additional biomass gained.
Depending on the Mob's Kill Points when consumed by a Proto Hivemind will depend on the amount of biomass she gains when removing their Kill Points.
A Hohlfresser with Kill Points will have his vomit projectiles appear to have bone pieces in the texture.
Evolution Points
Evolution Points can be obtained by different Mobs in a few different ways, albeit less than Kill Points, as they have less functions.
1 Evolution Point is granted to Spore Mobs whenever they kill a mob. This does not count when they infect a mob.
1 Kill Point is granted to Infected and Bairns (despite Bairns not being able to Evolve) when consuming Human Remains or Wall Remains due to Starvation.
1 Evolution Point is granted to Infected, Evolved, Hyper Evolved, Experiments, Bairns, Scampers, Gastgebers, and Calamities by Infested Spawners every 18 seconds in a 32 block radius.
This information is currently unconfirmed.
Any necessary Evolution Points required for Infected to Evolve have a 30% chance to be granted upon spawning if the Config option for Spore Mobs to attack other mobs is turned off.
If a mob from the Infected tier has at least 1 Evolution Point, they will Evolve after 5 minutes. If an Evolved mob that has a Hyper Evolved form has gained 7 Evolution Points, they will Evolve after 5 minutes.
Having Evolution Points prevents Infected and Bairns from Starvation.
Hyper Evolved will use Evolution Points to place Human Remains at their Nests.
Trivia
Spore Mobs can gain Kill Points and Evolution Points from killing other Spore Mobs, though making them fight is only achievable via commands or mods.
Checking for the amount of Kill Points prior to buffing only happens twice; before all logic is fired, and before checking combat-only logic. Due to this, it is possible for a Spore Mob to use more Kill Points than it has, resulting in a negative amount of Kill Points. Due to how the logic is constructed in the code, this is only possible if the Mob has only 1 Kill Point and grants itself Regeneration and Water Breathing in the same go.
The reason why Infected can instantly be granted Evolution Points if the Config option for Spore Mobs to attack other mobs is turned off is because of the Fear Nightfall modpack, which contains Fungal Infection: Spore and has that specific Config option disabled. People complained about the Spore's slow development, so the addition was made.