Zombies don’t respawn if you have killed them and pulped/butchered/burned their corpses. With wandering spawns on more zombies may come from another area giving the impression of them spawning, although they really just came from the neighboring town. In any case, it is perfectly possible to clean out an entire town of zombies if you are willing to spend the time and effort (depending on the size of the town this can be either trivial or require a lot of work).
Triffids, Fungi and Ants will spawn endlessly in a certain range around their home unless you kill their “Queen”.
Wildlife also spawns continuously IIRC at a very slow rate.
Evolution means that zombies grow stronger over time. For example; Brutes evolve into Hulks, Spitters into Corrosive Zombies, Boomers into Huge Boomers and ect.
If you want to see evolution in action you can debug a Zombie Master somewhere with a few zombies and watch what happens 