It is unknown for certain exactly the way that Pokémon are chosen to spawn, but it is very likely that Pokémon spawn in specific zone. For instance, water Pokémon have been found to spawn more often by lakes and oceans.
It is rumored that Pokémon spawners are static objects that have a list of Pokémon picked according to the zone rules below that they choose to spawn based on a weighted probability every 15, 30, 45, or 60 minutes.
A list of all the spawn zones as shown below was discovered in the game.
While most of these zones probably mark environments that specific Pokémon are require in order to spawn, it is also very likely that these zones also designate places that Pokémon are not allowed to spawn such as railways and runways, or designate places where they are allowed to spawn more often such as recreation areas and parks.
Some Pokémon can appear in the wild only in specific continents. These region-locked Pokémon may also be available from 10km eggs, but it is most likely that you can only hatch a region-locked Pokémon from an egg from that region. The rarest Pokémon are not yet available.
Aside from spawn zones, there is speculation that other factors such as the time of day, the weather (especially humidity), and more can affect Pokémon spawn rates. The folks at The Silph Road conducted a heavily data-fueled investigation of spawn zones as well as other factors to find a correlation, and their findings support many of these ideas.
- Golf Course
- Major Road
- Minor Road
- Nature Reserve
- Place of Worship
- Recreation Area
- Sports Center
- Urban Area