Currently, there is a working theory developed on identifying Biomes and how it works in Pokemon Go. The theory somehow allows players to understand the spawn algorithm of the augmented reality game from Niantic. Aside from the explanation regarding how Biome works, a chart has also been provided stating all the data of each specific location.

According to the report from Otakukart, each Pokemon spawn point is assigned on a single Biome which is based on location and local terrain. The working theory mentions that each Biome is represented by a Pokemon Set Spawn List. It also states that each Biome differs from each other which allows players to distinguish each Biome based on the frequency of spawn and common spawns. By assembling the same spawn points together, it would generate the original spawn list of each Biome.

Based on the theory above, reddit user homu has built a list and Spawn list that has a tier of common, uncommon, rare and very rare. Uncommon and common usually generate ¾ of the spawn list, while both rare and very rare take up for about 10 percent. In addition, it seems like there's a 5 percent chance of spawning anything that is completely random.

For further clarifications, here is a highlight of the said chat created by redditor homu. Based on the chart, in a Forest Location, Common spawns consist of 20 percent Pidgey, 15 percent Zubat and both Weedle and Spearow got 10 percent each. Uncommon spawns consist of both 5 percent Evie and Caterpie. Rare spawn, on the other hand, consists approximately 2 percent Paras, Nidoran Male and Female, Venonat, Krabby, Ekas, Staryu, Rattata, Goldeen and Poliwag. Lastly, a very rare tier has approximately 1 percent Oddish, Bellsprout, Meowth, Horsea, Jigglypuff, Golbat and Kakuna. In order to see the Biomes for each area, visit the page here.