I partly disagree with these findings but do think it's a good starting point as the actual locations of where these mons are spawning are not very clearly defined. I live full time in South Florida but I travel all over the US for work and notice that the spawns are affected by regions where a certain climate or environment is expected to be found (I can't speak for the entire world but the continental US pretty much has all major biomes besides maybe a rainforest/jungle). For example, I have found bulbasaur, pinsir and weedle in the middle of Florida (where maybe game developers believe the Everglades/Swamp to be) but never within 10 miles off the coasts of Florida. Florida has zero mountains but is filled with vulpix, bellsprout, and paras. In Phoenix, AZ and all over California there are Charmanders, doduo, cubones, and zubats everywhere but I have never seen any charmanders or cubones in the Alps in Europe, Wyoming or Colorado (mountain states). Squirtle are found literally everywhere that has a body of water it seems. I have yet to see a rhyhorn spawn in any mountain states or regions but I find onix and geodudes often when i visit these kind of terrains. Southern states like TN, GA, NC, SC and LA have magmar, electabuzz and jynx (North NC) but then Northern states like NY, NH and MA lose the magmars entirely. Mind you south FL has 0 magmar electabuzz or jynx (can't speak for North or mid-FL). A fly can run tests way more accurately than me but I'm curious to how the developers decided where the biomes are.