After the recent event where the original 150 Pokemon spawn rates were boosted, I assumed it might affect the spawn rates of those Pokemon. However, it appears it didn't have that effect. So far all I have seen of the originals are Pidgey and Rattata, which makes sense since they are a very common Pokemon.
I understand that bringing the other generation's spawn rates will saturate the field somewhat, but it appears that isn't the case.
What, if anything, affects a Pokemon's spawn rates?