"Evolve" doesn't well describe the very real possibility that you were/are more reactive to species popular in the Midwest, but not the Bay Area.

By "evolve", I thought you mean "change within the individual over their lifetime" - which also happens. I spent time in oxygen tents as a young child; I mostly suffer from (easily treated) sinus issues as an adult.

Also, our drugs have improved mightily, but that's obviously not relevant for an increase in allergies amongst the population (separate from the above meaning of "evolve").

Edit: I guess I'm not sure what you meant by the word.