The evolution is a result of local airborne yeasts slowly overcoming the
original. If you start with a San Francisco culture, and live there too, it
will stay as is for ever (except that local things evolve too!). If you
start in England with SF, as I did, it will slowly change. How slowly
depends on how you nurture the culture, but change it will. You can freeze
some of the original and reactivate at intervals, after throwing away the
one you have been using. That keeps near the original for much longer.
Cheers, Brian