This task stymied me for a while... Being that San Francisco is a tiny piece of land, I find it slightly unlikely that an organism has adapted to the city itself. I suppose the habits of certain pidgeons and rats could prove me wrong, but these may just be altered behavior patterns. I do know, rather well, of an organism that has evolved to the very specific cliamte of the foggy California coast, of which San Francisco is the Urban Crown Jewel.

The Coast Redwood (Sequoia Sempervirens) is the tallest tree in the world, and must to absorb 400 gallons of water a day, a need it cannot meet through rainfall alone. Instead, it captures fog condensation in it's needles, which drips down to the base of the tree, which allows it to live in a lower rainfall area. To collect as much water as possible, the roots extend a long distance, straight outward; the central root is never more than 10 feet deep, while the above ground portion can be upwards of 380 feet.

A single redwood cannot live on it's own for long, as it's roots cannot support it. Instead, they must exist in groves, so that the wide shallow roots can grow together in tangles. They can then depend on the combined strength of the group.

So in that way, the Redwood has evolved to those two ever present San Francisco conditions. Fog, and High Population Density.