[Cazador]

Here's an interesting thought... if the San Francisco garters diverged from other sympatric T. sirtalis in the San Francisco area to fill the niche available (i.e. specializing on toxic CA red-legged frogs), then the evolutionary arms race may have outpaced their ability to adapt (evolve) due to the bottleneck effect (limited genetic variation as a result of a relatively limited population size). Habitat degredation and destruction have no doubt been contributing factors, but I like the idea that the evolutionary arms race may have outpaced their ability to evolve, as Cascabel suggested. It's part of a very interesting hypothesis.