It's entirely possible I'm mistreating my yeast. I've read that when they are stressed, yeast begin producing spores (sort of an almost-sexual reproduction mode). That makes it possible for genes to recombine and introduce different characteristics in the off-spring. It isn't actually mutation, but I suspect that's what's actually happening when people refer to "yeast mutation".
Well, that wandered off-topic pretty thoroughly, didn't it?
So, after I've mistreated the yeast by pitching at 45F and leaving it in a freezer set to 50F, and once I suspect they've been fermenting for two or three days, I start moving the temperature up. I usually end the fermentation at about two weeks near the manufacturer's highest recommended temp. I guess that's a diacetyl rest or clean-up or fermentation completion or some such technical term. Seems to work OK.