content: Very high.  It happens all the time.  The mimic species, in this case the moth, gets protection from looking like a dangerous species, because potential predators avoid it.  Those that look most like a hornet are most likely to live and reproduce.  Those that look least like a hornet are less likely to reproduce.  Over just a few generations, they go from "kinda look like" to "almost exactly the same."  All they need is to start off looking vaguely similar.