I can see how probability would make no sense if each occurrence had infinite possibilites, but we can constrain the experiment so there are very limited possibilities. Looking at Virginia Whitetail deer for a moment, a generation is about three years. These deer have made no noticeable change since they were first described in the 1700's. So we might conclude that a significant change occurs in evolving species only once in about 100 generations or more. Virginia whitetails are subject to tremendous selection pressure from predators and hunting, and yet they remain the same.
Predators cull the weak and sick and poorly adapted.
Poorly adapted predators cannot find prey. Any change in a creature that did not convey a benefit would result in its selection by nature, and its genes would not be passed on. But we are ignoring that immense difficulty with evolutionary change for the moment, and are simply looking at the possibility that 1000 major changes could happen. If evolution, in its infinite variety of choices, brings a new creature to birth, it is subjected to the pressure of natural selection. Any creature that does not fit the current environment is ruthlessly exterminated.