Agreed. The only ones "chosen" were the mutants that were _not_ defective, that were more effective in the newly-available ecological niches. Because there are literally billions of offspring in even short terms (10,000 years.) Even a .1% mutation rate, with .1% of them being non-defectives, you end up with 1000 new organisms. Now, in a stable environment none of them make it, because the current organism in the current niche is close to ideal. But in a fluid environment many of them (say 1%) find that they have an advantage in this new environment. Maybe it's colder and they have more fur. Maybe it's warmer and they have less fur. Maybe trees have started to grow and they have looser skin so they get a little bit of a "parachute" effect when they jump from one tree to another. And so, in that fluid environment, you potentially have 10 new species after 10,000 years.