Sorry, I'm poisoning the well. I'll address your argument itself when I'm finished with my fallacy.

Here is an excerpt from a Texas Science Book



Now that that's over with, a "good" mutation connotes pervasiveness. Even if a good mutation is somewhat rare, when it occurs the fact that it is superior to the previous design means it only takes a few generations to become the norm. The advantage is simply too great for the previous design to compete. This is especially true when changes in the environment motivate the change. Most mutations are neither superior nor inferior, this is diversity. Other mutations are only inferior because of temporary environmental circumstances, such as the color of the trees the peppered moths live in. When the coal factories kill the lichen, the trees became darker the superiority switched from light colored peppered moths to dark ones, so the dark ones became the norm.