I remember seeing a documentary a couple years ago which showed some snakes in Arizona. 17 years prior to the documentary, there were no humans in the area. In that time, a town had been built and the newborn rattlesnakes had been born without rattles. I believe it wasn't just a small number of them, that it was almost the entire recorded population of that rattlesnake in the area.

Now, admittedly, maybe it had begun before people populated the area, and maybe the rattle-less rattlesnakes adapted to avoid being detected, or whatever it may have been. Wish I could find a clip of what I saw, but it was years ago and I don't remember much other than that.