Negative, evolution is driven by extinction. When a species is isolated from its counterparts, and local populations are driven to the verge of extinction, interbreeding between closely related members drives mutations, which accelerate evolution. There are simply too many people, and world-wide and cross-class interbreeding is so prevalant that evolution, upward or downward, is simply impossible. Only if groups of the human race become completely and totally isolated from others will groups evolve to a point where interbreeding becomes impossible. This simply won't happen on Earth, but possibly on extraterrestrial colonies isolated from Earth in the future.