They can't "take over as top ape" unless they somehow differentiate into a separate (ie, non-crossbreed-able) species. Until then, we're all the "same ape" even though we may look different, genetically we're all still the same (or close enough). The various differences we see now are mostly epigenetic, not genetic. Every single human has the genetics to be a range of shades, but only certain shades are expressed epigenetically depending on the environment. And per most genetic studies done this century, our evolution is speeding up, not slowing down...but this is due to the massive amount of people on Earth - which, as you pointed out, is a totally different yet much more pressing issue!
Evolution doesn't really occur in a "couple of generations" either. Certain genes get suppressed by enough generations over time and those suppressions become genetic (as opposed to epigenetic) and then the underlying DNA is altered to save energy in copying during procreation. When we see genetic divergence in species, it comes from after a period of epigenetic alteration (during which time there is only one species, even if they appear differently) before the change gets "locked in". Even then, often the "pre-divergence" genetic code is still there somewhere, shuffled off into that area we like to call "junk DNA" for future recall in case of environmental changes.
Check
this out, it's actually pretty amazing. Watch
this BBC documentary too...fascinating to see how the two-tiered system works!