Wouldn't that depend on the population dispersion and how easy it is for them to mingle? For example there are some differences between say a bengal tiger and a siberian tiger. They are relatively isolated by each other under pretty different conditions that probably applied different selection pressures across the whole of the populations each, so overtime they can become isolated but different.This, however, does not change the fact that, in order for any mutant to become dominant, the dominating species must extinct. This is still the foundation of evolution. No livin things can slowly mutate to gain or lose certain physiological features.
Are there certain selection stress that can alter the phenotype of a species without turning it into a different one? And where are those located on the DNA as opposed to genes that more strictly define intra-species compatibility?
Some maybe not extinction necessary but just have subsets of populations that later on become isolated from each other under different conditions, or an event that causes a genetic bottleneck. It's just that humans have become so well adapted and interconnected due to technological advances.