Global travel has no doubt caused the gene pool to become more homogenous, but yet there are still clear differences between people from many parts of the world.
Isolation (really meaning tendency not to inter-breed) doesn't have to be physical - it can be cultural too, and it seems this isn't limited to humans - many animal species that are on the cusp of speciation (lions/tigers, horses/donkeys, etc) share the same habitat and are just choosing to avoid each other.
In any case, "isolated" populations are only necessary for speciation, not for evolution. Even a single drifting gene pool is going to evolve to adapt to the changing environment, since individuals will still differ genetically. Whatever the environment throws at you will cause some individuals to do better than others (e.g. you survived the Ebola outbreak, others didn't), and these fitter genetics will then spread in the gene pool.
Isolation (really meaning tendency not to inter-breed) doesn't have to be physical - it can be cultural too, and it seems this isn't limited to humans - many animal species that are on the cusp of speciation (lions/tigers, horses/donkeys, etc) share the same habitat and are just choosing to avoid each other.
In any case, "isolated" populations are only necessary for speciation, not for evolution. Even a single drifting gene pool is going to evolve to adapt to the changing environment, since individuals will still differ genetically. Whatever the environment throws at you will cause some individuals to do better than others (e.g. you survived the Ebola outbreak, others didn't), and these fitter genetics will then spread in the gene pool.