We still have populations that seem quite isolated. Have there been any studies about whether the Congo pygmies or the Kalahari bushmen mingle reproductively with the surrounding populations? The same question applies to other isolated groups like Australian natives, Amazonian tribes, and highlanders in remote Indonesian areas. Even if there were occasional "melding events," it may be that the bulk of such populations continue to be reproductively isolated, and if this continues, it's conceivable that speciation could result in a few tens of thousand of years.
I don't know of any. The assault traditional lifestyles are under everywhere includes having the kids go off to school in the big city and marry outside the group. If the bushmen and pygmies aren't seeing that yet, they may soon.
The same question applies to other isolated groups like Australian natives, Amazonian tribes, and highlanders in remote Indonesian areas.
An interesting case is the Basque people. they have the highest rate of Rh- blood in the world. If the parents have different Rh types, (I forget the details), if the first child and the mother have different types the mother can build up antibodies, so that if a second child differs from the mother there is (often) a miscarriage.
I don't know if this has added to the isolation of the Basques or not, but if it were just a bit more extreme (no babies survive) then we'd have human speciation.