Sure there are. An easy example is Australian aboriginals. They were geographically isolated for tens of thousands of years. Their subgroups are more related to each other than to other subgroups.
That is one relatively isolated group. The existence of small isolated groups does not mean the whole species has a small number of distinct subgroups. The smallness here is intended to apply to the number of groups, with the size of the groups being correspondingly large.
The boundaries between groups may be blurred, but surely a person whose ancestors lived in location A for the last 10 thousand years will be genetically more similar om average to other people from that location than to people in location B on the other side of the planet.
There is, due to the way humans migrated and geographically isolated themselves over human history where founder effect, genetic drift, and evidence for introgression (both within our species and from hybridization with other species of hominids) is easily appreciated among populations even today.
Depends on the group. Oldie but goodie (1). Salient quote: "The Mormon gene frequencies are similar to those of their northern European ancestors. This is explained by the large founding size of the Mormon population and high rates of gene flow. In contrast, the religious isolates (Amish, Hutterites, and Mennonites) show marked divergence from their ancestral populations and each other, due to isolation and random genetic drift. "
As above, a few relatively isolated groups has no bearing on the truth of the proposition that all members fall into a small number of distinct groups with large numbers of members.
If you map out the reproductive connections betweeen these various small groups, they all connect quickly in terms of evolution.
