Even if pedigree collapse becomes endogamy, repeatedly occuring over many generation, the introduction of only one migrant per generation is sufficient gene flow to counteract the diversifying effects of genetic drift in a metapopulation.

It is one of the reasons why 90% of human genetic diversity is within groups and only 10% is between groups.

This being Finnish, one of the most isolated groups in Europe, what differs between me and someone also from Finnish descent is a similar quantity as what differs from say an indigenous Australian.

Humans are freakishly homogeneous compared to most species, with chimpanzees living on different sides of a river having more genetic diversity that the entirety of the human population.

Obviously historical low numbers of individuals at some point accounts for a lot of that, but cross deme gene flow even in fairly low rates is part why it is still true.

Here is a fairly accessible explanation of the above.

While I don't know if you are familiar with this, many people have the infinite population models in their head and the more realistic finite case can be counterintuitive.

https://www.nature.com/scitable/knowledge/library/natural-se...