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race = large group that can be identified by its genes that includes a number of nations, a thorough taxonomical analysis could identify
subspecies and race as one and the same and there would be a "nation" below that and an "ethnicity" below it
think that before 200 years ago large migrations to far away lands were few and since then they have still been relatively few permanent ones and almost all to the new world so ethnicity helps define regional differences in the same nation such as between different germans
if you consider the research into how it is possible that you dont have 16 billion ancestors all alive at the same time in the year 100bc then you can acknowledge that 2nd and 3rd cousin marriages were not only common but also not detrimental genetically - the oft cited anti incest rhetoric are royal families of middle ages into industrialization europe but most of them married exclusively first cousins … or closer
on the other hand if the average rate of marrying back into your own gene pool is about 3rd degree cousins meaning many are more distant than there is not only enough variety but also chances for genetic purging