It's unlikely that the Romans successfully exterminated any tribes, certainly they appear to have seriously reduced the numbers of Menapii and Eburones. However tribal identities certainly became extinct, but from what we know of Celtic socio-political systems this appears to have been a fairly frequent occurrence. The ginger gene itself is theorised to be the result of interbreeding with Homo sapiens neanderthalensis in the Palaeolithic, preserved Neanderthal hair has been found, among other colours, to be ginger. Its prevalence in northern regions is likely because during the neolithic there was not much of a migration by near eastern farmers into these regions, also reduced sunlight at higher latitudes, ironically, correlates with lighter hair colours.
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