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All the people in the world are the descendants of the first man Adam, so that all the varieties that you see among them were generated by the different climates they lived in. It is just genetic variation, not evolution.
Over thousands of years, the climate had an effect on individual genes in changing skin color, hair color, etc. In our days, we’ve known of several rare cases where a white-skinned child was born to blacks or a dark-skinned child was born to whites.
Another possibility is that non-Jews converted and joined the Jewish community, and their genes, which were more suited to the climate, spread through the local Jewish populace.
Assimilation was impossible, because Jews who assimilated over the generations forgot their Judaism and lost their Jewish identity. The only reason that you and I know that we are Jews, is because our ancestors married only with Jews.