A few centuries ago, humans were quite isolated and did not move around much. So ethnic groups acquired identifiable characteristics. When they started to travel to different areas and mix with other persons, the differences started to disappear. Notice that an ethnic group acquires its characteristics because gene exchange homogenizes the group, when external sources of variation are limited. In other words, gene exchange does not create diversity, it destroys diversity.
Diversity is generated by varied environmental conditions.
An alternative to the process of using gene exchange to remix characteristics is phenotypic variation. It is a mechanism used by yeasts because of their limited gene exchange. Yeasts are fungi which adapted to sugary solutions during the era of modern plants beginning about 65 million years ago. In sugary solutions, yeasts lost the ability to disseminate easily. External spores were converted to internal spores, so single cells could form spores.