Dr. Henry Morris explains how small populations generate distinct ethnic characteristics relatively quickly:

As each family and tribal unit migrated away from Babel, not only did they each develop a distinctive culture, but also they each developed distinctive physical and biological characteristics. Since they could communicate only with members of their own family unit, there was no further possibility of marrying outside the family. Hence, it was necessary to establish new families composed of very close relatives, for several generations at least. It is well established genetically that variations take place very quickly in a small inbreeding population. . . . In a small population, however, the . . . genes that may be present in its members . . . will have opportunity to become openly expressed and even dominant under these circumstances. Thus, in a very few generations of such inbreeding, distinctive characteristics of skin color, height, hair texture, facial features, temperament, environmental adjustment, and others, could come to be associated with particular tribes and nations. (The Genesis Record, p. 276)