We do not know when the flood happened, but presumably it was at least
several thousand years ago. I can only assume that human beings evolved
the different “racial” characteristics over the past several thousand
years. In a gene pool with a fairly broad genetic diversity, the physical
characteristics which lead to the greatest survival rate will become
dominant in an area in just a few generations. Clearly darker skin is a
strong survival trait in parts of the globe where the sun is nearly
directly overhead at noon the whole year, as UV radiation causes skin
cancer and other health problems for the light skinned. In regions of the
world where there is relatively little direct sun light, very dark skin
causes a low level of vitamin D to result, making lighter skin predominate
over just a few generations in parts of the globe farther from the
equator. Of course, skin color is not the whole story on “racial”
characteristics, but you can assume that genetic diversity and natural
selection has worked to produce the wide variety of physical
characteristics you see in humans.

John Oakes, PhD