Ok here's my theory. There were at least eight people who survived the flood. Noah, his wife, his sons Shem, Ham and Japheth, and their wives.
Shem was "African-looking." Ham was "Asian-looking." Japheth was "European-looking." Their wives had other extreme features. These six "primary-coloured" individuals went on to breed a wide variety of races and ethnic groups. After a few generations, someone decided to build the Tower of Babel. God confused the languages, the families became segregated and hocus pocus, we have the races and nationalities of today.
Also, semen contains millions of sperm cells, so variety isn't that hard to achieve.
