>>1
This is mainly due to a environmental adaptation, since blacks originated in areas where the sun is constantley beating down on them genitalia are larger allowing for better heat absorbtion and dispersal. Whites and asians developed in colder climates so their genitalia is reduced to conserve body temperature. The larger the genitalia the longer it takes blood to flow around it allowing it to cool off easier, the shorter the quicker the blood flow and the better heat conservation, also since Whites lived in rockier terrains than blacks the reduced genitalia would not be a hinderance when getting around.