Allelic and phenotypic frequencies tend to vary gradually across human populations. Definitions of race as a discontinuous category, reflecting clear ‘breaks’, are thus conceptually flawed: It is impossible to identify where one race begins and another ends. Skin color, as shown below, varies widely by latitude and degree of exposure to ultraviolet. Since Africa covers such a wide span of latitude, it is reasonable that African groups exhibit a wide range of skin colors that overlap tremendously with individuals from other continents (Jablonski, 2012).
In fact, most all traits vary continuously across individuals within a group (such as height) and across groups. If one were to walk from Oslo, Norway to Johanessberg, South Africa one would find that group average skin colors slowly and continuously increased to the equator and then slowly decreased again.