God created some mice and they began to breed. Soon there were lots of mice. Mice take up space, and they also need food, therefore not all mice can live in the same place. The group of mice will gradually spread out as more mice are born. Like all other animals, the children of one particular mouse will not be exactly the same, some may have ever so slightly shorter fur, some may have longer fur. For the mice that move north, where it is colder, the mice with longer fur ar more likely to survive. Therefore the further north you look, the longer fur mice should have. The opposite is true of the mice that move south.
Maybe you believe that God created lots of types of mice all at once. The mice he created in the cold north will have long fur to keep them warm, and the ones in the warm south will have short fur.
God only created one type of human.
What colour was their skin?