I believe the answer lies in allele (gene) combinations. Let's say both Adam and Eve had a double stranded DNA. This means 4 different genes are available to mankind in every gene location. There are about 20 000 gene locations in each human. The number of various gene combinations is therefore nearly infinite. The maths is 4x4x4x4x4x4x4x4x4x4x4x4x4x4x4x4x4x4x4x4x (until you have 20000 4s). Just up to the 18th gene we have 68 billion possible combinations from those 4 original DNA strands. As environmental conditions change, certain societies will show a different frequency of certain genes (alleles) in a certain gene location than other societies. This is because that gene combination will be especially suitable for the conditions they live in, and so more successful members of the tribe breed faster, increasing the gene frequency of that trait. Maybe they are hunter gatherers and endurance is important, and an endurance gene will be prevalent in that tribe. In another tribe the males are revered for their fighting strength and strength enhancing genes become prevalent. In another tribe a dark skin is better for the heat, and the darker members breed better. This can lead to an infinite number of varieties of humans, even over a short time frame.
Edited by ARGOSY, 09 January 2014 - 01:34 PM.