In Guns, Germs, and Steel, Diamond argues that geography is the major cause of cultural diversity. Does not genetics play a major role here?
Reasoning for this question being; Human civilisation originated somewhere in the Rift Valley of Central Africa and ultimately migrated into Europe where centuries of learning to adapt and survive in a harsh environment brought about the now well established practices of agriculture. This necessary adaptation has shaped our population expansions since those early days and continues to bring together urban populations into cohesive, structured societies where humans interact and realise their cultural identities within an ordered, learned environment. This ongoing movement and coordination of people through the centuries could have led to the genetic diversity that is seen in all races on our planet today, thus the evolvement of ethnicity. Basically, no agriculture, little cultural diversity as there would have been no ordered communities as such, rather small 'tribes' that would have wandered the regions they inhabited with very little genetic adaptations necessary, other than that which bestows survival fitness against the local climatic conditions.
I think, first of all, that you are starting this off with a faulty assumption. In your explanation, you tell us that people migrated to Europe where
... centuries of learning to adapt and survive in a harsh environment brought about the now well established practices of agriculture.
This is simply not true. Agriculture, of course, did not begin in Europe. Instead, it began independently in a number of places, none of which was in Europe. The first of these appear to have been the Fertile Crescent and China. It was only later that agriculture diffused to Europe. Therefore, you have a mistake in the premise of your question.
Second, the time between the discovery of agriculture and the present is in no way sufficient to allow for serious genetic change in populations, even if those populations had been kept separate from one another. Agriculture has only existed for 10,000 years, which is very little time in terms of evolution and genetic change. Furthermore, for much of that time, various populations have mixed and people in different parts of the world are not terribly genetically distinctive.
So I have a hard time seeing how genetics could play a part in the equation you posit here. Instead, it seems much more likely that geography had a great deal to do with the ways in which people had to/chose to live.