We have selective breeding now. College educated people tend to marry college educated people and have LESS children that uneducated people. Europe is dying out, at least the French, Italian, German original stock.
People have always selected mates based on cultural, economic, and geographical similarities. However, even in the most isolated communities and the most isolated times in the history of humanity, people haven't done this enough to speciate, which says a lot.
I don't know that European stock is dying out so much as it's being mixed with other "races" as it becomes easier to move around the world. Using the same argument you could say the African stock in the US died out because most people considered African-American in the US have a fair amount of European blood in them.
It is true that people who are college educated are having fewer children. Americans and Europeans are having fewer children than the rest of the world, not even counting level of education. But white people have always been a minority anyway. Eventually people will be browner than they are now. Oh well. The whole idea of "original stock" is kind of bunk anyway-- humans have been moving around and conquering each other forever.
Mother Nature is very harsh but simple when it comes to evaluting genetic favorabilty. Its this simple: the individual that leaves the most offspring for the next generation is the winner. Case closed.
Unless all of those offspring are killed in a calamity, or war, etc. Unless all of those offspring carry a genetic defect that was not expressed in said individual and increases rate of disease and/or infant mortality. Unless those offspring are, say, college educated and have 0-1 children each. Unless those genes all those offspring carry are selected against, and their line of descendants dies out quickly or slowly depending on the selection pressures. Unless there's some sort of mutation in the parent's gene that makes all the offspring sterile. I could go on, but you get the idea. It's not really case closed.
Take Jennifer Aniston. Pretty girl and if she has no children, a total genetic loser. Her line dies out today. Not one trace of her will be left for the rest of eternity.
Not really. She has siblings, cousins, and further removed family members who all carry a lot of the same genes as her. It's not really about the individual so much as it's about the gene pool. It's not the person who wins this race, it's the gene. The more a gene is expressed and selected for, the more common it is, and therefore wins the genetic race. In that way, it's less about individuals and more about genes and gene pools. Yes, people carry the genes, but it's more about the group carrying said gene and the selection pressures thereupon that make the difference of which genes become widespread and which genes die out.
The Selfish Gene by Richard Dawkins goes into this part of evolutionary theory. It's a good read. I don't claim to be an expert on evolution, but he is, and can probably express what I'm trying to better than I can.
Still, this doesn't have any bearing on the OP, really.