just imagine some embryo DNA modification which as a side effect comes without "backward compatibility" - i.e. impossibility to procreate (or more probably -

just imagine some embryo DNA modification which as a side effect comes without "backward compatibility" - i.e. impossibility to procreate (or more probably - with very bad results in case of procreation) between the people whose parents were able to afford the modification and the ones whose didn't. That is all it takes. No need for hundred thousand years of evolution.