Sorry to be pedantic, but Darwin's theory of evolution by natural selection doesn't have much to do with current human population shifts.  Humans are not in a natural state.  The weak and sick don't typically die and the "fittest" (whatever that means for humans) don't necessarily have more offspring.  

I am not making any racial or political judgments here, but the data show that economic prosperity is correlated with declining birth rates.  That is, as people (of any race) have more wealth, access to healthcare and education, they have fewer children.

filmat11