I think it's the ability to make tools that has reduced human genetic diversity. It has also reduced the need for genetic diversity.
Rather than waiting for time and evolution to kill off one genetic population and enhance another, humans make tools for themselves that work much faster. Before evolution can kick in, humans have already fixed the situation.
That means that a person's genetic makeup is less important than his ability to use his brain. "Genetically inferior" people can make contributions that overpower the evolutionary threat. Someone once said that if we outlawed eyeglasses, the gene for near-sightedness would disappear, along with the people that carried it. But the simple "tool" of eyeglasses lets these "inferior" people thrive and survive, and contribute more to society than what the glasses cost.
"Brain" short-circuits evolution because it works faster than evolution. Genetic differences become small and meaningless because everybody has the tools needed to survive, regardless of the genes one was born with. If the earth was hit by a killer asteroid, humans would get busy with making new tools, while the other creatures could only "ride it out", and hope that their genes are good enough to get some of them through.