studies show us that not only are Africans the most diverse race on the planet, Africans enslaved and brought over to the US were separated and kept from others that were related to them or even spoke the same language. Whites in the US still are genetically inbred because most of their DNA is based on inbreeding. Thats why you have so many degenerative diseases, low birth rates, and recessive genes.
Low birth rates in white communities are a direct result of extreme feminism and misandry ,abortion(over the span of dozens of generations), chemicals in our foods, the de-racialization of the general white populace, materialism and personal responsibility.
I could breed a couple hundred kids alone if I didn't give a damn about where they would end up in life and didn't care about a monogamous relationship.
Fertility is mostly only a factor for older white people trying to have kids, and unfortunately that share of the child rearing population keeps increasing.