I may add one thing to that last post. Another option is that instead of continuing to grow in numbers the human species shrinks. Then we may become more homogeneous. In fact that's probably what happened at least one time to our ancestors way back, and that may explain why we are one of the least genetically diverse species on earth, despite obvious regional physical differences.
So as a species we are faced with two main options (status quo is not an option): either we continue expanding, eventually overflowing out of this planet and ineluctably diverging genetically, OR shrinking in numbers, possibly becoming an endangered species, either by some large scale catastrophe, or very slowly, without even noticing it. Both preserving the earth, our nest, and swarming towards the stars are necessary elements of our survival as a species--and parent of species to be.