It strikes me that there must be some maximum human population. If, for instance, it takes 5000 generations for reproductive speciation then after a galaxy-wide population hits 2^500 members it would be impossible to stop humanity from splitting into separate species without some darwinian event culling branches.
So that should be our goal, 3.27E150 humans.
This is a massive number that is far greater than the atoms in the observable universe so we are going to need to create other universes.
My favorite theory about aliens visiting earth is that they are looking for novel genetic tricks to incorporate in their own genome.
But I also don't think we can go to space without speciation. Holding an entire solar system together as one species will be an engineering problem and a social problem.
