If it was caused through Darwin style survival of the fittest, then we wouldn't have the same family lines, parents and children in both universes.

In fact any point of divergence would lead to drastic changes with in 2-3 generations. It's kinda silly to try to make rhyme or reason from it.

It's almost better to just accept that the two universe are tethered together, and there must be an almost "Final Destination" style fate in one or both universes making things happen to keep them a mirror of them selves.

If we want to reason it out, we'd have to say that the Defiant crossing through interphasic space made the MU at that point in history and it's time traveled back and forth through history making the MU closes to PU at that time and diverges in both directions from the TOS era.