I believe you have the same misconception people usually have about MWI. People tend to say “ok, there are different branches, but why *I* am in this particular branch?” while all branches are symmetric and “you” in all branches are asking the variations of the same question “why I am in this branch?”
Exactly the same we have here. What do we call “real”? I take an apple. I touch the surface, I sense its smell and taste. It is all about the correlations between output signals from my brain (I give command to muscles to take an apple) and input signals (smell, taste).
Now imagine structures in different non-sterile Universes. Intelligent being there would definitely call their environments “real” for the very reasons I described above. They will also call other worlds “imaginary”. Finally these beings will ask your question: why my world is SU(3)? Or why my world is 93-dimensional? Why my geometry is non-commutative? Or why space in my world is made of pixels (is asking some creature living in the Conway’s game of life world)
So I not only refuse to answer your question about the selection rule, but I (after Max Tegmark) insist that there should not be any selection rule!
I’ve heard that neutrino oscillation experiments had ruled out the 4th generation
Lets say the total length of all formulas. I know, it is not perfect as it depends on the mathematical language used.