Good question! If it doesn’t have a regular evolution theory behind it (which I think would be the most probable, since there are so many isolated islands in the OP world with so many different environments), it could have something to do with the Void Century. But I think the former is true!
If evolution is the answer then the OP world must be extremely old.
Well, I think the OP world has an age similar to Earth!
It does make sense if the OP world was based on Planet Earth.
I think it is just regular evolution. The Calm Belt/Grand Line probably isn't man made. So the four Blues and not having any continents promotes the diversity. With everyone so isolated, diverging evolutionary trees naturally occurs. So it makes sense why people from different Blues look different.
This makes sense.