For example, look at all of the natural firstborns who were rejected from inheriting the promises and the ones chosen by God as firstborn to inherit the promises: Cain/Seth, Ishmael/Isaac, Esau/Jacob, Reuben/Ephraim. This applies to other things as well, eg, Saul/David. God is revealing a very important spiritual truth in this pattern that pervades scripture: first the natural, then the spiritual
This is not doctrine. You should familiarize yourself with the new covenant.