Before revision:  That both the D614G mutation and the K986P mutation increase the COVID-19 infectivity and why a large number of mutations, including those at interface residues, have not been selected in current SARS-CoV-2 pandemic are interpreted from the presented theory.
After revision:  That both the D614G mutation and the K986P mutation and the new variants 501Y in current SARS-CoV-2 pandemic are interpreted from the presented theory.