Original: Mayr 's formulation has been argued to have cleared up issues which Wagner had left unresolved : '' A new species develops if a population which has become isolated from its parental species acquires during this period of isolation characters which promote or guarantee isolation when the external barriers break down '' .
Simplified: Ernst Mayr 's formulation cleared up the issues which Wagner had left unresoved : '' A new species develops if a population which has become isolated from its parental species acquires during this period of isolation characters which promote or guarantee isolation when the external barriers break down '' .