Before revision:  Here, we show that selection acting on any relevant physical property of a protein, e. g. the elastic energy of an important conformational change , can give rise to such a sector.
After revision:  Here, we show that selection acting on any relevant physical property of a protein, represented by an additive trait, can give rise to such a sector. As an illustration of a selected trait, we consider the elastic energy of an important conformational change , can give rise to such a sector.