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. We demonstrate that the main signature of these physical sectors lies in the small-eigenvalue modes of the covariance matrix of the selected sequences.
After 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 within an elastic network model, and we show that selection acting on this energy leads to correlations among residues. For this concrete example and more generally, we demonstrate that the main signature of these physical sectors lies in the small-eigenvalue modes of the covariance matrix of the selected sequences.