Institute for Theoretical Biology, Humboldt University, Invalidenstrasse 43, D-10115 Berlin, Germany. s.kielbasa@biologie.hu-berlin.de|Institute for Theoretical Biology, Humboldt University, Invalidenstrasse 43, D-10115 Berlin, Germany. s.kielbasa@biologie.hu-berlin.de|Institute for Theoretical Biology, Humboldt University, Invalidenstrasse 43, D-10115 Berlin, Germany. s.kielbasa@biologie.hu-berlin.de