Institute for Theoretical Biology, Humboldt University, Invalidenstr. 43, D-10115 Berlin, Germany. h.herzel@biologie.hu-berlin.de|Institute for Theoretical Biology, Humboldt University, Invalidenstr. 43, D-10115 Berlin, Germany. h.herzel@biologie.hu-berlin.de|Institute for Theoretical Biology, Humboldt University, Invalidenstr. 43, D-10115 Berlin, Germany. h.herzel@biologie.hu-berlin.de