Institute for Theoretical Biology, Humboldt University, Berlin, Germany. p.westermark@biologie.hu-berlin.de|Institute for Theoretical Biology, Humboldt University, Berlin, Germany. p.westermark@biologie.hu-berlin.de|Institute for Theoretical Biology, Humboldt University, Berlin, Germany. p.westermark@biologie.hu-berlin.de|Institute for Theoretical Biology, Humboldt University, Berlin, Germany. p.westermark@biologie.hu-berlin.de