Nestedness in mutualistic networks emerges from a modified rule of   preferential attachment
  Replacement of papers q-bio/0605026 and q-bio/0607043 by a single manuscript (q-bio/0701029) follows suggestions from a magazine's referee 
Diego Medan, Roberto P.J. Perazzo, Mariano Devoto, Martin Zimmermann,   Horacio Ceva, Enrique Burgos and Ana M. Delbue