We investigate how the phylogenetic relationship between the species of each interacting guild in a mutualistic ecosystem influences its network of contacts. We develop a dynamical self organized model that reallocates contacts between mutualists, according to a contact preference rule (CPR) that takes into account phylogenetic distances. We conclude that a CPR that promotes phylogenetic proximity among the counterparts of the species of each guild leads to highly unrealistic contact patterns. We find that nestedness can instead be attributed to a general rule by which species tend to behave as generalists holding contacts with counterparts that already have a large number of contacts.