  A large class of phylogenetic networks can be obtained from trees by the
addition of horizontal edges between the tree edges. These networks are called
tree based networks. Reticulation-visible networks and child-sibling networks
are all tree based. In this work, we present a simply necessary and sufficient
condition for tree-based networks and prove that there is a universal tree
based network for each set of species such that every phylogenetic tree on the
same species is a base of this network. The existence of universal tree based
network implies that for any given set of phylogenetic trees (resp. clusters)
on the same species there exists a tree base network that display all of them.
