This refers to the generation of random initial trees for a heuristic search to find MP trees. In this case, a tree is generated by randomly selecting a sequence and adding it to the growing tree on a randomly-selected branch A branch is a line connecting either two internal nodes to each other or an external node to an internal node in a phylogenetic tree. The length of a branch denotes the genetic distance (e.g., number of substitutions per unit time) between the two taxa it connects..