Lecture 17 Determining Phylogenies by Parsimony February 27 , 1996 Lecturer : Joe Felsenstein Notes : Jim Fix

Phylogenies are branching diagrams that show the history of life. A phylogeny shows the ancestral relationships among a set of species. Any two species have some common ancestor. At a speciation event their common ancestor diverged into two different species, evolving independently of each other beyond this point. When describing the history of a collection of species, these ancestral relationships give rise to a tree-like structure called a phylogeny tree. Figure 17.1 shows a fictitious example of a typical phylogenic tree. Here, the species that we wish to compare occur as labels of the leaves of the tree. The root of the tree represents the common ancestor of all the species. The nodes represent species, and branch splits are the hypothesized speciation events where two species diverge.