Could be different sars viruses, different strains of hiv, or different species altogether; anything really. Tree of life project: trying to make phylogeny for all species on earth. Character can be almost any different observable feature of life, including behavior and dna. A database where all data goes in (taxa, characters and states, outgroup) Helps determine the pathway of change (character polarity) Quantify this using tee length and consistency index; in other words, to determine how parsimonious, we use these two measurements. Different numbers of characters used (aka different amount of data) causes conflict in using tree length as an indicator of parsimony so we use ci. Tree length = actual number of state changes in a phylogeny. Based on mutation rates of virus (+phylogeny), we can predict when the virus originated. Understand how traits are assembled in a lineage over time (aka how did you get a creature with that combination of traits) swordfish and birds.