Chapter 20 PHYLOGENY Introduction to Phylogeny Phylogeny is the evolutionary history of a species or group of related species Discipline of systematics classifies organisms and determines their evolutionary relationships Taxonomy is the ordered division and naming of organisms Cladistics Cladistics classifies organisms by common descent A clade is a group of species that includes an ancestor and all its descendents Using Derived Characters Characters can be used to infer evolutionary relationships. Molecular Clocks Molecular clocks use mutation data in related genes from different species Number of nucleotide changes is assumed to be proportional to the time since last common ancestry ie. Few changes = little time passed; many changes = lots of time passed