Systematics is a branch of biology that concerns itself largely with the classification and the reconstruction of the tree of life. Organisms are largely classified when two kinds of organisms share a feature because both inherited it from a common ancestor (ex: bird wings), not from features that two organisms share because of convergent evolution (ex: bat and bird wing). The relationships imply genetic relatedness, but since they are all phenotypical, this is an assumption. Now, since the early 1990ís, scientists are able to directly compare species genetically by analyzing their DNA. Much like you share more DNA in common with your cousin than you do with your friend, scientists have done this with DNA across species. So groups of animals that share similar DNA fall into related categories accordingly. Analyzing DNA (if available) along with morphology provides good data towards understanding the biological diversity on our planet.