Clades are diagnosed by advanced characteristics that set them apart from their ancestors. Clades are not family trees as the true ancestry of every animal would have to be known. Clades are hypotheses of ancestry, and are governed by the rules that govern all of science. A closely related science to Phylogenetic Taxonomy is called Cladistics. Cladistics is the science of discovering and representing the most parsimonious distribution of advanced characteristics among taxa.
Cladistics produces branching diagrams called cladograms.
Cladistics is used to test phylogenetic hypotheses, as the distribution of advanced characteristics should mirror the taxa's phylogeny, and the resulting cladograms are often used to represent the hypothesized phylogeny. Phylogenetic Taxonomy and Cladistics are therefore often used synonymously, although technically they are two different disciplines. The characteristics that diagnose a clade may change if new fossils that clearly belong in the clade do not have, either in full or modified form, the characteristics that diagnose the clade. In addition, scientists may disagree on what constitute advanced characteristics and therefore may differ in what they think diagnose the clade. However, the definition of the clade will always be the same no matter which scientist is speaking.