The study of the theory, practice, and rules of classification of living and extinct organisms. The naming, description, and classification of a given organism draws on evidence from a number of fields. Classical taxonomy is based on morphology and anatomy. Cytotaxonomy compares the size, shape, and number of chromosomes of different organisms. Numerical taxonomy uses mathematical procedures to assess similarities and differences and establish taxonomic groups. Modern taxonomy generally classifies organisms according to their evolutionary history (i.e. phylogeny), and valid taxa should reflect this by being monophyletic, i.e. containing only an ancestral form and all its descendants. See also systematics.