Phylogenetic systematics is based on the evolutionary concept that natural groups have a common ancestor and all descendents of that ancestor are members of the natural group. These natural groups, called monophyletic groups or clades, are produced using phylogenetic analysis based on the principl of parsimony or Ockham's razor. This scientific principle holds that the simplest explanation of observed phenomena is most likely correct. The clades produced by phylogenetic analysis are based on shared derived (evolved) characteristics of the organisms under study. Thus phylogenetic analysis produces the groups that are the basis of a classification system.
Taxonomy is the application of naming groups in a hierarchical system, i.e. species are grouped into genera, into subtribes, etc. The naming of these clades is governed by the International Code of Botanical Nomenclature. This code addresses issues of name priorities, valid publication, conservation of names, etc. These regulations, however, do not address the issues of species or generic level concepts. With biological concepts under constant discussion in the scientific community, molecular systematics is a valuable tool in defining the clades for taxonomic applications. Plant morphology, however, is most helpful when applying generic level concepts to produce a useful classification system. The purpose of a classification system is to provide a tool for plant identification. The tool should be “user friendly” and simple to apply based on visible characters of the plants. The system should allow the user to predict the placement of an unknown organism, based on the characteristics of known species.
Plant Systematics: A Phylogenetic Approach by Walter S. Judd, Christopher S. Campbell, Elizabeth A. Kellogg, and Peter F. Stevens. (1999)