Biological systematics is the study of the diversification of living forms, both past and present, and the relationships among living things through time. Relationships are visualized as evolutionary trees (synonyms: cladograms, phylogenetic trees, phylogenies). Phylogenies have two components, branching order (showing group relationships) and branch length (showing amount of evolution). Phylogenetic trees of species and higher taxa are used to study the evolution of traits (e.g., anatomical or molecular characteristics) and the distribution of organisms (biogeography). Systematics, in other words, is used to understand the evolutionary history of life on Earth.
Definition and relation with taxonomy
Systematic biology (hereafter called simply systematics) is the field that (a) provides scientific names for organisms, (b) describes them, (c) preserves collections of them, (d) provides classifications for the organisms, keys for their identification, and data on their distributions, (e) investigates their evolutionary histories, and (f) considers their environmental adaptations. This is a field with a long history that in recent years has experienced a notable renaissance, principally with respect to theoretical content. Part of the theoretical material has to do with evolutionary areas (topics e and f above), the rest relates especially to the problem of classification. Taxonomy is that part of Systematics concerned with topics (a) to (d) above.
The term "systematics" is sometimes used synonymously with "taxonomy", and may be confused with "scientific classification". Webster's 9th New Collegiate Dictionary of 1987 treats "classification", "taxonomy," and "systematics" as synonymous. According to this work the terms were coined in 1790, c. 1828, and 1888, respectively. Some[who?] claim systematics alone deals specifically with relationships through time, and that it can be synonymous with phylogenetics, broadly dealing with the inferred hierarchy of organisms, which means it would be as subset of taxonomy as it is sometimes regarded, but the inverse is claimed by others.[who?]
Europeans tend to use the terms "systematics" and "biosystematics" for the field of the study of biodiversity as a whole, whereas North Americans tend to use "taxonomy" more frequently.  However, taxonomy, and in particular alpha taxonomy, is more specifically the identification, description, and naming (i.e. nomenclature) of organisms, while "classification" is focused on placing organisms within hierarchical groups that show their relationships to other organisms. All of these biological disciplines can be involved with extinct and extant organisms.
Systematics uses taxonomy as a primary tool in understanding organisms, as nothing about an organism's relationships with other living things can be understood without it first being properly studied and described in sufficient detail to identify and classify it correctly. Scientific classifications are aids in recording and reporting information to other scientists and to laymen. The systematist, a scientist who specializes in systematics, must, therefore, be able to use existing classification systems, or at least know them well enough to skillfully justify not using them.
Phenetic systematics was an attempt to determine the relationships of organisms through a measure of similarity, considering plesiomorphies (ancestral traits) and apomorphies (derived traits) to be equally informative. From the late-20th century onwards, it was superseded by cladistics, which considers plesiomorphies to be uninformative for an attempt to resolve the phylogeny of Earth's various organisms through time. Today's systematists generally make extensive use of molecular biology and computer programs to study organisms.
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- Cladistics - a methodology in systematics
- Evolutionary systematics - a school of systematics
- Global biodiversity
- Phenetics - a methodology in systematics that does not infer phylogeny
- Phylogeny - the historical relationships between lineages of organism
- 16S ribosomal RNA - an intensively studied nucleic acid that has been useful in phylogenetics
- Phylogenetic comparative methods - use of evolutionary trees in other studies, such as biodiversity, comparative biology. adaptation, or evolutionary mechanisms
- Scientific classification and Taxonomy - the result of research in systematics
- Wilkins, J. S. What is systematics and what is taxonomy?. Available on http://evolvingthoughts.net
- Michener, Charles D., John O. Corliss, Richard S. Cowan, Peter H. Raven, Curtis W. Sabrosky, Donald S. Squires, and G. W. Wharton (1970). Systematics In Support of Biological Research. Division of Biology and Agriculture, National Research Council. Washington, D.C. 25 pp.
- Brusca, R. C., & Brusca, G. J. (2003). Invertebrates (2nd ed.). Sunderland, Mass. : Sinauer Associates, p. 27
- Fortey, Richard (2008), Dry Store Room No. 1: The Secret Life of the Natural History Museum, London: Harper Perennial, ISBN 978-0-00-720989-7
- Mayr, Ernst (1991). Principles of Systematic Zoology. New York: McGraw-Hill, p. 159.
- Mayr, Ernst (1991), p. 162.
- Schuh, Randall T. and Andrew V. Z. Brower. 2009. Biological Systematics: Principles and Applications, 2nd edn. ISBN 978-0-8014-4799-0
- Simpson, Michael G. 2005. Plant Systematics. ISBN 978-0-12-644460-5