Biological systematics is the study of the diversification of life on the planet Earth, 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.
"Systematic biology" and "taxonomy" (terms that are often confused and used interchangeably) were defined in relationship to one another as follows:
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". However, 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. However, systematics alone deals specifically with relationships through time, and can be synonymous with phylogenetics, broadly dealing with the inferred hierarchy of 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 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.
Systematics is fundamental to biology because it is the foundation for all studies of organisms, by showing how any organism relates to other living things (ancestor-descendant relationships).
Systematics is also of major importance in understanding conservation issues because it attempts to explain the Earth's biodiversity and could be used to assist in allocating limited means to preserve and protect endangered species, by looking at, for example, the genetic diversity among various taxa of plants or animals and deciding how much of that to preserve.
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