Systematics

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Systematics

Systematics (or biosystematics) is the classification of living organisms into natural, ordered in a hierarchical (or layered) fashion that emphasizes their phylogenetic (or evolutionary) relationships. Systematics is related to taxonomy, although the latter is more directly concerned with the theory and practice of naming and describing species and other taxonomic units (or taxa).

Modern systematists believe that all organisms can be divided into five major group, known as kingdoms ( these represent the highest level in the systematic organization of life. The five kingdoms are:

• Monerans, including bacteria and cyanobacteria (or blue-green bacteria), which are the simplest organisms, being single-celled and lacking a membrane-bounded organelle called a nucleus (i.e., they are prokaryotes; all other kingdoms have a nucleus, and are eukaryotes).
• Protista, encompassing a wide diversity of simple, eukaryotic organisms, including unicellular and multicellular species of protozoans, foraminifera, slime moulds, single-celled algae, and multicellular alga.
• Fungi, including yeasts, which are single-celled microorganisms, and fungi, which are multi-celled and filamentous in their growth form.
• Plantae, encompassing multicellular, photosynthetic organisms that manufacture their own food by using the energy of sunlight to synthesize organic molecules from inorganic ones (plants also differ from algae in having cell walls rich in cellulose, a mixture of photosynthetic pigments that includes chlorophylls a and b and carotenoids, and using starch as their principal means of storing energy).
• Animalia, which includes multicellular organisms that are mobile during at least some stage of their life history, and are heterotrophs that must ingest their food, ultimately consuming the photosynthetic products of plants or algae.

Below the level of kingdom, the hierarchical system used by systematists (and taxonomists) differs somewhat between zoology and botany. The major taxonomic elements (or taxa), listed in ascending order, are:

• Botany (twelve ranks): Kingdom, Division, Class, Order, Family, Tribe, Genus, Section, Series, Species, Variety, Form (some of the lower taxa are used infrequently, usually for genera containing very large numbers of species, or for economically important plants).
• Zoology (seven ranks): Kingdom, Phylum, Class, Order, Family, Genus, Species (additional groups may be recognized using the prefixes "super" and "sub," as in subspecies).
• The classification system can be illustrated using the following example, for the monarch butterfly of North America: Kingdom: Animalia; Phylum: Arthropoda; Class: Insecta; Order: Lepidoptera; Family: Danaidae; Genus: Danaus; Species: Danaus plexippus; Subspecies: Danaus plexippus plexippus.

Modern systematists examine as many biological characters as possible when trying to classify groups of organisms and determine their evolutionary relationships. Most commonly, structural (or anatomical) characters are used, because data on these are usually more readily available than for other elements. However, behavioral, biochemical, genetic, and ecological information may also be used in modern phylogenetic studies. To analyze their often large sets of data, systematists use various mathematical and statistical procedures. One frequently used procedure is cluster analysis, which divides taxa into groupings based on the similarities of their shared qualities. Species that cluster together in such an analysis are presumed to be more closely related and might (for example) be placed in the same genus.

The scientific goals of systematics are to develop catalogues of the world's species, to classify them into natural groups based on relatedness, and to understand their evolutionary relationships. Ultimately, this is founded in a desire to understand the patterns and processes of the natural world, and the pathways that species and other taxa have taken during the progression of life on Earth.