Taxonomy (biology)

• 1. Introduction

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  Taxonomy is the scientific study of naming, defining, and classifying groups of biological organisms based on shared characteristics. Every plant, animal, and microbe a person can name carries a hidden address in a vast filing system. That system has principal ranks: domain, kingdom, phylum, class, order, family, genus, and species. Behind those tidy categories sits a long argument about what the words even mean. The exact definition of taxonomy varies from source to source. Some treat it as a sub-area of systematics. Others invert that relationship. Some consider the two terms synonymous. There is even disagreement over whether the naming of organisms belongs inside taxonomy at all. How did a discipline this fundamental end up so unsettled about its own boundaries? Where did the idea of nesting groups within groups come from? And what happens to the whole hierarchy when scientists decide that classification should mirror evolution rather than mere resemblance? The answers run from Egyptian wall paintings to the cells without a nucleus.

• 2. A Discipline Of Definitions

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  Michener and colleagues, writing in 1970, split the work into lettered duties. Their systematic biology provides scientific names, describes organisms, preserves collections, supplies classifications and identification keys, investigates evolutionary histories, and considers environmental adaptations. Taxonomy, in their scheme, is the part concerned only with naming, describing, preserving, and classifying. The remaining duties, the evolutionary ones, sat outside it.

  The word taxonomy was introduced in 1813 by de Candolle, in his Théorie élémentaire de la botanique. John Lindley offered an early definition of systematics in 1830, though he wrote of systematic botany rather than using the term systematics itself. A cluster of related labels, including systematics, biosystematics, scientific classification, and phylogenetics, have carried overlapping meanings, sometimes identical, sometimes slightly different, but always intersecting.

  Europeans tend to favor systematics and biosystematics for the study of biodiversity as a whole. North Americans reach more often for taxonomy. Within that broad term lives a narrower one. Alpha taxonomy refers specifically to the identification, description, and naming of organisms, while classification focuses on placing them in hierarchical groups that show their relationships. The naming half travels under its own strict rules.

• 3. From Alpha To Omega

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  William Bertram Turrill introduced the term alpha taxonomy in papers published in 1935 and 1937, where he weighed the philosophy and possible future of the discipline. He described an increasing desire amongst taxonomists to consider their problems from wider viewpoints and to seek closer cooperation with their cytological, ecological, and genetics colleagues. He imagined a far-distant taxonomy built on as wide a basis of morphological and physiological facts as possible.

  Turrill borrowed the Greek alphabet as a yardstick of ambition. He accepted the older taxonomy based on structure, conveniently designated alpha, while glimpsing an ideal omega taxonomy at the far end. Ideals, he wrote, can never be completely realized, yet they act as permanent stimulants. Some taxonomists, he allowed, please themselves by thinking they are now groping in a beta taxonomy. He explicitly excluded ecology, physiology, genetics, cytology, and phylogenetic reconstruction from alpha taxonomy.

  Later writers bent the term to a new purpose. They used alpha taxonomy to mean the delimitation of species alone, using whatever investigative techniques are available, including sophisticated computational or laboratory methods. Ernst Mayr in 1968 defined beta taxonomy as the classification of ranks higher than species. He called the sorting of species into groups of relatives the second stage of taxonomic activity. Deciding how to define species in the first place earned its own name: microtaxonomy.

• 4. Before The System

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  Medicinal plant illustrations appear in Egyptian wall paintings from around 1500 BC, a sign that the uses of different species were understood and that a basic taxonomy was already in place. Distinguishing poisonous plants from edible ones is integral to the survival of human communities, and naming the surrounding world likely began with the onset of language itself.

  Aristotle classified organisms during his stay on the island of Lesbos, between 384 and 322 BC. He sorted beings by their attributes, such as having live birth, four legs, eggs, blood, or being warm-bodied. He divided all living things into two groups, plants and animals. Some of his categories survive in common use, including Anhaima, animals without blood, and Enhaima, animals with blood, along with groupings like the sharks and cetaceans.

  Theophrastus, who lived from 370 to 285 BC, carried the tradition forward in his Historia Plantarum, mentioning some 500 plants and their uses. Several plant genera trace back to him, including Cornus, Crocus, and Narcissus.

  The Aristotelian framework dominated the Middle Ages, fitted out with ideas like the great chain of being and the scala naturae, the Natural Ladder. It did not classify plants or fungi, partly because microscopes did not yet exist. In the Muslim world, Al-Damiri, who died in 1405, wrote Life of Animals, treating in alphabetic order 931 animals drawn from the Quran, Arab traditions, and proverbial literature.

• 5. The Renaissance Catalogues

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  Andrea Cesalpino, the Italian physician who lived from 1519 to 1603, has been called the first taxonomist. His magnum opus De Plantis appeared in 1583 and described more than 1,500 plant species. Two of the large plant families he first recognized, the Asteraceae and the Brassicaceae, remain in use today. Sophisticated optical lenses, which let scientists study morphology in far greater detail, helped make such ambitious work possible.

  John Ray, working in England between 1627 and 1705, produced many important taxonomic works. His Methodus Plantarum Nova of 1682 published details of over 18,000 plant species and rested on many combined characters, making his classifications perhaps the most complex any taxonomist had yet produced.

  Joseph Pitton de Tournefort, of France, lived from 1656 to 1708. His 1700 work Institutiones Rei Herbariae included more than 9,000 species arranged into 698 genera. That text directly influenced a young student who later upended the entire field, because it was the book he learned from.

• 6. Linnaeus Sets The Start Line

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  Carl Linnaeus, the Swedish botanist who lived from 1707 to 1778, is regarded as the founder of the current system of taxonomy. His major works were Systema Naturae, first published in 1735, and Species Plantarum in 1753, followed by the tenth edition of Systema Naturae. He implemented a standardized binomial naming system, an elegant solution to a chaotic and disorganized literature.

  His 1735 classification of animals carried a confident title, Systema Naturae, the System of Nature, implying that he believed it was more than an artificial system. Earlier taxonomy had leaned on arbitrary criteria, the so-called artificial systems, including Linnaeus's own scheme of sexual classification for plants. He introduced the standards of class, order, genus, and species, and made it possible to identify plants and animals from his book using the smaller parts of the flower.

  Plant and animal taxonomists treat his work as the starting point for valid names, set at 1753 and 1758 respectively. Anything published before those dates is labeled pre-Linnaean and is not considered valid, with one exception, the spiders published in Svenska Spindlar. Even names Linnaeus himself published before those dates fall outside the count. His authority echoes in everyday citation: in 1758 he named the Asian elephant Elephas maximus, written as Elephas maximus Linnaeus, 1758, often abbreviated to the single letter L.

• 7. Trees And Evolution

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  Dendrograms of the animal and plant kingdoms began appearing toward the end of the 18th century, representing the nested pattern Linnaeus had specified, and arriving well before Darwin published. The pattern of this Natural System did not require a generating process, yet it may have implied one, inspiring early transmutationist thinkers. Erasmus Darwin explored such ideas in Zoonomia in 1796, and Jean-Baptiste Lamarck did so in his Philosophie zoologique of 1809. Robert Chambers popularized the notion anonymously in Vestiges of the Natural History of Creation in 1844.

  Charles Darwin's On the Origin of Species, published in 1859, supplied a new explanation: classification should reflect common descent. Tree of life diagrams grew popular, with fossil groups folded in. Birds became one of the first modern groups tied to fossil ancestors. Using the newly found fossils of Archaeopteryx and Hesperornis, Thomas Henry Huxley argued that birds had evolved from dinosaurs, a group formally named by Richard Owen in 1842. The modern evolutionary synthesis of the early 1940s settled an essentially modern understanding of how the major groups arose.

  Cladistics emerged from the 1960s. Julian Huxley used the term clade in 1958, and Cain and Harrison introduced cladistic in 1960. The method arranges taxa in a hierarchical evolutionary tree, aiming for taxa that are monophyletic, meaning they include all the descendants of an ancestral form. Groups missing some descendants are paraphyletic; groups stitched from more than one branch are polyphyletic. Monophyletic groups are recognized through synapomorphies, shared derived character states. An alternative naming system, the PhyloCode, has been proposed to regulate the naming of clades, where Linnaean ranks are optional and carry no formal standing.

• 8. Kingdoms, Domains, And Counting Life

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  Carl Woese's three-domain system, first proposed in 1977, was not generally accepted until later. Its main feature is the separation of Archaea and Bacteria, organisms once lumped into the single kingdom Bacteria, also called Monera, with Eukaryota reserved for all organisms whose cells contain a nucleus. A small number of scientists prefer a sixth kingdom, Archaea, without accepting the domain method at all.

  Thomas Cavalier-Smith, who published extensively on the classification of protists, proposed in 2002 that the Neomura, the clade joining Archaea and Eucarya, evolved from Bacteria, specifically from Actinomycetota. His 2004 classification treated the archaeobacteria as a subkingdom of Bacteria and rejected the three-domain system entirely. Stefan Luketa went the other direction in 2012, proposing a five dominion system that added Prionobiota, acellular and without nucleic acid, and Virusobiota, acellular but with nucleic acid.

  Comprehensive treatments of all life remain rare compared with partial classifications. Adl and colleagues published surveys in 2012 and 2019 covering eukaryotes with an emphasis on protists, while Ruggiero and colleagues covered both eukaryotes and prokaryotes to the rank of Order in 2015. Database-driven efforts include the Catalogue of Life, which attempts to list every documented species. That catalogue recorded 1.64 million species across all kingdoms, claiming coverage of more than three-quarters of the species known to modern science. The rest of the tree is still waiting to be named.