- Botanists and mycologists use systematic naming conventions for taxa higher than genus by combining the Latin stem of the type genus for that taxon with a standard ending characteristic of the particular rank. (See below for a list of standard endings.) For example, the rose family Rosaceae is named after the stem "Ros-" of the type genus Rosa plus the standard ending "-aceae" for a family.
- Zoologists use similar conventions for higher taxa, but only up to the rank of superfamily.
- Higher taxa and especially intermediate taxa are prone to revision as new information about relationships is discovered. For example, the traditional classification of primates (class Mammalia-subclass Theria-infraclass Eutheria-order Primates) is challenged by new classifications such as McKenna and Bell (class Mammalia-subclass Theriformes- infraclass Holotheria-order Primates). These differences arise because there are only a small number of ranks available and a large number of proposed branching points in the fossil record.
- Within species, further units may be recognized. Animals may be classified into subspecies (for example, Homo sapiens sapiens, modern humans). Plants may be classified into subspecies (for example, Pisum sativum subsp. sativum, the garden pea) or varieties (for example, Pisum sativum var. macrocarpon, snow pea), with cultivated plants getting a cultivar name (for example, Pisum sativum var. macrocarpon "Snowbird"). Bacteria may be classified by strains (for example Escherichia coli O157:H7, a strain that can cause food poisoning).
Taxa above the genus level are often given names derived from the Latin (or Latinized) stem of the type genus, plus a standard suffix. The suffixes used to form these names depend on the kingdom, and sometimes the phylum and class, as set out in the table below.RankPlantsAlgaeFungiAnimalsDivision/Phylum -phyta-mycotaSubdivision/Subphylum -phytina-mycotinaClass-opsida-phyceae-mycetesSubclass-idae-phycidae-mycetidaeSuperorder -anaeOrder -alesSuborder -ineaeInfraorder -ariaSuperfamily -acea-oideaFamily -aceae-idaeSubfamily -oideae-inaeTribe -eae-iniSubtribe -inae-ina
- The stem of a word may not be straightforward to deduce from the nominative form as it appears in the name of the genus. For example, Latin "homo" (human) has stem "homin-", thus Hominidae, not "Homidae".
- For animals, there are standard suffixes for taxa only up to the rank of superfamily (ICZN article 27.2).
Classification of organisms is a natural activity of humans and may be the oldest science, as humans needed to classify plants as edible or poisonous, snakes and other animals as dangerous or harmless, and so forth.
The earliest known system of classifying forms of life comes from the Greek philosopher Aristotle, who classified animals based on their means of transportation (air, land, or water), and into those that have red blood and have live births and those that do not. Aristotle divided plants into trees, shrubs, and herbs (although his writings on plants have been lost).
In 1172, Ibn Rushd (Averroes), who was a judge (Qadi) in Seville, translated and abridged Aristotle's book de Anima (On the Soul) into Arabic. His original commentary is now lost, but its translation into Latin by Michael Scot survives.
An important advance was made by the Swiss professor, Conrad von Gesner (1516-1565). Gesner's work was a critical compilation of life known at the time.
The exploration of parts of the New World next brought to hand descriptions and specimens of many novel forms of animal life. In the latter part of the sixteenth century and the beginning of the seventeenth, careful study of animals commenced, which, directed first to familiar kinds, was gradually extended until it formed a sufficient body of knowledge to serve as an anatomical basis for classification. Advances in using this knowledge to classify living beings bear a debt to the research of medical anatomists, such as Hieronymus Fabricius (1537 - 1619), Petrus Severinus (1580 - 1656), William Harvey (1578 - 1657), and Edward Tyson (1649 - 1708). Advances in classification due to the work of entomologists and the first microscopists is due to the research of people like Marcello Malpighi (1628 - 1694), Jan Swammerdam (1637 - 1680), and Robert Hooke (1635 - 1702).
John Ray (1627 - 1705) was an English naturalist who published important works on plants, animals, and natural theology. The approach he took to the classification of plants in his Historia Plantarum was an important step towards modern taxonomy. Ray rejected the system of dichotomous division by which species were classified according to a pre-conceived, either/or type system, and instead classified plants according to similarities and differences that emerged from observation.
Two years after John Ray's death, Carolus Linnaeus (1707-1778) was born. His great work, the Systema Naturae, ran through twelve editions during his lifetime (1st ed. 1735). In this work nature was divided into three realms: mineral, vegetable, and animal. Linnaeus used four ranks: class, order, genus, and species. He consciously based his system of nomenclature and classification on what he knew of Aristotle (Hull 1988).
Linnaeus is best known for his introduction of the method still used to formulate the scientific name of every species. Before Linnaeus, long, many-worded names had been used, but as these names gave a description of the species, they were not fixed. By consistently using a two-word Latin name-the genus name followed by the specific epithet-Linnaeus separated nomenclature from taxonomy. This convention for naming species is referred to as binomial nomenclature.
Classification after Linnaeus
Some major developments in the system of taxonomy since Linnaeus were the development of different ranks for organisms and codes for nomenclature (see Domain and Kingdom systems, and Universal Codes above), and the inclusion of Darwinian concepts in taxonomy.
According to Hull (1988), "in its heyday, biological systematics was the queen of the sciences, rivaling physics." Lindroth (1983) referenced it as the "most lovable of the sciences." But at the time of Darwin, taxonomy was not held in such high regard as it was earlier. It gained new prominence with the publication of Darwin's The Origin of Species, and particularly since the Modern Synthesis. Since then, although there have been, and continue to be, debates in the scientific community over the usefulness of phylogeny in biological classification, it is generally accepted by taxonomists today that classification of organisms should reflect or represent phylogeny, via the Darwinian principle of common descent.A vertical orientation yields a cladogram reminiscent of a tree.
Taxonomy remains a dynamic science, with developing trends, diversity of opinions, and clashing doctrines. Two of these competing groups that formed in the 1950s and 1960s were the pheneticists and cladists.
Begun in the 1950s, the pheneticists prioritized quantitative or numerical analysis and the recognition of similar characteristics among organisms over the alternative of speculating about process and making classifications based on evolutionary descent or phylogeny.
Cladistic taxonomy or cladism groups organisms by evolutionary relationships, and arranges taxa in an evolutionary tree. Most modern systems of biological classification are based on cladistic analysis. Cladistics is the most prominent of several taxonomic systems, which also include approaches that tend to rely on key characters (such as the traditional approach of evolutionary systematics, as advocated by G. G. Simpson and E. Mayr). Willi Hennig (1913-1976) is widely regarded as the founder of cladistics.
- Hull, D. L. 1988. Science as a Process: An Evolutionary Account of the Social and Conceptional Development of Science. Chicago: University of Chicago Press.
- Lindroth, S. 1983. The two faces of Linnaeus. In Linnaeus, the Man and his Work (Ed. T. Frangsmyr) 1-62. Berkeley: University of California Press.