IDEAS + TERMS + MOVEMENTS Biosystematics The classification of living things; also known as taxonomy. Biosystematics was founded by Linnaeus, who introduced the familiar bionomial nomenclature in which each organism is given a generic (e.g. Homo) and a specific name (Homo Sapiens). The most majestic of taxonomic distinctions into plant and animal kingdoms (the subject-maters of botany and zoology respectively). Next in order of rank is the phylum, the members of which are united by a basic similarity of ground plan which may only be apparent at a relatively early stage of development. A phylum comprises a number of classes. Members of a class are united by a somewhat close degree of similarity than the members of a phylum -- extending to points of anatomical detail as well as to ground plan. For example, the phylum Arthropoda includes the classes Insecta and Crustacea, the Insecta being united by having three pairs of legs attached to the thoractic region of the body and respining by means of trachae; while within the Vertebrates (often accorded the rank of a sub-phylum within the phylum Chordata) fishes, reptiles, amphibians, birds and mammals are all graded as classes. Next below the class come successively the order, the family, and the genus (plural: genera}. It is not possible to specify the degree of resemblance that unites members of orders, families, or genera in a way that will apply to all animals and plants; it may be noted, however, that among mammals te whales and dolphins, the carnivores, the rodents, and the primates each form an order. Finally, below the genus comes the species; thus the big cats belong to the genus Panthera, in which Panthera leo (the lion) and Pantera tigris (the tiger) are species. Of the larger taxonomic sub-divisions it may be said that all phyla, classes and orders are intended to have the same value, i.e. to be of the same rank in the systematic hierarchy. Furthermore, each group within itself is intended to be monophyletic, i.e. to be such that the common ancestor of the members of the group is itself a member of the group. (There are exceeptions: notoriously, the class Reptilia is polyphyletic in the sense that reptiles are subdivided into two main streams -- one leading towards birds and one towards mammals -- both f which evolved out of amphibian ancestors.) On the other hand, taxonomics are not intended to be a dossier of extraordinary relationships; the purpose of taxonomy is to name reliably and consistently. Nevertheless, the fact of the evolutionary relationship imposes a certain pattern upon classification which is not o be avoided. Taxonomic characteristics, i.e. those upon which a systematic allocation may be made are, wherever possible, structural, since physiological and behavioural criteria are of little use to the museum taxonomist who normally has to handle dead specimens. Similar difficulties restrict the use of immunological criteria in determining blood relationships. They have, however, helped to show that whales are more nearly to akin to pigs than to other mammals that have been alleged to be their relations.
Posted on: Thu, 16 Oct 2014 10:58:57 +0000
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