Notes

Taxonomy is a branch of biology that identifies, names, and classifies living things. The study of taxonomy helps reconstruct the evolutionary progression of a species. It also provides information about the existing biological diversity on Earth.

Linnaeus's hierarchical system of classification included seven levels—kingdom, phylum, class, order, family, genus, and species. Each of these levels is called a taxon, or taxonomic category.

Linnaeus divided all living organisms into two kingdoms—Animalia and Plantae. These kingdoms were divided into phyla, and phyla into classes, and so on. These categories were based on shared physical characteristics, or phenotypes, within each group

Classification Cat Lion Tiger
Kingdom Animalia A malia Animalia
Phylum Chordata Chordata Chordata
Class Mammalia Mammalia Mammalia
Order Carnivora Carnivora Carnivora
Family Felidae Felidae Felidae
Genus Felis Panthera Panthera
Species catus leo tigris

The table on the left will help you understand the Linnaeus system of classification. Consider the three living organisms—a cat, a lion, and a tiger. All three organisms belong to the cat family.

Notice how the classification remains common up to the classification hierarchy of family. The cat is placed under the genus Felis, where as the lion and tiger, because of their ability to roar, are placed in the separate genus Panthera. The distinction between the lion and tiger is then made at the species level. The lion is placed under the species leo and the tiger under the species tigris.

According to this nomenclature, every organism is assigned a two–part scientific name. The first part of the name is derived from the genus, and the second part from its species.

Phylogeny can be defined as the mapping of the evolutionary development of an organism. It indicates the lines of descent of the organism and its relationship with other organisms.

Earlier classification systems used physical characteristics, but classification systems today use fossil data, physical characteristics, and DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) information to draw more accurate branching diagrams to represent the evolutionary relationships among organisms.

basis of taxonomy in the twenty-first century?=phylogenetic relationships

Whittaker's system recognized the two fundamentally different type of cells, prokaryotic (simple single–cell organisms) and eukaryotic (complex single–cell organisms), and set the prokaryotes apart from all eukaryotes by placing them in their own kingdom, Monera. He also indentified a separate kingdom, Protista, for complex single–cell eukaryotes. He then distinguished three kingdoms of multicellular Eukaryotes—Animalia, Plantae, and Fungi.

According to the six kingdom system of classification, Archaebacteria differ from the Eubacteria in biochemical processes and metabolic pathways. Due to this differentiation, Archaebacteria and Eubacteria are now placed in two separate kingdoms of their own

Recent molecular studies have led scientists to create a new taxonomic category—a domain, or superkingdom, that is more inclusive and larger than a kingdom. The three domains are Bacteria, corresponding to the kingdom Eubacteria, Archaea, corresponding to the kingdom Archaebacteria, and Eukarya, which includes the kingdoms Protista, Fungi, Plantae, and Animalia.

Domain Archaea is comprised of unicellular prokaryotes that have cell walls without peptidoglycan.