Biological classification, or scientific classification in biology, is a method by which biologists group and categorize organisms by biological type, such as genus or species. Biological classification is a form of scientific taxonomy.
Modern biological classification has its root in the work of Carolus Linnaeus, who grouped species according to shared physical characteristics. These groupings have since been revised to improve consistency with the Darwinian principle of common descent. Molecular phylogenetics, which uses DNA sequences as data, has driven many recent revisions and is likely to continue to do so. Biological classification belongs to the science of biological systematics.
Classification has been defined by Mayr as "The arrangement of entities in a hierarchical series of nested classes, in which similar or related classes at one hierarchical level are combined comprehensively into more inclusive classes at the next higher level." A class is defined as "a collection of similar entities", where the similarity consists of the entities having attributes or traits in common.
What makes biological classification different from other classification systems (e.g. classifying books in a library) is evolution: the similarity between organisms placed in the same taxon is not arbitrary, but is instead a result of shared descent from their nearest common ancestor. Accordingly the important attributes or traits for biological classification are those which are 'homologous', i.e. inherited from common ancestors. These must be separated from traits that are analogous. Thus birds and bats both have the power of flight, but this similarity is not used to classify them into a taxon (a "class")], because it is not inherited from a common ancestor. In spite of all the other differences between them, the fact that bats and whales both feed their young on milk is one of the features used to classify both as mammals, since it was inherited from a common ancestor.
Determining whether similarities are homologous or analogous or not can be difficult. Thus until recently, golden moles, found in South Africa, were placed in the same taxon (insectivores) as Northern Hemisphere moles, on the basis of morphological and behavioural similarities. However, molecular analysis has shown that they are not closely related, so that their similarities must be due to convergent evolution and not to shared descent, and so should not be used to place them in the same taxon. Read More