classification

The classification of the plant and animal kingdoms.

In biology, the arrangement of organisms into a hierarchy of groups on the basis of their similarities. The basic grouping is a species, several of which may constitute a genus, which in turn are grouped into families, and so on up through orders, classes, phyla (in plants, sometimes called divisions), and finally to kingdoms. The system that is used is one that reflects the evolutionary origin of the organisms. In other words, organisms belonging one group are thought to have evolved from a common ancestor at some time in the past.

All organisms are given a name formed from the species name and the genus name. This name is therefore called a binomial. The binomial name of humans is Homo sapiens. Homo is the genus name and sapiens the species name. Humans belong to the animal kingdom. The foundations of the binomial system were laid in the 18th century by Swedish naturalist Carolus Linnaeus.

The oldest method of classification, called phenetic classification, aims to classify organisms on the basis of as many as possible of their observable characteristics: their morphology, anatomy, physiology, and so on. Greek philosopher Theophrastus adopted this method in the 4th century BC, when he classified plants into trees, shrubs, undershrubs, and herbs.

Awareness of evolutionary theory, however, led to the development of phylogenetic classification, which aims to classify organisms in a way that mirrors their evolutionary (see evolution) and genetic relationships (see genetics). Species are grouped according to shared characteristics believed to be derived from common ancestors (care being taken to exclude shared characteristics known to be due to convergent evolution – such as the wings of bats and birds). In practice, most present-day systems of classification compromise between the phenetic and the phylogenetic approaches.

Present day classification often uses computers to analyse the degree of similarity between organisms, including the similarity of their DNA. Cladistics is a recent phylogenetic method that applies a strict, objective procedure, often assisted by computer analysis, to classify species according to characteristics derived from a common ancestor.