Fundamentals of Biological Classification and Taxonomy

Core Principles of Biological Classification

Taxonomy

Taxonomy is a subdiscipline of systematic biology that studies the relationships among organisms and their evolutionary history. Today, taxonomy aims to resolve the phylogenetic tree of the organisms studied. This means organizing them into clades, or evolutionary branches, based on their relationships.

Systematics

Systematics is the study of the classification of species according to their evolutionary history (phylogeny).

The Hierarchical System of Taxonomy

Taxon

A taxon is a group of related organisms that, in a given classification, have been grouped together. The group is given a Latin name, a description, and a "type." For a species, the type is a specific, exemplary specimen.

Taxonomic Category

Taxa, the groups into which living things are classified, are structured in a hierarchy of inclusion. In this system, a group includes other smaller groups and is, in turn, subordinate to a larger one. Each group is assigned a taxonomic rank or taxonomic category, which accompanies the group's name. Some well-known examples are:

• Genus: Homo
• Family: Canidae (dogs)
• Order: Primates
• Class: Mammalia (mammals)
• Kingdom: Fungi (fungi)

Phylum and Division

A phylum is a taxonomic category placed between the Kingdom and the Class. It is used for the animal kingdom, protist kingdom, and bacteria. In Botany (Kingdom Plantae) and for the Kingdom Fungi, the term division is used instead; both terms are equivalent.

Understanding Evolutionary Relationships

Homology

Homology is the relationship between two different parts when their genetic determinants share the same evolutionary origin. Furthermore, homologous structures may have diverged in function to fulfill different roles.

Analogy

In biology, two structures are considered analogous if they fulfill similar functions by similar means, without necessarily having the same evolutionary origin. If they share the same evolutionary origin, they are homologous, not just analogous.

Approaches to Biological Classification

Natural System

A natural system is an organizational system that emerges as a property of nature. The concept of a natural system is opposed to an artificial system, in which the classification of elements depends on an artificial criterion adopted by convention.

Phylogenetic Classification

Phylogenetic classification is a scientific classification of species based solely on their evolutionary relationships. It aims to reconstruct the history of their diversification (phylogenesis) from the origin of life to the present day.

Modern Classification

Modern classification is rooted in the system of Carolus Linnaeus (1753), who proposed a hierarchical classification of species based on shared physical characteristics and standardized their naming.