The Linnaean hierarchy has long served as the basis for biological classification, but there have been other systems, and there are new systems in development. There are three main philosophical questions about biological classification. The first is theoretical: what is represented? The second is about the formal structure: what are the categories and how are they related? The third is operational: what principles are used to generate the classification? Historically there have been theoretical approaches based on pragmatic interests, natural kinds and essences, Divine ideas, evolutionary history, phenetics and the phylogenetic branching of the evolutionary tree. Currently there is some consensus that classification should represent evolutionary history in terms of phylogeny, but there are competing proposals both for the formal structure of classification, and operational methods to reconstruct phylogeny.

Key Concepts:

• The theoretical basis of a classification is what it represents. The formal structure of a classification involves the nature and organization of the categories. The operational procedures of a classification are the rules for generating the classification.