A Higher Level Classification of All Living Organisms

RESEARCH ARTICLE A Higher Level Classification of All Living Organisms

Abstract

We present a consensus classification of life to embrace the more than one point six million species already provided by more than three thousand taxonomists' expert opinions in a unified and coherent, hierarchically ranked system known as the Catalogue of Life. The intent of this collaborative effort is to provide a hierarchical classification serving not only the needs of the Catalogue of Life's database providers but also the diverse public-domain user community, most of whom are familiar with the Linnaean conceptual system of ordering taxon relationships. This classification is neither phylogenetic nor evolutionary but instead represents a consensus view that accommodates taxonomic choices and practical compromises among diverse expert opinions, public usages, and conflicting evidence about the boundaries between taxa and the ranks of major taxa, including kingdoms. Certain key issues, some not fully resolved, are addressed in particular. Beyond its immediate use as a management tool for the Catalogue of Life and Integrated Taxonomic Information System, it is immediately valuable as a reference for taxonomic and biodiversity research, as a tool for societal communication, and as a classificatory "backbone" for biodiversity databases, museum collections, libraries, and textbooks. Such a modern comprehensive hierarchy has not previously existed at this level of specificity.

Introduction

Biological classification (taxonomy) aims to simplify and order the immense diversity of life into coherent units called taxa that have widely accepted names and whose members share important properties. It synthesizes information concerning a great variety of characters (e.g.,

morphological; molecular: genes, metagenome, and metabolome; etho-ecological). There is currently no consensus among the world's taxonomists concerning which classification scheme to use for the overall hierarchy of life, in part because of the confusion resulting from Hennig's redefinition of previous terminology of classification, which has not been universally accepted; the separate goals of cladification and classification; and conflicting or unresolved evidence for phylogenetic relationships. The continuing advances in the use of specialized analytical tools from many different fields and their resulting conclusions and assumptions require regular updates as advances in knowledge are made.

Biological classification can integrate diverse, character-based data in a phylogenetic framework, which allows a broad user community to utilize the disparate knowledge of shared biological properties of taxa. Phylogeny is, therefore, the basis for these biological classifications but there is still strong debate over their accounting for evolutionary divergence or information content other than the branching pattern. Accordingly, classifications have often been labeled either phylogenetic or evolutionary, depending mainly upon whether or not they reject paraphyletic groups.

While the type of classification to be used to support further exploration and analysis of any biological scenario may be important, it is not the subject of this paper. The proposed classification does not address detailed phylogenetic questions and, while hierarchical and reflective of phylogeny, is not itself a phylogenetic tree. The aim of this classification is to be a pragmatic means of managing the ever-increasing knowledge of the diversity of life, its relationships, characteristics, and properties. Indeed, the past two decades have witnessed an explosion in biodiversity research and informatics, emphasizing the need for a quality list of accepted scientific names of the more than one point nine million described living species and for greater consensus on how to classify them at higher taxonomic ranks. Since two thousand one, Species two thousand and the Integrated Taxonomic Information System have worked with their respective contributors to complete a comprehensive species list, called the Catalogue of Life. The Catalogue of Life Annual Checklist already contains more than one point six million valid or accepted species names provided by more than one hundred forty taxonomic databases involving more than three thousand taxonomists. More than eighty-two percent of the global species databases are provided at the rank of class or below (includes one point three million species), and more than sixty-three percent are provided at the rank of order or below (includes one point zero million species). Owing to the heterogeneity in higher level classification among the contributed databases, the Catalogue of Life managers sought a practical and coherent hierarchical classification that could serve as a framework for data integration. Here we explain the rationale behind the consensus higher level classification that we propose for Catalogue of Life use.

Our goal, therefore, is to provide a hierarchical classification for the Catalogue of Life and its contributors that (a) is ranked to encompass ordinal-level taxa to facilitate a seamless import of contributing databases; (b) serves the needs of the diverse public-domain user community, most of whom are familiar with the Linnaean conceptual system of ordering taxon relationships; and (c) is likely to be more or less stable for the next five years. Such a modern comprehensive hierarchy did not previously exist at this level of specificity. In this sense it summarizes overarching aspects of the tree of life, including both paraphyletic and monophyletic groups, both being important in facilitating meaningful communication among scientists and between the scientific community and society.

The most recent higher level classification to this level was published more than thirty years ago, before the advent of modern molecular analysis. Beyond the immediate use for Catalogue of Life, the hierarchy is valuable as a reference for taxonomic and biodiversity research, as a tool for societal communication, and as a stable classificatory "backbone" for biodiversity databases, museum collections, libraries, and textbooks, to name a few applications.