Taxonomy and the Tree of Life

Learning Objectives

Explain the Linnaean system of hierarchical classification.

List the major taxonomic ranks in order from most inclusive to least inclusive.

Distinguish between the three domains of life: Bacteria, Archaea, and Eukarya.

Interpret a simple phylogenetic tree to determine evolutionary relationships.

Organizing Life's Diversity

Taxonomy is the science of naming, defining, and classifying groups of biological organisms on the basis of shared characteristics. The system used today is based on the work of Carl Linnaeus.

The Linnaean Hierarchy

Linnaeus proposed a hierarchical system of classification, where organisms are grouped into increasingly inclusive categories. The standard ranks are:

Domain (Most inclusive)

Kingdom

Phylum

Class

Order

Family

Genus

Species (Least inclusive)

A useful mnemonic is 'Dear King Philip Came Over For Good Soup.'

Every species is given a unique two-part scientific name, a system called binomial nomenclature. The name consists of the Genus (always capitalized) and the species (always lowercase), and is written in italics. For example, humans are Homo sapiens.

The Three Domains of Life

Based on molecular data, particularly ribosomal RNA sequences, all life is now classified into three domains:

1.Bacteria: These are prokaryotic organisms (lacking a nucleus). They have cell walls containing peptidoglycan. They are the most widespread and diverse group of organisms.

2.Archaea: Also prokaryotic, but they have distinct biochemistry and genetics from bacteria. Their cell walls lack peptidoglycan, and their cell membranes are composed of different lipids. Many are extremophiles, living in harsh environments like hot springs or salt flats.

3.Eukarya: These are all eukaryotic organisms (cells contain a nucleus and other membrane-bound organelles). This domain includes the kingdoms Protista, Fungi, Plantae, and Animalia.

Phylogenetics: The Tree of Life

Modern classification aims to reflect evolutionary history. Phylogenetics is the study of evolutionary relationships among biological entities. These relationships are often depicted in a branching diagram called a phylogenetic tree.

How to read a phylogenetic tree:

The root of the tree represents the most recent common ancestor of all organisms in the tree.

Each branch point (or node) represents a divergence event, where a single ancestral lineage split into two or more.

The tips of the branches represent the organisms being studied.

Organisms that share a more recent common ancestor (a closer branch point) are more closely related than those that share a more distant common ancestor.

The length of the branches can sometimes represent time or the amount of genetic change.

Key Terms

Taxonomy: The science of classification of organisms into an ordered system that indicates natural relationships.
Binomial Nomenclature: The system of nomenclature in which two terms are used to denote a species of living organism, the first one indicating the genus and the second the specific epithet.
Phylogenetic Tree: A branching diagram showing the inferred evolutionary relationships among various biological species or other entities based upon similarities and differences in their physical or genetic characteristics.
Prokaryote: A microscopic single-celled organism that has neither a distinct nucleus with a membrane nor other specialized organelles. Prokaryotes include the bacteria and archaea.
Eukaryote: An organism consisting of a cell or cells in which the genetic material is DNA in the form of chromosomes contained within a distinct nucleus.