Introduction

Taxonomy is the science of the classification of organisms, with the goal of showing relationships among organisms.

Taxonomy also provides a means of identifying organisms.

The Study Of Phylogenetic Relationships

Phylogeny is the evolutionary history of a group of organisms.

The taxonomic hierarchy shows evolutionary, or phylogenetic, relationships among organisms.

Bacteria were separated into the Kingdom Prokaryotae in 1968.

Living organisms were divided into five kingdoms in 1969.

The Three Domains

Living organisms are currently classified into three domains. A domain can be divided into kingdoms.

In this system, plants, animals, fungi, and protists belong to the Domain Eukarya.

Bacteria (with peptidoglycan) form a second domain.

Archaea (with unusual cell walls) are placed in the Domain Archaea.

It appears that the universal ancestor split into three lineages, giving rise to Archaea, Bacteria, and eventually the eukaryotes.

Infoldings in the plasma membrane of a prokaryotic cell may have produced both a nuclear membrane and endoplasmic reticulum.   On the other hand, lipid analysis of prokaryotes, archaea, and eukaryotes suggests that the nucleus may have come from endokaryosis: a gram-negative bacterium may have hosted an intracellular organism generally assumed to be a member of Domain Archaea, which became a membrane bound nucleus within a cell that would be called a protoeukaryote.   Endosymbiosis with other bacteria could then have provided membrane-bound organelles.

Cyanophora paradoxa - eukaryotic host and bacterium require each other for survival.

A Phylogenetic Hierarchy

Organisms are grouped into taxa according to phylogenetic relationships (from a common ancestor).

Some of the information for eukaryotic relationships is obtained from the fossil record.

Prokaryotic relationships are determined by rRNA sequencing, although some fossils of prokaryotes exist.

Fossilized prokaryotes