How Animals Are Classified

For centuries, the practice of naming and classifying living organisms into groups has been an integral part of the study of nature. Aristotle (384BC-322BC) developed the first known method of classifying organisms, grouping organisms by their means of transport such as air, land, and water. A number of other naturalists followed with other classification systems. But it was Swedish botanist, Carolus (Carl) Linnaeus (1707-1778) that is considered to be the pioneer of modern taxonomy.

In his book Systema Naturae, first published in 1735, Carl Linnaeus introduced a rather clever way to classify and name organisms. This system, now referred to as Linnaean taxonomy, has been used to varying extents, ever since.

About Linnaean Taxonomy

Linnaean taxonomy categorizes organisms into a hierarchy of kingdoms, classes, orders, families, genera, and species based on shared physical characteristics. The category of phylum was added to the classification scheme later, as a hierarchical level just beneath kingdom.

Groups at the top of the hierarchy (kingdom, phylum, class) are more broad in definition and contain a greater number of organisms than the more specific groups that are lower in the hierarchy (families, genera, species).

By assigning each group of organisms to a kingdom, phylum, class, family, genus, and species, they can then be uniquely characterized. Their membership in a group tells us about the traits they share with other members of the group, or the traits that make them unique when compared to organisms in groups to which they do not belong.

Many scientists still use the Linnaean classification system to some extent today, but it is no longer the only method for grouping and characterizing organisms. Scientists now have many different ways of identifying organisms and describing how they relate to each other.

To best understand the science of classification, it will help to first examine a few basic terms:

• classification - the systematic grouping and naming of organisms based on shared structural similarities, functional similarities, or evolutionary history
• taxonomy - the science of classifying organisms (describing, naming, and categorizing organisms)
• systematics - the study of the diversity of life and the relationships between organisms

Types of Classification Systems

With an understanding of classification, taxonomy, and systematics, we can now examine the different types of classifications systems that are available. For instance, you can classify organisms according to their structure, placing organisms that look similar in the same group. Alternatively, you can classify organisms according to their evolutionary history, placing organisms that have a shared ancestry in the same group. These two approaches are referred to as phenetics and cladistics and are defined as follows:

• phenetics - a method of classifying organisms that is based on their overall similarity in physical characteristics or other observable traits (it does not take phylogeny into account)
• cladistics - a method of analysis (genetic analysis, biochemical analysis, morphological analysis) that determines relationships between organisms that are based solely on their evolutionary history

In general, Linnaean taxonomy uses phenetics to classify organisms. This means it relies on physical characteristics or other observable traits to classify organisms and does consider the evolutionary history of those organisms. But keep in mind that similar physical characteristics are often the product of shared evolutionary history, so Linnaean taxonomy (or phenetics) sometimes reflects the evolutionary background of a group of organisms.

Cladistics (also called phylogenetics or phylogenetic systematics) looks to the evolutionary history of organisms to form the underlying framework for their classification. Cladistics, therefore, differs from phenetics in that it is based on phylogeny (the evolutionary history of a group or lineage), not on the observation of physical similarities.

Cladograms

When characterizing the evolutionary history of a group of organisms, scientists develop tree-like diagrams called cladograms. These diagrams consist of a series of branches and leaves that represent the evolution of groups of organisms through time. When a group splits into two groups, the cladogram displays a node, after which the branch then proceeds in different directions. Organisms are located as leaves (at the ends of the branches). 

Biological Classification

Biological classification is in a continual state of flux. As our knowledge of organisms expands, we gain a better understanding of the similarities and differences among various groups of organisms. In turn, those similarities and differences shape how we assign animals to the various groups (taxa).

taxon (pl. taxa) - taxonomic unit, a group of organisms that has been named

Factors That Shaped High-Order Taxonomy

The invention of the microscope in the mid-sixteenth century revealed a minute world filled with countless new organisms that had previously escaped classification because they were too tiny to see with the naked eye.

Throughout the past century, rapid advances in evolution and genetics (as well as a host of related fields such as cell biology, molecular biology, molecular genetics, and biochemistry, to name just a few) constantly reshape our understanding of how organisms relate to one another and shed new light on previous classifications. Science is constantly reorganizing the branches and leaves of the tree of life.

The vast changes to a classification that have occurred throughout the history of taxonomy can best be understood by examining how the highest level taxa (domain, kingdom, phylum) have changed throughout history.

The history of taxonomy stretches back to the 4th century BC, to the times of Aristotle and before. Since the first classification systems emerged, dividing the world of life into various groups with various relationships, scientists have grappled with the task of keeping classification in sync with scientific evidence.

The sections that follow provide a summary of the changes that have taken place at the highest level of biological classification over the history of taxonomy.

Two Kingdoms (Aristotle, during 4th century BC)

Classification system based on: Observation (phenetics)

Aristotle was among the first to document the division of life forms into animals and plants. Aristotle classified animals according to observation, for example, he defined high-level groups of animals by whether or not they had red blood (this roughly reflects the division between vertebrates and invertebrates used today).

• Plantae - plants
• Animalia - animals

Three Kingdoms (Ernst Haeckel, 1894)

Classification system based on: Observation (phenetics)

The three kingdom system, introduced by Ernst Haeckel in 1894, reflected the long-standing two kingdoms (Plantae and Animalia) that can be attributed to Aristotle (perhaps before) and added third kingdom, Protista that included single-celled eukaryotes and bacteria (prokaryotes).

• Plantae - plants (mostly autotrophic, multi-cellular eukaryotes, reproduction by spores)
• Animalia - animals (heterotrophic, multi-cellular eukaryotes)
• Protista - single-celled eukaryotes and bacteria (prokaryotes)

Four Kingdoms (Herbert Copeland, 1956)

Classification system based on: Observation (phenetics)

The important change introduced by this classification scheme was the introduction of the Kingdom Bacteria. This reflected the growing understanding that bacteria (single-celled prokaryotes) were very much different from single-celled eukaryotes. Previously, single-celled eukaryotes and bacteria (single-celled prokaryotes) were grouped together in the Kingdom Protista. But Copeland elevated Haeckel's two Protista phyla to the level of kingdom.

• Plantae - plants (mostly autotrophic, multi-cellular eukaryotes, reproduction by spores)
• Animalia - animals (heterotrophic, multi-cellular eukaryotes)
• Protista - single-celled eukaryotes (lack tissues or extensive cellular differentiation)
• Bacteria - bacteria (single-celled prokaryotes)

Five Kingdoms (Robert Whittaker, 1959)

Classification system based on: Observation (phenetics)

Robert Whittaker's 1959 classification scheme added the fifth kingdom to Copeland's four kingdoms, the Kingdom Fungi (single and multi-cellular osmotrophic eukaryotes)

• Plantae - plants (mostly autotrophic, multi-cellular eukaryotes, reproduction by spores)
• Animalia - animals (heterotrophic, multi-cellular eukaryotes)
• Protista - single-celled eukaryotes (lack tissues or extensive cellular differentiation)
• Monera - bacteria (single-celled prokaryotes)
• Fungi (single and multi-cellular osmotrophic eukaryotes)

Six Kingdoms (Carl Woese, 1977)

Classification system based on: Evolution and molecular genetics (Cladistics/Phylogeny)

In 1977, Carl Woese extended Robert Whittaker's Five Kingdoms to replace Kingdom bacteria with two kingdoms, Eubacteria and Archaebacteria. Archaebacteria differ from Eubacteria in their genetic transcription and translation processes (in Archaebacteria, transcription, and translation more closely resembled eukaryotes). These distinguishing characteristics were shown by molecular genetic analysis.

• Plantae - plants (mostly autotrophic, multi-cellular eukaryotes, reproduction by spores)
• Animalia - animals (heterotrophic, multi-cellular eukaryotes)
• Eubacteria - bacteria (single-celled prokaryotes)
• Archaebacteria - prokaryotes (differ from bacteria in their genetic transcription and translation, more similar to eukaryotes)
• Protista - single-celled eukaryotes (lack tissues or extensive cellular differentiation)
• Fungi - single and multi-cellular osmotrophic eukaryotes

Three Domains (Carl Woese, 1990)

Classification system based on: Evolution and molecular genetics (Cladistics/Phylogeny)

In 1990, Carl Woese put forth a classification scheme that greatly overhauled previous classification schemes. The three-domain system he proposed is based on molecular biology studies and resulted in the placement of organisms into three domains.

• Bacteria
• Archaea
• Eukarya