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Life Science: Session 2

Taxonomic Classification

How are organisms classified to the species level?

A hierarchical system is used for classifying organisms to the species level. This system is called taxonomic classification. The broadest classifications are by domain and kingdom; the most specific classification is by genus and species. The hierarchical groupings in between include phylum, class, family, and order.

What is a species?

Species are the basic unit of classification. While there are different views on what defines a species, in sexually reproducing organisms, a species has traditionally been defined by the ability of its members to reproduce together to form fertile offspring. This definition is trickier in asexually reproducing organisms like bacteria, archaea, and protists, where scientists look at the similarity in DNA among individuals to tell whether they are in the same “species.”

When identifying an organism, familiar names — like human, fruit fly, or maple tree — are most likely the names you use. However, each type of organism has a scientific name — humans are called Homo sapiens, for example. Scientific names are derived from the genus and species names in a system known as binomial nomenclature (“bi” = two; “nom” = name). When species names appear in print, the genus is always capitalized and the species name is not, and both names appear in italics (or underlined).

This system allows for a common language with which biologists can both classify and compare organisms. It also provides a basis for biologists to communicate their findings with other scientists. A key aspect of taxonomy is the ability to characterize each level of description with unifying features, thus relaying information about the organism(s).

For example, for Homo sapiens, the classification would break down as follows:

Classification level Name Characterized by:
Domain Eukarya Nucleus, organelles
Kingdom Animalia Ingests food, multicellular, no cell wall
Phylum Chordata Spinal cord
Subphylum Vertebrata Segmented backbone
Superclass Tetrapoda Four limbs
Class Mammalia Nurse offspring
Subclass Theria Live birth
Order Primates High level of intelligence
Family Hominidae Walk upright
Genus Homo Human
Species H. sapiens Modern human

Below is an additional example of the classification of an animal. This is for an invertebrate, the fruit fly, which is used by many biologists to study genes and heredity.

Classification level Name Characterized by:
Domain Eukarya Nucleus, organelles
Kingdom Animalia Ingests food, multicellular, no cell wall
Phylum Arthropoda Hard exoskeleton, paired legs, segmented body
Class Insecta Terrestrial, six legs, antennae
Order Diptera Two-winged
Family Drosophilidae  
Genus Drosophila  
Species D. melanogaster Common fruit fly

Furthermore, similar schemes are used for plants. Below is the description used to characterize a tree, the red maple.

Classification level Name Characterized by:
Domain Eukarya Nucleus, organelles
Kingdom Plantae Makes own food, multicellular, cell wall
Phylum Tracheophyta Tissue-level organization
Class Angiospermae Flowering
Order Sapindales Produces sap
Family Aceraceae  
Genus Acer  
Species A. rubrum Red maple

As additional organisms are described and additional features are found which differentiate these groups, biologists have added taxonomic levels to distinguish groups from one another. As seen in the Homo sapiens table, there are “sub” and “super” groupings which come before or after the traditional taxonomic level. This further level of detail gives scientists more power to describe organisms.

Not surprisingly, there are levels “below” species. Particularly common in bacteria and plants, “subspecies” are described which can produce viable offspring but possess one or more features which distinguish them from other members of that species. In plants, these sub groups are referred to as different “varieties” or “subspecies,” while in single-celled organisms, they are referred to as “strains.” Within the animal kingdom, subspecies may be termed “races” or “breeds.” For example, all domesticated dogs are grouped in the same species, Canis familiaris, though we all know there is a huge diversity of subspecies, or breeds.

Ideally, the taxonomic classification reflects evolutionary relationships among organisms. For example, it is accepted that all organisms included in the phylum Chordata descend from a common ancestor. Currently, many biologists are using DNA sequence data to verify taxonomic descriptions of organisms and determine the patterns of evolution within and between different groups.

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