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Taxonomic rank of organisms

From its Latin roots, the word species means “kind” or “appearance” (Campbell & Reece, 2005). In biology, the word species refers to a unit of classification given to a group of closely related organisms (Purves, Sadava, Orians, & Heller, 2004). There are various methods used to classify populations as individual species, and no single method has been universally adopted over the others leaving the scientific community without a concrete definition of “species” (Wayne & Gittleman, 1995). 

Species Concepts[]

Below you will find a few of the concepts used by researchers to classify organisms into different species.

Morphospecies Concept - Categorizes different organisms based on similarities in their appearance, or morphology, this was one of the first ways that scientists started to distinguish different species (Purves, Sadava, Orians & Heller, 2004).

Biological Species Concept – Stipulates that to be considered a species, members of the studied population must be able to produce fertile offspring with each other, but not with other species (Campbell & Reece, 2005; Freeman & Herron, 2007). This concept hinges on reproductive isolation and cannot be applied to bacteria.

Ecological Species Concept – Defines species based on the ecological niche they occupy and resources they use (Campbell & Reece, 2005).

Pluralistic Species Concept – Combines both the Biological and Ecological Species concepts (Campbell & Reece, 2005).

Genealogical Species Concept – Describes individuals with a closely related genetic history as members of the same species (Cambell & Reece, 2005).

Phylogenetic Species Concept – Defines a species as the smallest group of descendants from a common ancestor (monophyletic group) (Freeman & Herron, 2007).

Why We Care About The Definition[]

The Endangered Species Act only extends its protection to distinct species, thus appropriate classification could be a matter of life or death for many organisms.


Campbell, M. A., & Reece, J. (2005). Biology (7th ed.). San Francisco, CA: Pearson Benjamin Cummings.

Freeman, S. Herron, J. (2007). Evolutionary Analysis (4th  ed.). San Francisco, CA: Pearson Benjamin Cummings.

Purves, W., Sadava, D., Orians, G., Heller, H. C. (2004). Life the Science of Biology (7th ed.). Sunderland, MA: Sinauer Associates, Inc.

Wayne, RK and Gittleman, JL. (1995). The problematic red wolf. Scientific American. 273(1): 36–39.