What is a Species?

It seems there are members who are unsure what a species is, why we use this term, and why scientific nomenclature is preferred to common names. The use of scientific nomenclature dates to the 1700’s with Carl Linnaeus, the father of modern taxonomy. Linnaeus was the first to organize and assign names to specific organisms within a hierarchy. This hierarchy has specific levels that all known life is sorted into: Domain, Kingdom, Phylum, Class, Order, Family, Genus, and finally Species (Table 1). A species is the lowest level of this basic classification and is always paired with the Genus as a species name (Ex: Crotalus horridus, Pantherophis alleghaniensis, Diadophis punctatus). This two named classification is known as binomial or scientific nomenclature.

The way a species has been classified has changed over the years. When Linnaeus started his classification system, he based his groupings on the morphological species concept. That is, a species is a group of organisms that shared morphological features. Over the years we have expanded or changed this definition such as with the biological species concept (Species are groups of interbreeding organisms), the recognition species concept (Species are groups of organisms that can potentially recognize each other as mates), or the phylogenetic species concept (A species is the smallest branch of a phylogenetic tree). In the modern era, we commonly use several of these methods coupled with genetic testing to determine what makes a species.

Table 1: Taxonomic hierarchy of the common garter snake

Domain Eukarya
Kingdom Animalia
Phylum Chordata
Class Sauropsida
Order Squamata
Family Natricidae
Genus Thamnophis
Species Thamnophis sirtalis
Thamnophis sirtalis (Linnaeus, 1758)
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There are a couple rules we use when talking about species. First, when using a species name the Genus is always capitalized. Next, when a species name is typed it is italicized, and when it is written it is underlined. If you’re using the same species name repeatedly, you can use abbreviations in the writing. When you have already referenced a species name you can abbreviate the genus (Ex: T. sirtalis). Additionally, if you’re talking about multiple species in one genus you may refer to them by using spp. For instance, if you wanted to reference Thamnophis sirtalis, T. brachystoma, and T. sauritus you could simply say Thamnophis spp. If you want to refer to a subspecies within a known species complex you could simply refer to them as ssp. For instance, you could refer to Coluber constrictor constrictor simply as C. constrictor ssp. Lastly, you will commonly see a name and date referenced after a species name (Ex: Thamnophis sirtalis (Linnaeus, 1758)). This refers to the first author(s) that described the species.

In the sciences we prefer this binomial nomenclature to common names as there will always be one single name assigned to one organism, while an individual species may have several common names. For instance, the eastern garter snake is also called a black snake, gardener snake, or garden snake depending on the locality but using the Linnaean system it will only be Thamnophis sirtalis. Additionally, there are species out there that do not have a common name associated with them.

Finally, how we view this hierarchy has changed over the years and newer levels have been added, with each grouping being further broken down with groupings like super families, subfamilies, infraorders, subclasses, etc. In the table below you can see that Suborder, Infraorder, two clades, subfamily, and subspecies have been added. The last grouping, subspecies, further divides the species classification. This occurs when portions of a species become geographically or reproductively isolated but are not distinct enough to be classified as a new species.

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Table 2: Taxonomic hierarchy of the southern Pacific rattlesnake

Domain Eukarya
Kingdom Animalia
Phylum Chordata
Class Sauropsida
Order Squamata
Suborder Serpentes
Infraorder Alethinophidia
Clade Macrostomata
Clade Caenophidia
Family Viperidae
Subfamily Crotalinae
Genus Crotalus
Species Crotalus oreganus
Subspecies Crotalus oreganus helleri
Crotalus oreganus helleri (Ashton et. al, 2001)


Jon currently lives in southern New York where he is continuing his research on timber rattlesnakes. Jon received his M.S. from East Stroudsburg University of Pennsylvania where he conducted research on how human disturbance effects population size of timber rattlesnakes. His research background includes anthropogenic habitat disturbance, timber rattlesnakes, fence lizards, worm snakes, pollinators, and ecology. Jon is currently teaching online for the Upward Bound program as a biological instructor where he has taught biology, chemistry, physics, herpetology, and general science. In his free time Jon enjoys board games, video games, hiking, photography, and helping with various educational groups on FaceBook.

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