What is a species?

What Is A Species?


Photo credit: Arthur Chapman, Breutelia affinisCC BY-NC-SA

Biodiversity can be measured in a number of ways, depending on what aspect is being examined. There is the diversity of functions (e.g., decomposition, nutrient cycling, predation) that organisms can perform in their ecosystems, the diversity of genes in a population of an organism, or even the diversity of ecosystem types in a particular area. Most of the time when people talk about biodiversity, though, they’re referring to the number of different species (or “types”) of organisms. Biodiversity inventories, such as BioBlitzes, or catalogues, such as the Encyclopedia of Life, are organized by species.

Ursus americanus is the genus species name for the American black bear. The first part of the name is the genus, a group of related species (in this case bears); the second part of the name is the species within the genus. In some cases, a subspecies can also be designated to further distinguish groups within a species, indicated by the addition of a third name. Ursus americanus has several subspecies, for example, Ursus americanus californiensis and Ursus americanus altifrontalis; these subspecies names refer to the geographical locations in which they’re found.

Because evolution is an ongoing process, new species form continually over time, which means that there is no perfect, definitive way of classifying organisms. However, taken together, there are multiple “species concepts” that provide guidelines.

Biological species concept

Most broadly, a species is a group of organisms with a shared, closed gene pool. In many organisms, the way to pinpoint a species is to apply the biological species concept. According to this definition, a species is made up of all the organisms that are able to mate with each other and produce fertile offspring. For example, lions produce offspring with other lions, and tigers with other tigers; if lions and tigers are brought together in captivity they can occasionally produce a hybrid called a liger or tigon, but these hybrids are not themselves able to reproduce. It then follows that lions and tigers are separate species. (Another term used for the inability to interbreed is reproductive isolation.)

Morphological species concept

Another way that organisms can be classified is on the basis of morphology, or physical features. This definition is also often useful: sometimes it is not possible to observe whether two organisms interbreed (e.g., different types of whales in the wild) but examining their physical features can help determine how closely they’re related. In general, if carefully chosen, morphological features can indeed be an indicator of organisms having the same common ancestor.

There are many problems with this approach:  domestic dogs, for example, can look very different but are actually all one species capable of interbreeding, whereas mouse lemurs look very similar to each other but are actually at least 18 distinct species.

Genetic species concept

For organisms that have neither sexual reproduction nor many distinctive morphological features in common—for example, bacteria—species are often defined as clusters of genetically similar organisms (with “similar” somewhat arbitrarily defined). Using genetic markers as traits defining species is also the principle behind DNA barcoding.


Winston, Judith.E. 1999. Describing Species: Practical Taxonomic Procedure for Biologists Columbia University Press, New York.

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Alexandra Mushegian, University of Basel

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Species Concepts are concepts that were developed to help taxonomists define a species. There are many different concepts such as the Biological Species Concept, Morphological species concept,Phylogenetic species concept and the Genetic species concept.