Drawing the line regarding species classification

By Nitara Wijayatilake

Providing a definition for a ‘species’ is in no way, simple. Defining this term is of such value because it acts a unit of comparison, relevant in the classification of groups of living organisms and the measurement of biological diversity. Mayden (1999) identified at least 22 species concepts being used in modern times. Arguably, the most commonly referred to definition comes from the ‘Biological Species Concept’- that species are groups of interbreeding populations in nature. One of the main reasons there is dispute concerning the species concept is due to Darwinian theories surrounding speciation. Darwin (1859) published the theory of Natural Selection, explaining that species change so slowly it becomes difficult to specify the exact time when one species diverges into two. One scientist may group organisms into ‘species’ using genetic or morphological similarities while another may consider reproductive compatibility or phylogenetic relationships.

Firstly, when considering how one may organise groups of organisms into species, the initial logic is by physical traits. This is the ‘Morphological Species Concept’. However, the problem that arises is the similarity of phenotypes between species. A clear advantage of using this concept to individualise species is that organisms can be grouped easily using observable traits, without causing harm to life. When judging morphology, boundaries become extremely fuzzy. Some intraspecific differences among organisms due to individual genetic variation may be mistaken as different species or two similar species may be classed as one (Wheeler & Meier, 2000). For example, when examining a group of worms at hydrothermal vents, it can be seen that at hot vents the worms appear as short and fat tubes, whereas at cool diffuse vents they are long and skinny. Initially, these were described as two separate species- R. Piscesae and R. phaeophiale but were actually found to be plastic morphs of just one species, R. Piscesae (Brazaeu, 2019). Genetic testing will often differentiate two organisms that have similar observable characteristics as different species, making the morphological species concept insufficient as a way to define ‘species’. Although morphology is a seemingly easy way to classify organisms, a more comprehensive concept is required to accurately assign a species definition.

The ‘Biological Species Concept’ defines species as groups of interbreeding natural populations that are reproductively isolated from other groups (Mayr, 1942). Mayr further explains this through organisms being able to recognise individuals of the same species and produce fertile offspring. The reproductive barrier of different species being unable to interbreed means that they possess separate gene pools with no gene flow, gradually making them more and more genetically different over time. Indeed, this concept faces its own issues as it excludes the most abundant organisms on earth, bacteria. There is no consideration for prokaryotic species within this definition, making it an insufficient explanation to account for all life (Hey, 2009). Moreover, organisms that do not necessarily need to interbreed such as, some angiosperms are capable of self-fertilisation. They, therefore, are excluded from this definition too (University of Toronto, 2013).

The ”Phylogenetic Species Concept” attempts to define the term ‘species’ by considering relationships to other species. It is described as a group of organisms that have shared unique character states and exhibit a pattern of phylogenetic ancestry and descent (Eldredge & Cracraft, 1980). This concept puts the term ‘species’ in terms of its evolutionary history which allows the grouping together of asexual organisms as prokaryotes have their own distinct evolutionary history observable through character traits. Unfortunately, extremely closely related species are difficult to differentiate, and this may lead to an over-estimate of species numbers. In fact, using the Phylogenetic Species Concept inflated 15 amphibian species into 140 (Maclurin & Sternly, 2008). This can be dangerous as overestimating the number of species means genetic diversity is inaccurately recorded.

So why should we care? The Species Concept Problem may just be considered an arbitrary biology problem, more blamed upon the systematics of language than a real scientific concern. Some even argue that the term ‘species’ should be discarded all together. Indeed, even Darwin in 1859 said this issue is “trying to define the indefinable”. However, this problem does affect many aspects of everyday life. A farmer needs to tell the difference between crop plants and weeds to maximise agricultural yield and a doctor needs to differentiate bacterial species to prescribe effective antibiotics (Hey, 2009). Differentiating species is especially important for conservation biologists to be able to identify species on the verge of extinction. The North Pacific Right Whale only fairly recently was classified as its own species after examining of its mitochondrial DNA sequence. Once identified as its own species, it was realised that its population is so small that the group must be considered endangered (Federal Regulations, 2008). Such identification means that necessary steps can be taken to protect this endangered species.

Indeed, this problem is a conceptual one that faces barriers imposed by the schematics of language. This requires a conceptual solution. Arguably, the idea of eliminating the term ‘species’ is not a suitable solution. Although, the discrepancies in what the true species definition is causes problems, using this word is still important in biological study. This debate is especially significant to aid conservation efforts. What is a species? That is a question yet to be answered.


Mayden, R. (1999) Consilience and a Hierachy of Species Concepts: Advances Toward Closure on the Species Puzzle. Journal of Nematology 31 (2): 95-116.

Darwin, C. (1859) On the origin of species by means of natural selection, or preservation of favoured races in the struggle for life. London, John Murray

Wheeler, Q and Meier, R. (2000) Species Concepts and Phylogenetic Theory. New York, Columbia University Press

Brazeau, M. (2019) Species and Speciation. [Lecture] Imperial College London, 17th October.

Mayr, E. (1942) Systematics and the Origin of Species. New York, Columbia University Press

Hey, J. (2009) Why Should We Care about Species? Nature Education 2(5):2

University of Toronto. (2013) Self-fertilizing plants contribute to their own demise. ScienceDaily. Available from: http://www.sciencedaily.com/releases/2013/06/130610095148.htm Accessed [28.02.2020]

 Eldridge, N., & Cracraft, J. (1980) Phylogenetic patterns and the evolutionary process. New York, Columbia University Press

 Maclurin, J. and Sterenly K. (2008) What is Biodiversity? Chicago, University of Chicago Press

 Federal Register. (2008) Endangered and Threatened Species; Endangered Status for North Pacific and North Atlantic Right Whales. Available from: https://www.federalregister.gov/documents/2008/03/06/E8-4376/endangered-and-threatened-species-endangered-status-for-north-pacific-and-north-atlantic-right Accessed [24.02.2020]

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