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What is a species

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Last updated on 24-Jan-2020

Juan Miguel Artigas Azas,

Classification: Taxonomy and phylogeny.

" For several centuries the coining of a simple and definitive definition of what a species is has been widely searched, with several definitions being produced. Unfortunately, to our days no definition is satisfactory for every case treated "

The need for the species concept

In our constant effort to understand life forms around us being for academic, medicinal, or commercial purposes, we have the need to be able to tell apart the different organisms and recognize them in a way that either if we talk or write about them we are being unequivocal of what organism we are talking about. With this important aim humans have developed the concept of the species.

Although the classification achieved by naming different species may sound straightforward and evident when we talk about very different forms, let’s say and elephant and a giraffe, things get significantly more complicated when we are referring to closely related organisms, and particularly so after we realize the extent of both individual and geographical variation on many of them. When should we stop considering two slightly different forms as varieties of one single species and name them different species? This by no means is a trivial question, and has been in the mind of biologist since the very beginning of species classification.

In “On the Origin of Species by Means of Natural Selection, Or, The Preservation of Favored Races in the Struggle for Life” in 1859, Charles Darwin recognizes this difficulty and is not able to offer better advice than (p. 47): “Hence, in determining whether a form should be ranked as a species or a variety, the opinion of naturalists having sound judgement and wide experience seems the only guide to follow”.

Darwin considered species as the fundamental units of evolution, emphasizing the fact that species could be produced rapidly if the conditions were appropriate and in the absence of such conditions, species might remain unchanged for a long time.

For several centuries the coining of a simple and definitive definition of what a species is has been widely searched, with several definitions being produced. Unfortunately, to our days no definition is satisfactory for every case treated.

The earliest systematist to achieve a species definition was the English scholar John Ray (1627–1705). In his major work Historia Plantarum (1686) he stated: “No matter what variations occur in the individuals or the species, if they spring from the seed of one and the same plant, they are accidental variations and not such as distinguish a species permanently; one species never springs from the seed of another nor vice versa”.

This latter definition however does not account for many cases we see in nature. Let’s think for a moment about two classic cichlid species: The angelfish Pterophyllum scalare and the discus Symphysodon discus. Most people would agree at once that they are two different species, no doubt about it. Things get however significantly more complicated when we are trying to tell apart two of the three different species of angelfish, Pterophyllum scalare and P. altum, with discussions extending to our days. Being P. scalare so variable, is P. altum really a different species? How can we prove it?

Species concepts

Young adult at Mante River Herichthys carpintis. Photo by Juan Miguel Artigas Azas. (10-Feb-2007). determiner Juan Miguel Artigas Azas
Adult at San Juan River Herichthys carpintis (picture above) and H. cyanoguttatus (picture below), different species or different geographical variations?. Photo by Juan Miguel Artigas Azas. (10-Jul-2006). determiner Juan Miguel Artigas Azas

To be able to establish the limits between different species, given the difficulty to obtain a universal definition, taxonomists have established different concepts of a species, concepts to be used either individually or in combination when naming a new species. There are at least 26 different species concepts nowadays, and probably more will be coined in the future. I list here four of them:

Biological species concept

Probably the most widely used species concept, coined by the German zoologist Ernst Mayr (1904-2015) in 1942. He defined a species as: “groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups”. Groups of related species do not interbreed when growing in the same area in nature. However this is inapplicable to asexual organisms, it is also impractical in instances of geographically isolated populations of the same species. We also know that introduced species may sometimes interbreed with the local forms. Take for example the initially relatively high hybridization rate of the introduced cichlid Herichthys carpintis with H. labridens in the valley of Rioverde in San Luis Potosi, Mexico — it seems however that although initially high, hybridization has significantly decreased nowadays.

In talking about variation in different populations, let’s consider two widely accepted cichlid species: Herichthys cyanoguttatus (The Texas cichlid) and H. carpintis. They live in different river systems, and visually differ in the size of the green spots found scattered throughout the body and fins. They do not interbreed in nature because they are geographically isolated, but if they were put together, they would certainly interbreed and create individuals of mixed spots sizes, this has been experienced by aquarists for many years now. The hybrid (?) fry of such mixed pairs subsequently recognize each other and reproduce among themselves. Are then H. carpintis and H. cyanoguttatus different species? Given that they do not abide to the biological species concept? Opinions vary and there is not a definitive answer. Already Alvarez del Villar (1970) proposed them just as different subspecies, which are no more than officialized geographical varieties, I tend to agree.

Morphological species concept

In this concept, coined by Arthur Cronquist (1919-1992) in 1978, a species is characterized by body shape and other structural features, with organisms classified as the same species if they appear identical by morphological criteria. It is applied to asexual and sexual organisms alike and it is useful when information on gene flow is unknown.

However, many times morphological characteristics to be selected as invariable are subjective and depend on expert opinion to select. To make this worse, in some cases species can be morphologically indistinguishable but still different. Several different species of birds may appear identical and even overlapping in their distribution and are just differentiated by their songs and behavior. Let’s also consider two Lake Malawi cichlid species: Cynotilapia afra and C. zebroides, they are morphologically identical and can just be differentiated by minor aspects of their coloration, clearly they could not be catalogued as different species based in the morphological species concept. The same can be said about the three Symphysodon (discus) species of South America.

Ecological species concept

Leigh Van Valen (1935-2010) in 1976 established the Ecological Species Concept, where a species is a lineage (or a closely related set of lineages) which shares the exact same ecological niche. Although this concept recognizes the role of the environment in controlling morphological development, the ecological species concept has complications since it may be difficult to precisely describe a niche, also and more important, some species may start their development in one ecological niche and subsequently move to a different one. Think about salmons starting their lives in freshwater creeks and subsequently moving to the sea. Species may also exploit alternative resources (different niches) when a resources becomes plentiful or scarce. Being plentiful, two different species may occupy (at least temporarily) the exact same niche as another one. Most cichlid species are opportunistic and will not doubt to take advantage of a suddenly abundant resource. Furthermore, we are familiar with both lacustrine and riverine populations of a single species (e.g. Herichthys labridens), which are morphologically different but readily interbreed if put together.

Phylogenetic species concept

This concept was proposed by the paleontologists Niles Eldredge (1943-) and Joel Cracraft (1943-) in 1980. The phylogenetic species concept simply defines species as a group of organisms that share a common ancestor, species are determined by means of uncover the genealogical relationships between populations, obtaining monophyletic groups that include all the descendants of a common ancestor. In other words species are individuals that show a high degree of resemblances in many unique traits which produce monophyletic clusters based on discriminative phenotypes. A key to this concept is diagnosability of species. That is, a set of unique characters should be shared by all individuals in a species (or any other major taxonomic group) that differentiate them from other.

However clear it is, this concept has its own practical problems, as it is necessary to obtain the evolutionary path of a group and in many cases it is not easy to determine the set of characteristics that define it. Morphology is variable between populations of a single species and selecting the wrong attributes my lead to excessive splitting of a phylogenetic lineage into supposed species which are not reproductively isolated. Consider the great morphological variation of humans in several continents, but humans are not reproductively isolated. Many cichlids species exhibit likewise great morphological and coloration variation and many species inhabit through separate drainages, it is my belief that in many case the morphological variation has been excessively used to define inexistent species. In any case, the diagnosis of a species must be applicable to every individual of that species, and nothing more. New DNA comparison techniques and a better knowledge of the variation and relative importance of shared morphological features will certainly make this concept more useful.

Pluralistic species concept

Although not one of the 26 species concepts, this pseudo-concept proposes the idea to use more than one species concept when necessary. This recognizes, basically, that “the factors that are most important for the cohesion of individuals as a species vary”.

Deciding on a species

Up until the late 1970s all taxonomic decisions and their resulting papers were supported in what we know as morphology based taxonomy, which is the comparison of the shape of the body and the placement of the different parts of an organism. In the case of fishes two particular sets of measurements were the base of most of the comparisons between species: meristics and morphometrics.

Meristics relates to the counting of quantitative features of a fish, such as the number of spines in different fins or the number of rows of scales in different parts of the body. Since rarely all fish specimens belonging to a species, even in the same population, have exactly the same number of each feature, they were normally expressed in ranges, or in more recent times with statistical values such as mean, mode, standard deviations, and others.

Morphometrics relates to the measurements of distinct parts of the body as a relation to a base measurement, normally the length of the body, which is expressed in two different manners: the standard length (SL), meaning the size of the fish from the tip of the snout to the base of the caudal fin; that is, the length excluding the tail, and the total length (TL), which as you can infer, includes the length of the caudal fin.

Early fish collection expeditions in the eighteenth, nineteenth, and early twentieth centuries targeted areas sometimes difficult to travel, and had to cope with transportation conditions which in many cases were not optimal for the perfect preservation of the collected specimens. As you can imagine, the tail (without hard spines) and other soft fins were the first to get damaged during collection, preservation and transportation. For this and other reasons the standard length was often used as the base for morphometric proportions.

Early ichthyologists would take many measurements of different features in specimens of what they thought was a single species, and compared them with others they thought a closely related species. They suspected this relatedness because of their shared common features, features that later defined major taxonomic ranks like families. When sufficient differences were found between two groups of similar organisms, they would name new species based on these differences in publications where they offered a diagnosis which comprised a detailed description of the organism, and if the means were available, with beautiful illustrations.

Most of the early ichthyologists were, however, academics and museum curators, and were rarely familiar with the live organisms they studied and with their habitats. In most cases they never saw live examples of their study subjects, and so they trusted in their measurements and comparisons to establish the relationships among the studied organisms, often concluding differently from those relations we now obtain using modern methods.

One of the problems taxonomy faced was the sometimes small number of available specimens and their different stages of development. When you compare specimens of a different age, you are faced with what is known as ontogeny, which is the development of an organism from birth to adulthood. During this development changes occur in body proportions, so a morphological relation (e.g. head length / standard length) may be very different in a young individual compared to an adult one. In our days some mathematical algorithms are used to correct partially for these variations, but in the early days this was not possible, since sufficient specimens / data and the proper methods were lacking.


Taking all this into consideration, a modern species description should contain the species concept upon which it is defined, a description of the species and its geographical range and variation and a clear differentiating diagnosis that sets it apart from all other species.


Special thanks to Rico Morgenstern for reviewing this article and giving me valuable comments.

References (5):


Artigas Azas, Juan Miguel. (January 23, 2020). "What is a species". Cichlid Room Companion. Retrieved on November 28, 2021, from: