Defining A Species: The Biological Species Concept

Biological Species Concept

Biological Species Concept: Every day, news about the discovery of novel organisms and even endangered species are being reported. However, when one talks about a species, what does he actually pertains to?

As it turns out, the answer to this simple question is actually very complex.

More often than not, concepts about species (can refer to as either singular or plural) are very complicated. Even before Charles Darwin had launched his idea about how organisms could change through time, biologists have already struggled with questions pertaining to the definition of what a species is.

Everything became even harder when he showcased evidences that support evolution.

There have been several attempts to define what a species is, in fact, a lot of explanations regarding it were proposed. Some tried to delineate based on physical appearance, others on ancestry.

However, in this article, we will explore the most famous and widely accepted concept – the biological species concept.

What is the Biological Species Concept?

Ernst Mayr (Berlin Museum)
Ernst Mayr Statue in Berlin Museum (Source: Wikimedia)
Apparently, the biological species concept[1] is the most accepted and widely used concept that explains the concept of species. For instance, biologist Ernst Mayr (1940)[2], has defined species as “groups of interbreeding populations (both naturally and possibly), that are isolated from other groups in terms of their reproduction patterns“. In simpler terms, Mayr’s definition of a species is confined to the aspects of interbreeding or reproduction with others of the same kind.

  • In addition to that, Mayr also claimed that species of the same kind can identify their potential partners and if ever they mate, they are able to produce fertile offspring.
  • On the other hand, Mayr believed that different species will not even try to mate with each other. But if they do, they will only produce an offspring that is infertile.
  • Ernst Mayr was the first biologist to emphasize the use of the idea of reproduction (both shared and lack of) in delineating between different species. Later on, he called the ideas as the infamous Biological Species Concept.

Importance of Biological Species Concept

The uncertainty about the identification of species is not just a problem for scientists. Everyone even those not in the scientific field must be able to distinguish among different kinds of organisms[3].

The following reasons explain why the concept of species is of great importance.

1) Defines why individuals of same species are alike

Homo Sapiens Interbreeding happens when individuals of the same species tend to breed only among themselves. As this process occurs repeatedly, the genes that the parents pass on to their offspring are continuously being rearranged around the species’ gene pool (set of different genes in an interbreeding population). This gene pool is then what gives a sort of identity to the species.

For example, humans who are also known as the species Homo sapiens. Basically, the biological species concept can be used to differentiate humans from other species by looking at the set of genes (genome) and their gene pool.

2) Explains the importance of reproduction in the perpetuation of species

Reproduction For species to continue to survive, they need to undergo reproduction. And that is what the biological species concept is mainly about: reproduction. According to the concept, individuals continually look for other individuals as potential partners in order to continue the existence of their species.

3) Has an important role in taxonomy

Taxonomy Taxonomy, or the science that deals with the identification, naming, describing, and classifying living organisms, traces its principles from the biological species concept. The application of the definition of a species becomes important in the field as it sets parameters of how to classify each organism. By having a “specific” definition, it will be much easier to delineate organisms that do not meet that certain criteria.

Limitations/Problems of Biological Species Concept

But despite the widespread acceptance of the biological species concept, the very definition of the idea is somewhat criticized by other scientists. Sometimes the definition of the concept is limited and tends to be complicated by various factors[4]. Below are some of the following:

A) Overlooks variations that occur within species

Galapagos Sometimes, individuals of the same species are unable to reproduce among themselves due to the difference in geographical location.

  • Scientists then argue that if they follow the definition from the concept, wouldn’t the resistance of some organisms (even though they have the same morphological characteristics with the others) to mate due to geographic locations make them a different species?
  • Example-1: One good example for this variation caused by geographic location is the case of the female Galapagos ground finch Geospiza difficilis. Interestingly, they only respond to the songs of males of the same island and tend to ignore the songs of the males of the same species from other locations. Certainly, the biological species concept cannot provide an explanation to this phenomenon.

B) Does not take into consideration the changes that happen over time

Dinosaurs By merely observing mating patterns between organisms, the Biological Species Concept tends to overlook the fact that such concept is inapplicable to the study of fossils.

  • The very concept seems to be widely problematic especially when applied to the field of paleontology.
  • Paleontologists study organisms that are separated by thousands of years hence it would be difficult to determine whether such organisms were once part of the same reproductive community.
  • Example: In case of the dinosaurs, since no more species of dinosaurs are existing at present, it would be absolutely impossible to trace their mating pattern or part in the reproductive community. Hence, field of paleontology requires a different concept to define a species (i.e. phylogenetic species concept)

C) Tends to ignore hybridization

Donkey and Mule This phenomenon occurs when there is a mating between two genetically distinct species that creates a new organism[5].

  • The offspring, often infertile (unable to reproduce), then has inherited the characteristics of both of its parents.
  • Example: A hybrid between a horse and a donkey – a mule. Basically, these infertile species are incapable of mating with either of the parent species. And as alluded earlier in the previous items, the definition of a “species” based on the concept is invalid and inapplicable to this phenomenon.

D) Has problems regarding classifying non-bisexual organisms

Bacteria In the kingdoms of biological organisms, it is apparent that some organisms are classified as neither “males” nor “females“.

  • Several modes of uni-parental (single parent) births have already been recorded and such have been geared of evolutionary significance. The definition of a species according to the biological species concept tends to be inapplicable in many cases in the biological world.
  • Example: Many species of bacteria reproduce through asexual process (either by mere conjugation or binary fission) and are assigned to each species based on their chemical and structural characteristics. Again, the Biological Species concept requires organisms to mate with each other and exchange their genetic material in order to be called a species.
  • Bacteria deviate from this definition, hence, other species concept should be used.

Other Species Concepts

As alluded to earlier, the biological species concept certainly cannot address to its limitations. As a result, a lot of concepts to define a species were proposed. Below are just some of them.

Recognition Species Concept (RSC)

Recognition Species Concept Similar to the biological species concept, the recognition species concept revolves around the mating behavior and breeding patterns of organisms.

  • According to this concept, organisms that can recognize other organisms as their potential partner are considered as same species. Such phenomenon is termed as a “shared mate recognition system“.
  • Take for example the species in the Class Insecta of the Kingdom Animalia. Using this concept, insects like butterflies, bees, and ants can be classified as a species because they can recognize other “insects” as their potential mate despite their vast number.

Phenetic species concept (PhSC)

Phenetic Species Concept This concept dwells on the phenotypic (physical) characteristics that exist among organisms. According to this concept, organisms are classified to be under the same species only when they are physically distinct from other organisms.

  • In contrast to the recognition species concept, the phenetic species concept proposes that individuals who look physically similar but tend to avoid each other during mating are still considered as the same species.
  • The human species Homo sapiens can be used as an example for this concept. Humans can be classified as a single species because they look similar – same body parts and organs. This is of course regardless of some variations like skin color, height, and other traits.

Phylogenetic Species Concept (PSC)

Phylogenetic Species Concept This concept has a different approach as compared to the two previous ones. According to this concept, individuals belong to the same species if and only if they have descended from a common ancestor.

  • By having been descended from a common ancestor, each individuals of the same species have a certain defining (distinguishable from others) and derived (common trait inherited from ancestor) traits.
  • One good example for this concept is the species of dogs (e.g. Shi tzu, bulldog, pug) Canis familiaris, which all descended from their common ancestor: wolves. While they have shared characteristics, they also developed certain unique traits. Hence, species are considered to be individually distinct from each other.

More Species Concepts

According to ‘A hierarchy of species concepts: the denouement in the saga of the species problem‘ written by Dr. R.L. Mayden[6], there are other less common species concepts as well which are listed below:

  • Agamospecies Concept (ASC)
  • Cohesion Species Concept (CSC)
  • Cladistic Species Concept (CISC)
  • Composite Species Concept (CpSC)
  • Ecological Species Concept (EcSC)
  • Evolutionary Significant Unit (ESU)
  • Evolutionary Species Concept (ESC)
  • Genealogical Concordance (GCC)
  • Genetic Species Concept (GSC)
  • Genotypic Cluster Definition (GCD)
  • Hennigian Species Concept (HSC)
  • Internodal Species Concept (ISC)
  • Morphological Species Concept (MSC)
  • Non-dimensional Species Concept (NDSC)
  • Polythetic Species Concept (PtSC)
  • Recognition Species Concept (RSC)
  • Reproductive Competition (RCC)
  • Successional Species Concept (SSC)
  • Taxonomic Species Concept (TSC)

While the biological species concept remains to be questioned by some, it is undeniable that it provides the most essential and correct definition of what a species is. It is important to know that unanswered questions about species tend to have an impact especially when it comes to their preservation and possible restoration.

But as far as the definition of what a species is concerned (and after knowing the limitations if the concept), do you agree with Mayr’s? What do you think?

Cite this article as: "Defining A Species: The Biological Species Concept," in Bio Explorer, November 30, 2016,


  • [1] – “Evolution – A-Z – Biological Species Concept.” Evolution – A-Z – Biological Species Concept. Accessed November 30, 2016. Blackwell Publishing.
  • [2] – “Species Concepts and the Definition of “Species”” Untitled. Accessed November 30, 2016.
  • [3] – Hey, J. (2009) “Why Should We Care about Species?” Nature Education 2(5):2 – Accessed November 30, 2016. Nature
  • [4] – “What is a species?” – Accessed November 30, 2016. BBC
  • [5] – “Hybridization and “Genetic” Extinction” – Can and do we preserve the genetic integrity of species, and if so, how? – Accessed November 30, 2016. PDF.
  • [6] – Dr. R.L. Mayden – A hierarchy of species concepts: the denouement in the saga of the species problem‘. Last Accessed November 30, 2016. PDF