Introduction

A species is a basic unit of classification. However, the concept of species is not so easily defined. Species are the central unit of comparison in all fields of biological study from behaviour to anatomy and from  ecology to morphology; fields like evolution, physiology, genetics and paleontology all make use of the species as a unit of classification.

Biologists assign a two-part name when naming an organism. The first part is the genus and the second part is the species. This is done for all species except viruses. For example, in Homo sapiens, ‘Homo’ is the genus and ‘sapiens’ is the species.

However, there has been no clear definition of species for nearly three centuries. Biologists have tried repeatedly to come to some consensus about the factors that define species. Still, the living world is too dynamic and  full of changes to be pinned down to any universally fundamental distinctions. This is where the problem of the concept of species arises. 

Concept of Species

Over the centuries, biologists have tried to define what is meant by the concept of species. But each attempt to bring clarity to this idea unearths even more questions. These attempts to define what a species is and its limiting factors should be are called the species concept. There are several species concepts currently under study today. This study of the species problem is called microtaxonomy. There are almost 26 species concepts, but we will limit ourselves to the most popular concepts. They are as follows:

1.     Biological species concept

• Ernst Mayr gave the biological species concept in 1942. It is based on reproductive isolation
• Today, it is the most widely applied concept of species
• Mayr famously defines it as “groups of actual or potentially interbreeding natural populations, which are reproductively isolated from other such groups”
• Biological species concept means that if two related populations of organisms exist together, they will be classified as two different species if they cannit mate sexually and produce infertile offspring
• One problem with this concept is that it cannot be applied satisfactorily to species that reproduce asexually
• Another problem is that related species of animals exist on a continuum. On one end are species that interbreed freely and on the other, strictly isolated species, but most populations can move up and down this continuum as ecological and evolutionary conditions change
• One example of this problem in the biological concept of species is the instance of the Ensatina salamanders. Their many subspecies interbreed freely with their adjacent populations except two subspecies that live side by side in Southern California
• Then there is the problem of chronospecies. These are the representative stages in the same evolutionary lineage. It does not make sense to classify each stage into a different species

• Allopatric populations also present a problem if the letter applies this definition. For example, just because two groups of frogs of the same species live in geographically separate areas, they will not interbreed. But this does not make them members of different species

2.     Morphological species concept 

• As the name suggests, the morphological species concept distinguishes species based on their morphological (anatomical) characteristics.
• In the words of Regan, “a species is a community or several related communities, whose distinctive morphological characters are, in the opinion of a competent systematist, sufficiently definite to entitle it or them, to a specific name”
• There are some definite advantages to the morphological species concept. Using this definition, even asexually reproducing species can be classified
• This method of distinguishing populations is also helpful for paleontologists to classify species based on fossils
• As can be seen from the definition given by Regan, this kind of classification is subject to “expert” opinion. In other words, the distinctions may be at risk of being subjective rather than objective

• Conversely, two population members may differ significantly anatomically, as happens in bee or ant colonies, yet belong to the same species

3.     Ecological species concept

• According to a definition given by Valen, “A species is a lineage (or a closely related set of lineages) which occupies an adaptive zone minimally different from that of any other lineage in its range and which evolves separately from all lineages outside its range”.
• But Colinvaux explains it thus “A species is several related populations the members of which compete more with their kind than with members of other species”.
• So, to put it simply, a species is a population or a group of closely related folks whose needs overlap. So they are more likely to develop similar characteristics and occupy the same ecological niche.

4.     Evolutionary species concept 

• This concept defines species as “a single lineage of ancestor-descendant populations of organisms which maintains its identity from other such lineages [in space and time] and which has its evolutionary tendencies and historical fate”

• This concept was developed to include asexual species and species discovered in fossils, specifically for species that could not be classified by biologically species concept

5.     Cohesion species concept

• Defined as “an evolutionary lineage that serves as the arena of action of basic microevolutionary forces, such as gene flow (when applicable), genetic drift and natural selection”
• It is distinct from the evolutionary concept of species in that it stresses the current phenotypic similarities within species

6.     Pluralistic concept of species

• As can be seen from the above classifications, the concept of species is a complicated and challenging process
• There are too many factors in flux to pin down any one set of elements for distinction
• This gave rise to the need to have a pluralistic species concept
• Basically, this means using an amalgamation of all the species concepts at our disposal
• Campbell and Reece put it as “the most important factors for the cohesion of individuals as a species vary”
• These factors can be judged based on any species concepts, which are most cogent on a case by case basis

Conclusion

These are just some of the concepts in use today. Many others (such as composite, internodal, genetic, etc.) take into account other biological nuances. However, their use is contingent upon the aim of the classification project. If the project’s objective is to classify fossil species concepts that deal in the phenotypes (when genetic material is available) or morphological or even ecological species concepts are employed. What is clear is that the living world is vast and ever-changing. It’s not a simple task to create distinct categories of classification.

The most helpful guide in this mammoth task is “sufficiently distinct”. Taxonomists limit themselves to this parameter, enabling them to work with whatever knowledge about the organism under study is available.