Species are often defined as the largest group of organisms in which two individuals can produce fertile offspring, typically by sexual reproduction. To differentiate between types of living organisms, species are used as the principal natural unit in biology.
Four most important concepts on species are:
- Typological Species Concept: In this concept , there are a number of varieties of living organisms of finite types that exist on the earth. These types don’t have any relationship with each other. These types are termed as species. This inequality is regarded as an unimportant and irrelevant phenomenon.
- Aristotle and Plato stated about this concept in their philosophies. In the year of 1954 and 1956, Cain regarded the Typological species concept as the morphospecies concept. As the members of the species or a taxon can be identified by their essential characteristics, a group of scientists refers to this as essentialist species concept.
- Morpho species concept defines the species can be differentiated from other species by their physical features and can be identified by their morphological attributes. So, this is called as the morphological species concept.
- Nominalistic Species Concept: Nominalistic species concept is the concept of Occam and his followers, who believed that nature produces individuals only. Species are man’s own creation and they have no definite existence in nature. They are just mental concepts. Species have been invented so that we may refer to great numbers of individuals collectively. In France during the 18th century, this concept was in demand and even now is used by some botanists.
- Biological Species Concept: In the middle of 18th century, a fresh concept called biological species concept appeared. This concept was accepted in the later half of the nineteenth century after Darwin’s “Origin of Species” was published (in 1859). So, this is also known as Newer Species concept. K. Jordan first formulated this concept in 1905. Later in 1940, Mayr supported this concept. As per this concept, ”a species is a group of interbreeding natural population that is reproductively isolated from other such groups”. Mayr described that the members of a species show the following properties:
- Reproductive community: For the purpose of reproduction the individuals of a species recognize each other as potential mates.
- Ecological Unit: The species members differ from each other due to many attributes but all the members cooperatively form a unit. They interact with other species as a unit in any environment.
- Genetical unit: Species consists of a large, inter-communicating gene pool, although the individual is simply a non-permanent vessel holding a small part of the contents of the gene pool for a short period of time.
- Evolutionary Species Concept: The flaws of biological species concept had led the paleontologists to formulate the evolutionary species concept.
- Simpson (in 1961) had defined it as “an evolutionary species is a lineage (an ancestral- descendant sequence of populations) evolving separately from others and with its own unitary evolutionary role and tendencies”.
- Wiley (in 1978) had provided a revised definition of evolutionary species concept. He stated that “an evolutionary species are a single lineage of ancestral-descendant population which draws its identity from other such lineages and has its own evolutionary tendencies and historical fate”.