Meaning of Species:
The term species is the basic unit of classification. Over time, several concepts of species have been proposed to explain diversity among living organisms.
Currently accepted meanings of species:
1. Morphological species concept:
According to this concept, a species is a group of organisms that resemble each other in morphological characters such as size, shape and structure. This concept is useful in the classification of fossils and organisms with limited biological information.
2. Biological species concept:
Proposed by Ernst Mayr, it defines a species as a group of interbreeding natural populations that are reproductively isolated from other such groups. This is the most widely accepted concept for higher plants and animals.
3. Ecological species concept:
According to this concept, a species occupies a specific ecological niche and is adapted to a particular environment. Different species may coexist by occupying different ecological niches.
4. Phylogenetic species concept:
This concept defines a species as the smallest group of organisms sharing a common ancestor and forming a distinct branch in evolutionary history. It is based on evolutionary relationships and genetic similarities.
5. Genetic species concept:
According to this concept, species are groups of organisms that share a common gene pool and show significant genetic similarity. Modern molecular techniques are used to identify species under this concept.
Species concept in higher plants and animals:
In higher plants and animals, the biological species concept is most appropriate.
Individuals belonging to the same species can interbreed and produce fertile offspring. Reproductive isolation acts as an important barrier between different species.
Thus, species in higher organisms are defined mainly by reproductive compatibility.
Species concept in bacteria:
In bacteria, the biological species concept is not applicable because:
• Bacteria reproduce asexually.
• There is no interbreeding.
• Horizontal gene transfer occurs frequently.
Therefore, bacterial species are defined using:
• Morphological characters
• Biochemical properties
• Genetic similarity (DNA–DNA hybridization, rRNA sequencing)
A bacterial species is considered as a group of strains showing high genetic similarity and common physiological traits.
Conclusion:
Thus, the concept of species varies among different groups of organisms. While reproductive isolation defines species in higher plants and animals, genetic and biochemical criteria are used to define species in bacteria.