Concept of species and Taxonomic Hierarchy:


·         Species is the fundamental unit of taxonomic hierarchy.

·         Davis (1978) called them building bricks in biological classification.

·         According to Biological Species Concept (Ernst Mayer), a species is a group of organisms reproductively isolated from other organisms.

·         It is the lowest category of hierarchy which is consistently used and recognized by all botanists.

·         According to Stobbins (1977) species is the basic unit of evolutionary process.

·         Taxonomy is the science of discovering naming and describing the species diversity of plant.

·         It is the arrangement of various categories in successive levels of the biological classification.

·         Each of this level or hierarchy is called as the taxonomic category or rank.

·         Examples: Birds are group of organism which shows common charactestics like feathers and flight. Thus, based on the common characterstics, they can be classified into a taxonomic category.


Study of taxonomy has lead to the taxonomic categories- Kingdom, Phylum, Class, Order, Family, Genus and Species. Now let us see how all the organisms are classified into the hierarchy.

·         Species: It is the lowest level of classification and shows the high level of similarities among the organisms. One species can be distinguished from other closely related species based on distinct differences in morphology.

 Example: Plasmodium vivax and Plasmodium falciparum, both are malaria causing parasite, but have different effects on the human (patient). Plasmodium is the name of the genus and has number of species which show distinct morphological characterstics.

·         Genus: The taxonomic group composes of multiple species which have similar characterstics but different from that of species from other genus.

Example: Genus of tiger, leopard, lion i.e. Panthera and the genus of cats i.e. Felis are grouped together in the family Felidae.

·         Order: The classification starts from order has less similarities, they are categorized based on aggregates of characterstics. A group of families, showing somewhat few similarities forms an order.

Example: Carnivora includes families like Felidae and Canidae.

·         Class: A group of orders which share a few similarities form a class.

Example: Orders of primates and carnivores are grouped together in the class of mammals.

·         Phylum: This is the next level of classification where along a number of classes are clubbed up to form one phylum.

Example:  Fishes, Reptiles, Aves, Amphibians and mammals together form the phylum of vertebrata and chordates.

·         Kingdom: This is the highest level of classification. We have kingdoms like Plantae, Animalia, Fungi, Protista and Monera.

Taxonomy is the science of the classification of life forms. The rules that have to be followed for a valid description of taxa are laid down in nomenclature codes by the respective taxonomic societies. Taxonomy is governed by long term traditions. Therefore, the assignment of organisms to a nomenclature code doesn’t always make sense from an evolutionary point of view. There are following types of taxonomy:

1.       Alpha taxonomy: This identifies new forms. Eg: A Greenland seal is distinct from a harbor seal.

2.       Beta taxonomy: It shows discontinuity from other forms. Eg: Greenland seals have harped shaped marking.

3.       Gamma taxonomy: This taxonomy applies formal name, according to rules.

Different Species concept:

         i.            Ernst Mayer invented the biological species concept in 1972.

       ii.            Phylogenetic species concept, uses molecular markers- usually DNA sequences to identify closely related taxa. It is an example of standard species concept in bacteria, but has been tested also for eukaryotic taxa.

Taxonomic classification divides species in a hierarchical system beginning with a domain and ending with a single species.

·         Categories within taxonomic classification are arranged in increasing specificity.

·         The most general category in taxonomic classification is domain, which is the point of origin for all species; all species belong to one of these domain: Bacteria, Archae, and Eukaryotes.

·         Within each of the three domains, we find kingdoms, second category within taxonomic classification followed by subsequent categories that includes phylum, class, order, family, genus, and species.

·         At each classification category, organism becomes more similar because they are more closely related.

·         As scientific technology advances, changes to taxonomic classification of many species must be altered as inaccuracies in classification are discovered and corrected.

How are organisms classified to the species level?

A hierarchical system is used for classifying organism to the species level. This system is called taxonomic classification. The broadcast classification is by domain and kingdoms, the most specific classification are by genus and species. The hierarchical groupings in between includes phylum, class, family, order.



What are species?

Species are the basic unit of classification. In sexually reproducing organism, a species has traditionally been defined by the ability of its members to reproduce together to form fertile offsprings.

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