The following points highlight the seven obligate taxonomy categories of organisms. The categories are: 1. Species 2. Genus 3. Family 4. Order 5. Class 6. Division or Phylum 7. Kingdom.
Category # 1. Species:
Species (used both as singular and plural) is a natural population of individuals or group of populations which resemble one another in all essential morphological and reproductive characters so that they are able to interbreed freely and produce fertile offspring.
Mango is species indica of genus Mangifera (Mangifera indica). Potato is species tuberosum of genus Solarium (Solarium tuberosum). Lion’s species is Leo of genus Panthera (Panthera Leo) while Tiger’s species is tigris of genus Panthera (Panthera tigris).
Each species is also called genetically distinct and reproductively isolated natural population. Mayr (1964) has defined species as “a group of actually or potentially interbreeding populations that are reproductively isolated from other such groups”. However there are two objections for using interbreeding as a sole criterion for delimitation of a species.
(i) Occasional hybrids occur in nature due to breakdown of mechanical, physiological, spatial and seasonal barriers between species. Interspecific hybrids have been obtained since ages artificially by man, e.g., mule. Hybrids between closely related species are often obtained in captivity. These hybrids are generally sterile, but some are fertile as well.
A few examples are given below:
(a) Mule (sterile) — Female horse and male donkey
(b) Hinny (sterile) — Male horse and female donkey
(c) Tigon (fertile) — Male tiger and female lion
(d) Liger (fertile) — Male lion and female tiger
(ii) Sexual reproduction is absent in prokaryotes and some protists. In such cases and fossils morphological differences, cytotaxonomy and chemotaxonomy are resorted.
Principles for Delimiting a Species:
(i) The different members of a species whether morphologically similar or dissimilar are able to interbreed freely and produce fertile offspring.
(ii) All the members whether present in one population or different populations found in remote areas of the globe are derived from a common ancestor,
(iii) The members resemble one another more than they resemble individuals of any other species,
(iv) There is a complete anatomical similarity,
(v) All the members of a species have similar karyotype (cytotaxonomy)— there is similarity in the number, size, shape and behaviour of meiotic chromosomes,
(vi) All the individuals of a species contain similar genetic material,
(vii) There is a broad similarity in morphological characters,
(viii) At the molecular level there is similarity in the types of proteins, enzymes, hormones and other bio-chemicals (chemotaxonomy).
Category # 2. Genus:
It is a group or assemblage of related species which resemble one another in certain correlated characters. Correlated characters are those similar or common features which are used in delimitation of a taxon above the rank of species. All the species of genus are presumed to have evolved from a common ancestor.
It is not essential for a genus to have several species. There are genera which have only one species. They are called monotypic. Others are known as polytypic. Thus the genus Solanum has a large number of closely related species, e.g., S. tuberosum (Potato), S. melongena (Egg Plant, Brinjal), S. nigrum (Black Night-shade), S. surratense.
Similarly Lion, Tiger, Leopard, Jaguar are closely related species which have been placed in the genus Panthera.
The animals are respectively named as Panthera leo, P. tigris, P. pardus and P. onca. These ferocious animals are related with some differences to cats included in the genus Felis, e.g., F. domestica (Common Cat), F. bengalensis (Leopard Cat), F. marmorata (Marbled Cat), F. chaus (Jungle Cat), F. viverrina (Fishing Cat) and F. temminki (Golden Cat).
Category # 3. Family:
It is taxonomic category which contains one or more related genera. All the genera of a family have some common features or correlated characters. They are separable from genera of a related family by important and characteristic differences in both vegetative and reproductive features. Thus the genera of cats (Felis) and leopard (Panthera) are included in the family felidae.
The members of family felidae are quite distinct from those of family canidae (dogs, foxes, wolves). Similarly, the family solanaceae contains a number of genera like Solatium, Withania, Datura, Petunia and Nicotiana. They are distinguishable from the genera of the related family convolvulaceae (Convolvulus, Ipomoea).
Category # 4. Order:
The category includes one or more related families. Thus the family solanaceae is placed in the order polemoniales along with four other related families (convolvulaceae, boraginaceae, hydrophyllaceae and polemoniaceae). Similarly, the families felidae and canidae are included under the order carnivora along with hyaenidae (hyaenas) and ursidae (bears).
Category # 5. Class:
A class is made of one or more related orders. For example, the class dicotyledoneae (dicotyledonae, dictoyledons) of flowering plants contains all dicots which are grouped into several orders (e.g., rosales, passiflorales, polemoniales, sapindales, ranales, etc.)
Likewise, class mammalia of animals includes all mammals which range from bats (order chiroptera), kangaroos (order marsupialia), rodents (order rodentia), whales (order cetacea), and carnivores (order carnivore) to great apes and man (order primata).
Category # 6. Division or Phylum:
It is a category higher than that of class. The term phylum is used for animals while division is commonly employed for plants. A division or phylum is formed of one or more classes.
The phylum chordata of animals contains not only the class mammalia but also aves (birds), reptilia (reptiles), amphibia (amphibians), cyclostomata, chondrichthyes, osteichthyes (fishes), etc. The division spermatophyta in plants has similarly seven classes of gymnosperms and two classes of angiosperms.
Category # 7. Kingdom:
It is the highest taxonomic category. All plants are included in kingdom plantae while all animals belong to kingdom animalia.