Biological Classification - Taxonomy

Earth is considered as the home to more than about 8 million different species. To truly appreciate this number, one must also keep in mind that this is not quite all; it is an ever-expanding number since there are still species to be discovered especially in the tropics. With these vast numbers, it becomes necessary to classify them into groups otherwise the sheer number of different species becomes alarmingly difficult to study. Hence, the taxonomists ( a group of biologists) have devised a carefully developed plan to organize these myriad species into one group. It all began in the mid-1700s when Carolus Linnaeus, a Swedish physician, and botanist, classified organisms with shared characteristics. He relied on the method called binomial nomenclature for biological classification.

Biological Classification

Biological classification

Further onwards, the art of biological classification was introduced which basically puts organisms into groups. This is a part of Scientific Taxonomy. The classification system starts with a group with a wide variety of organisms and becoming more selective as the groups get more specific, as most classifications go. Linnaeus classified about 4,000 species using this method. He classified organisms into seven groups, based on their appearance.

The Seven Groups

There are common eight groups comprising of

  1. Domain,
  2. Kingdom,
  3. Phylum,
  4. Class,
  5. Order,
  6. Family,
  7. Genus,
  8. Species.


A domain is the highest rank of organisms. The three-domain system of taxonomy was designed by Carl Woese in the year 1990. Taxonomic ranks should always be capitalized, except for species as it assists people to distinguish between bacteria and Bacteria ((the organisms and the domain).

The three domains of life are Bacteria, Archaea, and Eukaryota.


The Kingdom happens to be the highest classification after domain into which living organisms are grouped. At present, there are five known kingdoms. All the living species are grouped into specific kingdoms based on factors like the types of cells they are composed of, mode of their nutrition and the total number of cells they hold.

The different kingdoms are Animalia, Plantae, Fungi, Protista, and Archaebacteria.


The phylum or phyla is grouped next level after kingdom in the biological classification. The purpose of this level is to find some physical similarities among various organisms within a kingdom.There are 35 phyla in the kingdom Animalia.

The different kingdoms are phylum Arthropoda, phylum Porifera phylum Chordata, etc.


The organisms of a phylum are further divided into various classes.The classes were the most common rank proposed by Linnaeus. There are about 108 different classes in the kingdom Animalia that were introduced after the 19th century, proposed by Linnaeus which are still followed today.

The different classes are class Aves, class Mammalia, class Plants, class Reptilia and many others.


The organisms of a particular class are further distributed into orders.It is much more accurate than the classes.There are about 18-25 orders of mammalians which are based on the classification of organisms. The sources differ. The art of taxonomy is mostly used to determine which order an organism belongs to.

The different orders are Order Primates, Order Chiroptera, Order Carnivora, Order Cetaceans, etc.


After the classification of organisms into orders, they are further grouped into families.It is the 8th major taxonomic rank in biological classification which can be subdivided into subfamilies.There are totally 12 families in the order Carnivora and 620 families in the class Plants.

The different families are family Canidae, family Ursidae, family Felidae,  family Mephitidae, etc.


The Genus or genera. It is even more specific compared family and other groups.Genus is the first part of an organism’s scientific name using binomial nomenclature followed by the second part is the species name.

The scientific name of an organism is always italicized, followed by the genus name which should be capitalized.  Both Genus and species are the only taxonomic ranks that are italicized.

For example –Homo sapiens – it is the scientific name given to humans.

Homo – genus name and sapiens -species name.


Species is the last and major taxonomic rank which are subdivided into subspecies in certain cases only. There are more than 8.7 millions of different species of organisms on planet Earth. As each genus name is unique, the same species names can be used for different organisms.The species name should be always italicized, but never capitalized. The species of an organism would determine the second part of its binomial nomenclature.

For example –  Bufo americanus is the American toad and  Ursus americanus is the American black bear and

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