Taxonomic Hierarchy

Over several millions of years, a huge variety of species have evolved on earth and time immemorial many attempts of classification of things have been made. The life forms that surround us range from small ants to big trees, colorless insects to brightly colored flowers or birds. For ease in studying and identifying, the idea of biological classification was put forth but for the applicable classification, a fundamental basis of classification became a necessity.

Initially, things around us were segmented into living and non-living things and later, when the variety of living things started to grow, there came the necessity for the biological classification. The classification of living organisms based on similarities and dissimilarities is known as biological classification. Each of the biologists who classified organisms, have done it by separating them into different groups according to various criteria. It took years for researchers to decide the most basic characteristics for the process of classification. 

The Beginning of the Basis of Classification

The history of biological classification began with Aristotle, the Greek thinker. He classified animals on the basis of their habitat i.e., air, water, and land but that classification was not justified. Animals in each of these groups had nothing in common, other than their habitat. A fish and turtle could not be put in the same group. Hence, habitat could not be a criterion for their classification.

Later age scientists began to work on classifying living organisms on the basis of their characteristics, which could be explained in many ways. Characteristics are the appearance and behavior of organisms. For example, a dog has limbs but a snake does not, a dog and a snake can move whereas a plant cannot. These are characteristics of various organisms. 

Every classification starts from a particular point. The fundamental characteristic will form the basis for the broadest divisions i.e., main groups among living organisms. The characteristics at the next level are decided sub-groups based on the basic one. Thus, a chain of command of classification is built.

Some of the Characteristics that are used Presently to Classify Organisms are:

• Prokaryotic or Eukaryotic cell.

• Unicellular or Multicellular.

• Autotrophs (Photosynthetic) or Heterotrophs (Non-photosynthetic).

• The level of organization and growth of organs.

The characteristic to classify a plant and an animal is on the fundamentals of their body design. The next level of the hierarchy is regarding the plant being a tree or a shrub. On the basis of different characteristics, more sub-categories will be formed.

The method of arranging a variety of classes, groups, and other categories into consecutive levels of the biological classification in a chain either in an increasing or decreasing order from species to the kingdom or vice versa is known as Taxonomic hierarchy. Each of these hierarchies is known as the taxonomic category or rank. In the system of classification, the Kingdom is always ranked highest and is succeeded by division, class, order, family, genus, and species which is ranked the least in the Hierarchy always. Taxonomic Hierarchy Categories were first introduced by Linnaeus, therefore it is also called Linnaean hierarchy.

This concept can be better understood with an example: birds are a group of organisms that show common characteristics like feathers and flight. Hence, based on the common characteristics, they are classified into a single taxonomic category.

Earth is considered as a home to more than about 8 million kinds of species. To truly appreciate this amount, we should also remember that it is an ever-expanding amount since there are still species to be discovered especially in the tropical regions. With these vast numbers, it becomes necessary to classify them into groups lest the sheer number of different species becomes very hard to study. Hence, the taxonomy biologists have prepared a carefully developed plan to organize these countless species into a group. It began in the mid-1700s when Carolus Linnaeus, a Swedish botanist, and physicist, separated organisms with common characteristics. He depended on the method called binomial nomenclature for classification in biological sciences.

Biological Classification

The art of biological classification was introduced which essentially puts organisms into groups which is a part of Scientific Taxonomy. The classification system begins with a group with a wide range of organisms and as they become more selective, the groups get more specific. Linnaeus classified about 4,000 species using this technique. He classified organisms into seven groups, based on their exterior appearance.

There are common eight groups are namely

1. Domain

2. Kingdom

3. Phylum

4. Class

5. Order

6. Family

7. Genus

8. Species

1. Domain

A domain is the uppermost rank of organisms. The Three-domain system of taxonomy was created by Carl Woese in 1990.

The three prominent Domains of life are Archaea, Bacteria, and Eukaryota.

2. Kingdom

The Kingdom happens to be next in classification after the domain into which living organisms are separated. Currently, there are five known kingdoms and all the living species are sorted into specific kingdoms on the basis of factors like the types of cells they are made of, their mode of nutrition and the total number of cells they have. This is also the highest level of classification in some classification systems. 

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

3. Phylum

The phylum (or phyla) is grouped right after the kingdom in the classification hierarchy. The purpose of this level is to find certain physical similarities amongst the various organisms within a kingdom. For example, there are 35 phyla in the kingdom Animalia.

Example –fishes, reptiles, mammals, and amphibians, aves combined to form the phylum of Vertebrates or Chordata.

The various kingdoms are phylum Chordata, phylum Arthropoda, phylum Porifera, etc

4. Class

The organisms of a phylum are further sorted into more classes. The class was one of the most common ranks put forth by Linnaeus. There are around 108 different classes in the kingdom Animalia that were introduced post 19th century, proposed by Linnaeus that are followed even today.

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

5. Order

The organisms of a particular class are further separated into orders. It is considered much more accurate than the classes. There are around 18-25 orders of mammalians that are based on the classification of organisms. For instance, the order of carnivores i.e., Carnivore accommodates families such as Felidae and Canidae.

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

6. Family

After the division of organisms into orders, they are further divided into families. It is considered the 8th major taxonomic rank in the classification hierarchy which can be divided further into subfamilies. There are 12 families on the whole in the order Carnivora and there are 620 families in the class plants. 

This category of taxonomy, the living organisms share some resemblance among themselves. For example, the genus of tiger, leopard, and lion are in Panthera and the genus of cats, that is, Felis are found together in the family Felidae.

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

7. Genus

The Genus or genera is even more specific when compared family and other tiers. The genus forms the first part of an organism's scientific name in the binomial nomenclature followed by the species name. The scientific name of any organism is italicized always and it is followed by the genus name which has to be capitalized. For example –Homo sapiens – it is the scientific name given to humans. Homo is the genus name and sapiens is the species name.

This taxonomic group consists of several species which have similar characteristics but are different from that of species and from another genus. Considering the example of Plasmodium, it is a genus with multiple species that are similar to each other but differ from the species of another genus.

8. Species

Species are the last and major taxonomic rank which is split even further into subspecies in certain cases. There are greater than 8.7 million different species of organisms on our planet Earth. Each genus name is exclusive and so are species names that can be used for various organisms. The species name should be italicized and never capitalized. The name of the species of an organism would occupy the latter half of its binomial nomenclature.

It is the bottommost rung of classification but they show the most number of similarities amongst all the organisms in any other rung. A species can be differentiated from other strongly related species based on the unique differences in their morphologies. For example, Plasmodium vivax and Plasmodium falciparum are both malaria-causing parasites but they each had different effects on their patients. Plasmodium is the genus name that has several species which exhibit unique morphological characteristics. Another relevant example is Bufo americanus which is also called American toad and Ursus americanus which is also known as American black bear.