Zoological Classification

Zoological Classification

Definition of classification:

In general classification means “the grouping of objects into classes owing to their shared possession of attributes” or “The arrangement of entities in a hierarchical series of nested classes, in which similar or related classes at one hierarchical level are combined comprehensively into more inclusive classes at the next higher level” (Mayr and Ashlock, 1991). Here class is defined as ‘a collection of similar entities’.

Definition of zoological classification:

The zoological or biological classification is the science of identifying, naming, and grouping of living organisms.

According to Simpson, 1961, “Zoological classification is the ordering of animals into groups or sets on the basis of their relationship that is of association by contiguity, similarity or both”

According to Mary, 1969, “A biological classification is the ordered grouping of organisms according to their similarities and consistent with their inferred descent”

According to “The Free Dictionary and World English Dictionary, 2011” a biological classification is “The placing of animals and plants in a series of increasingly specialized groups because of similarities in structure, origin, molecular composition, etc, that indicate a common relationship”.

The steps for construction of classification:

Three major steps are involved in the procedure of construction of classification –

1. Assembling of phena into populations and population into species.

2. Gathering species into species groups and formal taxa.

3. Ranking taxa into hierarchy of categories.

The need of classification:

Presently over one and half million species of animals are known in addition to 12,000 species of fossils. The process of describing species is continuing and it is estimated that 3 to 10 million of living and fossil species still wait discovery. The vast numbers of animal species exhibit a great deal of variation in form, structure, mode of life and in various other aspects. The variations and similarities among the organisms point to the necessity of having a classification system. So, it would be practically impossible to work with them unless they were organized around a standard. Classification performs the tasks of converting this chaos into order; otherwise it would be practically impossible to study the vast diversity of organisms.

The importance of zoological classification:

Dealing with a complex biodiversity requires an ability to recognize similarities and differences among the organisms. Classification system helps to clarify relationship among organisms. It also helps us to remember the identity and trait of organisms. The importance of biological classification can be explained as follows –

1. It is impossible to remember the characteristics of vast diversity of organisms unless we classify them into categories. The members of categories share many characteristics.

2. Classification system improves our predictive power. As for example female mammalian species produce milk for their offspring, then we can quite certain about a newly obsevered mammal that it will possess other mammalian traits such as presence of hair, constant body temperature etc.

3. Classification system improves our ability to explain relationships among organisms. The construction of evolutionary pathways establishes the relationship among the organisms.

Process of zoological classification:

A classification is constructed by grouping the animals on the basis of degree of similarities among them. The process of classification is not static but changes continually in the light of new information and ideas and the addition of new species are going on continually. There are two distinct processes in the classification, which are frequently carried out at the same time, ie, the establishment of equivalent features of the object to be classified and their grouping on the basis of their features.

The function (purpose) of classification: -

In pre-Linnaean days, the sole purpose of classification was to serve as a identification key. Today a classification serves multiple purposes, both practical and theoretical (Mayr and Ashlok 1991). The main purpose of classification is to keep track of the diversity and numbers of more than one million species and provision for the discovery of new ones.

System of classification: -

The similarity of organisms is due to the fact that similar species and genera are descendant from common ancestors. Thus the first and foremost step in the classification is simply the grouping together of individual organisms on the basis of relationships or association among them. These groups then have to delimited and arranged in an orderly manner. For this it is necessary to understand the terms like hierarchy, taxon, category and rank which are components of biological classification.

Taxon is a taxonomic group and distinguished by name. The ordinal levels of taxa are called ranks. The taxa of given rank constitute category. And the arrangement of taxa into an ascending series of over increasing inclusiveness is called hierarchic system of classification.

Animals occur in nature as distinct species. The individuals of a species show similar features and recognizably distinct from other species. The species which share most of the features are placed into a large and more inclusive taxa called genera. Genera similarly are included into families and so on.

In classical taxonomy two systems of classification are recognized, ie, artificial system of classification and natural system of classification. The basic differences between the two approaches lie in the types of characters used and their methodical approach. The artificial systems were based on a priori (theoretical or deductive) and the natural system on a posteriori (empirical or inductive) approaches. The former involves thinking out beforehand what kinds of attribute should be more important in the organisms to be classified, while the later involves first observing the actual distribution of characters and then constructing a classification.

Artificial system of classification:

Artificial system of classification is also called Aristotelian system of classification. It is based on superficial characteristics of organisms, such as form, habitat etc.

Drawbacks of artificial system of classification:

1. The system does not reflect the natural relationships among the organisms.

2. The system does not reflect evolutionary relationships among the organisms.

3. Sometime distantly related organisms are placed in the same groups on the basis of their habitats, eg., – whales and the fishes in the same group.

4. Closely related organisms have been placed in different groups because of the differences in their habitat, feeding habit etc.

Natural system of classification: -

With the advancement of knowledge, scientists started looking for more and more details about different groups of organisms. As a result, more and more similarities and differences started emerging between the different groups of organisms. This led to the more systematic and scientific approach to classification, which is now known as the natural system of classification.

Advantages of natural system of classification: -

The natural system of classification has specific advantages over the artificial system of classification. It was initiated by English biologists, Jhon Ray (1627 – 1705) and was placed on affirm footing by the Swedish biologist, Carolus Linnaeus (1707 – 1778)

1. It avoids the heterogeneous grouping of unrelated organisms.

2. It helps in placing the only related group of organisms together.

3. It indicates the natural relationships among organisms.

4. It provides a clear picture on the evolutionary relationships among the different groups of organisms.

Classical System of Classification:

Theories of classification: -

The classical taxonomists wanted to established ‘natural groups’ or they wanted to group together species on the basis of their essential characters or showing natural affinity. Such vague and seemingly noncommittal phrases are actually the brief expression of philosophies of taxonomic theories.

Followings are the five theories of classification: -

1. Essentialism

2. Nominalism

3. Empericism

4. Cladism

5. Evolutionary classification.

The first three are Pre-Darwinian and the last two are Post-Darwinian.

1. Essentialism: -

It is also known as Aristotle’s Natural System. According to Karl Popper (1950) “it is task of pure knowledge or science to discover and to describe the true nature of things, i.e., their hidden reality, or essence”. This view was also held by Plato. It can be summarized as “to discover the true nature of things and their hidden property or essence”. This theory postulates that al the members of the taxon should exhibit some essential characters or in other words that they conform to the same type. This is why the essentialist ideology is also called typology.

Drawbacks:

1. It is difficult to assess which character is essential.

2. Why other characters cannot be considered as essential?

3. According to the essentialism taxon will be monothetic (single essential character), but higher taxon contains certain species which lack common essential nature.

4. Member of a population having same heritage cannot be placed together if they lack essential character.

2. Nominalism: -

According to this theory only individual exists. All the groupings are artifact of human beings. According to this theory classification of all organisms and inanimate objects based on the will of the classifier to serve his own purpose of study.

Drawbacks:

1. Ignore the fact that there is difference between inanimate objects and the organisms.

2. Ignores the fact that organisms of same class has same heritage.

3. Misinterpret the relation between similarity and relationship.

3. Empiricism: -

Adanson (1727–1806) is the founder to introduce empiricism in taxonomy. According to this theory there is no need for any theory of classification. Observed and record as many characteristics of organisms as possible and then grouped them according to the majority of shared characters. A species consists of individuals with a maximum number of shared characters. A genus consists of species with a maximum of shared characters and so on. The necessary system of precedence of characters and rank of taxa is thus built up by step. Thus, if characters are evaluated properly a natural system of classification will automatically emerge.

Drawbacks:

The principle of empiricism in this form lacks the interpretation and evaluation to be supplied by evolutionary taxonomy. Even though the working taxonomist usually proceeds on the basis of this principle, he feels that the resulting classifications would be biologically meaningless if not supplied with a theoretical foundation (Mayr, 1969).

4. Cladism:

According to this theory organisms are ranked and classified according to the “recency of common descent”. Here, categorical system depends upon the position of branching point of the phylogenetic tree. The more recent the common ancestry of two species or taxa, the more characters in common they should have. Category of taxon is determined by fixing branching point of Geological Time Scale (GTS). Using the philosophy of cladism a standard method for fixing the ranks used in the hierarchical classification is presented in the following table.

The whole concept of evolutionary classification is based on Darwinism. Darwin and his followers believe that the groups are created through evolution. They classify organisms rather than characters. It is based on the simple fact that easily delimitable groups such as birds, bats, beetles etc. are exist in nature. The existence such groups demand the explanation from the taxonomists for the improvement of classification. It is the Darwin who first explained that closeness of descent is the only known cause of similarity of organisms. Now the taxonomists become the discoverer of groups made by evolution. Now a biologist understands that he is classifying populations, not inviduals, or phena. The groups or populations are polyphetic (multicharactered) and the result of divergent evolution. All living animal species are related to one another by the way of evolutionary descent and this type of relationship helps us to establish correct systematic groupings.

Drawbacks: Fig.1: Fig: Mosaic evolution.

1. Evolutionary classification is subjective and not scientific because decisions are not testable hypothesis.

2. The claims of evolutionary classification are doubtful in examples of reticulate evolution resulting from the fusion of previously separated lines or when total convergences leads to groupings with unexplained polyphyletic origin.

Omnispective classification:

Here an experienced taxonomist includes all the readily available features of the organisms but only those are used for classifactory purposes, which are helpful and distinctions. This practice is currently used by most taxonomists.

Biological classification:

Biological classification may be defined as “the arrangement of organisms into taxa on the basis of inferences concerning their genetic relationship”

But unfortunately it is difficult to study the genetic relationship especially in case of higher taxa. So, biological classification is practical at species level to some extent.

Linnaean Hierarchy/ Taxonomic Hierarchy or Five kingdom Approach

The first and foremost step in the classification is simply the grouping together of individual organisms on the basis of relationship or association among them. These groups then have to delimited and arranged in an orderly manner.

Animal occur in nature as distinct species. The individuals of species are reproductively isolated from other individuals of different species. The species which share most of the features are placed into a large and more inclusive taxa called the genera. Genera, similarly are included into families and so on. This sort of arrangement of taxa into an ascending series of over increasing inclusiveness is called hierarchic system of classification. “A hierarchy is thus a systematic framework for Zoological classification with a sequence of classes (or sets) at different levels in which each class except the lowest includes or more subordinate classes (Simpson, 1960)”.

Linnaean hierarchy: - Hierarchy was developed mainly in the 17^th and 19^th century and reaches nearly definitive form in the 10^th edition, 1758, of the “Systema Naturae” by Linnaeus. So, it is called Linnaean hierarchy. Linnaeus recognizes only five hierarchic levels within the animal kingdom. So, this is also termed as “Five kingdom approach”. These were ‘classis’ (Class), ‘ordo’ (order), genus, species and variety. Later two additional categories –family (by Butscheli in 1870) and phylum (by Hackel in 1886) were added. The six categories – phylum, class, order, family, genus and species form the basic taxonomic hierarchy of animals. The term variety was eventually discarded. More recently two other categories have become widely used. These are ‘tribe’ between genus and family and ‘cohort’ between order and class. The increase in the number of known species together with the degree of relationship between them influenced taxonomists to assign precise taxonomic position to species. This resulted in splitting of original basic categories and also addition of some among them. With the result there are as many as 33 categories presently in use in the hierarchic classification. Of these 33, only 18 (marked with underlined) are generally followed.

Kingdom Sub order

Sub kingdom Infra order

Infra kingdom Super family (–oidea)

Super phylum Family (-idea)

Phylum Sub family (-inae)

Sub phylum Infra family

Infra phylum Super tribe

Super class Tribe (-ini)

Class Sub tribe

Sub class Infra tribe

Infra class Super genus

Super cohort Genus

Cohort Sub genus

Sub cohort Super species

Infra cohort Species

Super order Sub species

Order