Classification and phylogeny, Biology tutorial


Classification or categorization means recognizing similarities and differences among various types of organism and then placing identical or similar organisms in one group and various types of organisms in various groups.

Taxonomy might therefore be stated as the science of categorization of organisms into categories, maintaining some rules. Early taxonomists categorized organisms according to the morphological characteristics or features only. Once the theory of organic evolution was accepted, taxonomists start to draw evolutionary relationships among various sorts of organisms. This was known as systematic. Nowadays taxonomy and systematic are treated as synonymous since for categorization, both morphological and biochemical resemblances and even those of between molecules like DNA and RNA are studied to establish the evolutionary relationships. Phylogeny is the evolutionary relationship between organisms. Preferably, categorizations reflect phylogeny in that it states how organisms are associated via evolution and common ancestry. Species in the similar genus are more closely associated than species in the separate genera and so on as we carry on from genus to domain. 


Classification comprises assigning species to a hierarchy of categories: species, genus, family, order, class, phylum and kingdom. Though classifying an organism, it is assigned to categories which exhibit its evolutionary relationship by other groups of organisms. Each and every level or category is known taxon (plural-taxa). The lowermost category is species. The other categories are ordered above species in such a way that there is a hierarchy of categories.

1) Taxonomic categories:

The different taxonomic categories are illustrated below:

Species: Group of individuals of one type that can interbreed to produce fertile offspring.

Genus: Group of species similar to each other in some features or traits pointing common ancestry.

Family: Group of genera (singular-genus) resembling one other. Example: Felis domestica (that is, the cat) and Panthera Tigris (that is, the tiger), both fit in to family Felidae.

Order: Comprises families exhibiting similar traits or characteristics.

Class: Comprises related orders.

Phylum: Comprises related classes.

The different phyla fit in to their respective kingdoms. There are five kingdoms. Illustration:

Kingdom: Animalae (Animals)

Phylum: Chordata (Animals having notochord or backbone)

Class: Mammalia (Animals which suckle their young ones.)

Order: Primates (Mammals having bigger brains and binocular vision)

Family: Hominidae (Humans and human resembling ancestors)

Genus: Homo (Fossil men and modern man)

Species: H.sapiens (Modern man)

2) Scientific naming of organisms:

A simplified system of naming organisms termed as binomial nomenclature has been the standard for more than two centuries now. It was stated by the Swedish biologist, Carolus Linnaeus (1707-1778). Binomial nomenclature merely signifies two-name system of naming. The name of each and every type of organism consists of two parts, that of the genus followed by that of species. The generic name is written by a capital letter and the specific name by a small letter. Example: Homo sapiens are the scientific name of modem man, Mangifera indica is the biological name of the mango.

Three main features or characteristics of biological naming are as follows:

a) The scientific name, by convention, is printed in italics or underlined whenever hand-written.

b) Scientific naming is according to the set of scientific laws of nomenclature.

c) Scientific names are generally in Greek and Latin. They are understood all over the world and have made communication regarding organisms simpler.

3) The Five Kingdoms of Organisms:

Till recently there were just two kingdoms for classification: Plantae and Animalae. Such a two kingdom categorization had some drawbacks, example: bacteria and fungi were kept all along by plants however they are much different.

R.H. Whittaker in the year 1969 recommends the five kingdom categorization which is based on three criteria:

i) The presence or absence of a well-stated nucleus.

ii) Unicellular or multicellular.

iii) Mode of nutrition.

Name of Nature of what type of Kingdom nucleus unicells or nutrition multicells:

a) MONERA Prokaryotic Unicellular Diverse kind of Blue green algae nutrition and bacteria.

b) PROTOCTISTA Eukaryotic Unicellular Diverse type of Algae and nutrition Protozoa.

c) FUNGI Eukaryotic Multicellular Saprophytic (Moulds and so on). Mainly feed on dead and decaying matter.

d) PLANTAE Eukaryotic Multicellular Autotrophic (that is, all green plants). They Synthesize food by using the process of photosynthesis.

e) ANIMALAE Eukaryotic Multicellular Heterotrophic (Animals). Mainly based on others for their food.


The history of life is depicted as a branching tree, termed as a phylogeny. All the forms of life, comprising most of the extinct forms which represent dead branches, will join to this tree anywhere.


Phylogenetics is the modern manner in which organisms are categorized and arranged in evolutionary trees. Phylogeneticists arrange species and higher categorization classes into clades. Clades might be represented on a diagram termed as a cladogram. A clade includes a most recent general ancestor and its entire descendant species-the general ancestor is supposed and not recognized.

Just the lamprey, the so-called 'out-group', lacks jaws; however the other six groups of vertebrates are in the similar clade as they all encompass jaws, a derived feature relative to their ancestors. On the other hand, the vertebrates beyond the shark are all in the similar clade as they encompass lungs and so on.

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