Historical background of Classification
In the past, scientists used to classify organisms based on personal conveniences. They heavily relied on very few observable features. There was no standard classification system as each and every scientist would classify organisms in a way that would suit his intentions.
In particular, living organisms were simply classified as plants or animals. The plants were also classified as herbs, trees and shrubs. Animals were also grouped into herbivores, carnivores and omnivores.
Some biologists would also classify plants as: Edible or non edible and Flowering or non-flowering.
The modern classification systems take into account the evolutionary relationships between living organisms. It has overcome the many weaknesses of the artificial (traditional) classification systems.
From the original parents, arose new groups of organisms that went under structural changes that enabled them to live in different habitats. The structural changes account for the great diversity of living organisms observed today.
Carolus Linnaeus, a Sweddish biologist is largely credited for his significant contribution to the development of the modern classification system.
Taxonomic units in classification
Taxonomic units of classification refer to the groups or taxa into which organisms are placed as a matter of convenience.
Grouping of organisms in these groups is based on easily observable characteristics that are common in that group.
Living organisms which share a lot of characteristics are placed in the same group.
Each taxonomic unit reflects the position of an organism in relation to the others in the classification scheme.
In a classification scheme, a hierarchy of groups is recognized and it proceeds from the first largest and highest group, the kingdom to the smallest and lowest unit, the species.
There are seven taxonomic units of classification.
All living organisms are classified into five major kingdoms:
a) Kingdom Monera- This is composed of microscopic unicellular organisms mainly bacteria e.g amoeba.
b) Kingdom Protoctista/Protista- This kingdom is comprised of members who are microscopic. Though, some are large enough to be seen with the naked eyes. Members of this kingdom include algae and protozoa.
c) Kingdom Fungi- Members of this kingdom comprises the mushrooms, toadstools, moulds and yeast.
c) Kingdom Plantae- This kingdom comprises the moss plant, ferns, maize plants, hibiscus, meru oak tree etc.
c) Kingdom Animalia- Members of this kingdom include the tapeworms, hydra, fishes, human beings, lizards, earthworms etc.
In hierarchy of classification, a kingdom is further divided into several phyla (plural of phylum) or divisions (in plants). Within the phyla or divisions, organisms are further sorted out into groups known as classes based on their similarities and mode of life. Each class is further subdivided into small groups called orders based on structural similarities. Orders subdivide into families which subdivide into genera (plural for genus).Genera are then subdivided into smaller units of classification called the species.
Species is the smallest unit of classification whose members share many similarities and can freely interbreed to give rise to fertile or viable offsprings.
Members of a particular species can, however, exhibit various differences e.g. differences in skin colour or body forms. Within the species, organisms can further be classified based on the differences in colour or forms.
In humans, this gives the races, in animals the term used is breed while in plants, variety is preferred. In bacteria, the term strain is used to describe the variant forms.
Members of different but very closely related species can breed but the resulting offspring will be sterile (infertile). In particular, a mule is a sterile offspring between a horse and a donkey.