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INTRODUCTION
Have you ever imagined how many living things there are in the world? Indeed there is a greater variety of organisms in the world. How can we organize them so that they are easily recognizable? The first thing to do is to sort them into smaller and simpler groups. For example, if you were given a collection of books and asked to put them into two groups. How would you do it? What characteristics would you use? Biologists use the same practice of putting things into groups of related organisms. This is called classification.
Living organisms are classified into groups depending on their structure and characteristics. This system was developed in the eighteenth century by Carl Linnaeus.
The classification of species allows the subdivision of living organisms into smaller and more specialised groups.
The Need to Group/Categorize Living Organisms
Activity; SORTING BOOKS IN THE LIBRARY
Key question
How can you classify the books in the library?
What you do
Imagine you are in charge of a new library and government has delivered a number of different kinds of textbooks to your school. Someone is going to put the books on shelves in the school library, and she needs instructions on how to sort them. Write, in not more than 50 words, the instructions to be followed in sorting the books so that the library users can easily get access to the books they need.
Living organisms are classified mainly to avoid confusion, to make study of organisms easy and learn how various organisms are related to each other.
Levels of Classification
Classification is the act of putting together living organisms into groups based on their common/similar characteristics. Each group of similar organisms is called a taxon (taxa-plural). The branch of biology that deals with classification of organisms is called taxonomy.
Living things can then be ranked according to:
phylum
class
order
family
genus
species
Phylum follows Kingdoms and has many different organisms, including three examples below:
Chordata, which have backbones
Arthropod, which have jointed legs and an exoskeleton
Annelids, which are segmented worms
Class is an additional sub-division, which for example, results in the Chordata phylum being divided into:
Mammals
Birds
Amphibians
Fish
Reptiles
Order follows class and as an example, mammals can be further sub-divide into a variety of different groups such as:
Carnivores
Primates
Orders are broken down into families. Here are a few examples of which carnivores can be divided into:
Canidae - dogs
Felidae - cats
Genus, the Felidae family can be further sub-divided into four genus examples:
Acinonyx - cheetah
Panthera - lion and tiger
Neofelis - clouded leopard
Felis - domestic cats
Species is the final classification stage, and the genus Panthera can be divided into:
Panthera leo (lion)
Panthera tigris (tiger)
As an example, the complete breakdown of the classification of lions:
kingdom - animal
phylum - vertebrate
class - mammal
order - carnivorous
family - cat
genus - big cat
species - lion
There are many ways to remember this order, for example using the mnemonic:
Living things can then be ranked according to:
phylum
class
order
family
genus
species
Phylum follows Kingdoms and has many different organisms, including three examples below:
Chordata, which have backbones
Arthropod, which have jointed legs and an exoskeleton
Annelids, which are segmented worms
Class is an additional sub-division, which for example, results in the Chordata phylum being divided into:
Mammals
Birds
Amphibians
Fish
Reptiles
Order follows class and as an example, mammals can be further sub-divide into a variety of different groups such as:
Carnivores
Primates
Orders are broken down into families. Here are a few examples of which carnivores can be divided into:
Canidae - dogs
Felidae - cats
Genus, the Felidae family can be further sub-divided into four genus examples:
Acinonyx - cheetah
Panthera - lion and tiger
Neofelis - clouded leopard
Felis - domestic cats
Species is the final classification stage, and the genus Panthera can be divided into:
Panthera leo (lion)
Panthera tigris (tiger)
As an example, the complete breakdown of the classification of lions:
kingdom - animal
phylum - vertebrate
class - mammal
order - carnivorous
family - cat
genus - big cat
species - lion
There are many ways to remember this order, for example using the mnemonic:
The Two-Name Naming (Binomial) System of Living Organisms
An office/restaurant/bank in a town can easily be located using only two small groups of the address i.e. the name of the street and plot number.
In the same way all species are named from the last two groups in the taxonomic levels i.e. the genus and the species.
In the two-name naming system every living organism has a unique twopart name that consists of two words from the genus name and species name.
The first name is Genus, the second name is species. Scientific names of organisms are written in Latin, printed in italics or underlined separately when handwritten.
The genus name always starts with a capital letter, and the species name is always written in small letters.
The binomial system is important because of the following reasons:
(i) Clarification: each organism has a unique name that is specific to that organism and can be identified.
(ii) Universal: using same name everywhere to identify the specific organism.
(iii) Education: names are short and easier to remember and learn.
(iv) Classification: organisms are more easily categorized and the categories are easier to understand.
Using a Flow Chart for Biological Classification
The features/characteristics of organisms can be used to classify them using a flow chart. The chart usually begins with two distinct features that distinguish a group of organisms. Then other features/characteristics can be used to further separate the organisms until each individual in the group is identified independently. Take a look at the chart below and try to figure out the domestic animals based on their characteristics.
1. Which one of the domestic animals is a:
i) duck
ii) cow
iii) cock
iv) dog
v) pig
2. Using the domestic animals in (1) above, create your own flow chart but with features/characteristics different from those given above.
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