CHAPTER THREE; CLASSIFICATIoN

1.5 Classification of organisms and its importance

Activity 1.8

In groups

1. Look at the photograph below. It shows inside a supermarket.

With your friend, identify the order in which things are arranged. Why do you think such arrangement is necessary?

What will happen if the items were not properly arranged?

2. Now, look at the picture below.

Which organisms can you see? 

Name them. What does this show you about the nature of organisms in various habitats?

3. Do you think it will be better if the organisms are put in some forms of groups? Why?

Different habitats have millions of organisms hence the need for grouping organisms for ease of identification and study. Scientists previously grouped organisms using artificial means. For  example:

• If the organisms were useful or harmful to human beings.

• If the organisms were edible or not.

• If the organisms lived on land, water or air.

However, such features could not form meaningful groups for study. A scientist called Carl Von Linne’ (Carolus Linnaeus), in 1753 introduced a modern way of classifying organisms.

Activity 1.9

In groups

1. You are provided with a mixture of seeds that include: maize, beans, groundnuts, peas, rice, wheat and millet. Try to count each seed in the mixture.

• What challenges did you face while counting each seed in the mixture?

2. Now, separate the seeds into individual groups say maize alone, beans alone, etc as shown below.

3. Try counting the maize and the bean seeds separately.

• Is it now easier to count the separated seeds? Why is this the case?

Therefore there are two types of classification.

• Natural classification

• Artificial classification

Natural classification is based on the natural order or common ancestors that is supposed to exist and that is informed by science e.g. the cat family. Artificial classification on the other hand is based on superficial characteristics, imaginations or human uses e.g parasites or pests.

Work to do

Based on the descriptions above, come up with a table showing the differences

between natural and artificial classification methods. Did your table look like this?

Table 1.1 Comparison between natural and artificial classification methods

Activity 1.10

In groups

1. Take a walk outside the classroom and collect different types of living organisms.

Caution! Some organisms can sting! Others can bite! Therefore do not use your bare hands to handle such organisms. Use protective gloves.

2. Take the collected organisms back to class for study.

3. Put the organisms into various groups.

You can use these features;

• Number of legs

• Presence of wings

• Presence of antennae

• What covers the body

4. Share your findings with the rest of the class.

Importance of classification

There are about 1.8 million species of known organisms. It is also believed that there could be many more undiscovered species in the forest ecosystems and in deep seas.

1. Classification puts information in an organized manner to avoid chaos and confusion among scientists.

2. It enables scientists to place organisms in their correct groups for ease of study.

3. It allows scientists to better understand the phylogenetic relationships among organisms i.e. how organisms are interrelated.(Organisms that have more common characteristics are more closely related).

4. Classification allows scientists to identify, group and properly name a newly discovered organism.

1.6 Taxonomy hierachy of classification

Activity 1.11: Discussion corner

In groups

1. Give the names of the animals below in your native language.

(i) Cow 

(ii) Cat

(iii) Elephant 

(iv) Lion

2. Do you think people from other parts of the world can recognize the names you gave in 1 above?

3. What impact do you think this will have on the study of living organisms?

It is sometimes difficult to identify living things using local names. This is because of the existence of several local languages.

To assist scientists from different parts of the world to communicate, one scientific name is given.

• This creates no confusion as to which organism is being referred to.

• Scientific names rarely change.

• Scientific names are written in the same language around the world.

When grouping organisms, it is necessary to have an organized order. This enables organisms to be identified easily depending on the level of classification they are placed. Different levels of classification in which organisms are  placed form a taxonomic unit. Each taxonomic unit is referred to as a taxon (plural taxa). The order in which the taxa are arranged starting from the highest to the lowest is taxonomic hierarchy of classification.

Work to do

In the following table, write the correct classification against the address information. The first one has been done for you.

From your work above, The country has the highest number of people just like the kingdom which has the largest number of organisms. On the other hand, the village has the lowest number of people which corresponds to the species level in the classification hierachy. Therefore, organisms placed in the same

kingdom share fewer basic features as compared to those at species level. For example, all multicellular organisms possessing chlorophyll and synthesise organic food substance are placed in Kingdom Plantae, those that are multicellular but feed on already made organic food substances are placed in Kingdom animalia. Kingdom is the taxon with more members and it is split further into phyla (singular-phylum) or division (when referring to plants). Phylum or division is further split into classes, classes into order, order into family, family into genus and finally genus into species. As you progress from Kingdom to the lower levels of classification; the features shared by the organisms become more, while the number of organisms become ewer. As such, species being the lowest level of classification comprises of closely related organisms that share many characteristics. A species can therefore be defined as a group of organisms which can naturally interbreed to give rise to a viable offspring.

The most commonly used criteria for classification of organisms is use of observable features.

Observable features can be external

or internal. External physical features

of organisms are mainly used to put

organisms into different taxonomic unit.

These features include number of legs, number of eyes, types of leaves, among others. Organisms that are closely related have more common features and are therefore grouped together. Those that are distantly related have less common features and are grouped separately. 

Study table 1.4 below.

Summarise the features that were used to rank the organisms.

1.7 The binomial system

Activity 1.14

In groups

Discuss these questions with your friends.

1. Are there students with similar names? How do you identify them?

2. Are the students with similar names related?

3. Are there students known by their nicknames? How did they acquire them?

4. How do you name children in your community? Do their names have meanings?

Do a presentation to the rest of the class on your findings above.

Research activity

1. Find out from your parents or guardian about the source and meaning of your name.

2. Ask about the meaning of the names of your siblings.

3. Share your findings with the rest of the class members.

Activity 1.15: Reading

In pairs

Read the following story then discuss the study question.

Do you remember the long lost cousin you were looking for? You have finally located the house where this cousin lives. However, you have been told that your cousin is at school. To assist you find your cousin the teacher tells you that there are five learners with the same name as your cousin’s first name. The teacher assisting you asks you if your cousin is tall or short. You say tall. You are told that of the five learners, who share your cousin’s first name, three are tall. The teacher asks you if your cousin has a light skin complexion or dark skin complexion.You say dark skin complexion.You are told that of the three tall learners, two are dark- skinned. The teacher then asks you if your cousin has straight or curly hair. You say that your cousin has curly hair. The teacher then says that your cousin is in Secondary 3 East. The teacher then goes to fetch your cousin of whom you are pleased to see again after a long time.

Study Question

What characteristics did you follow to find your lost cousin? Write them down. 

Naming of organisms is called nomenclature. The system of naming organisms whereby an organism is given two scientific names is called Binomial nomenclature. The two names represent the Genus and the Species of the organism. This system of naming is conventional i.e. it is accepted and is used by scientists all over the world irrespective of their language used.

Rules of Binomial Nomenclature

1. The genus name is written first followed by the species name.

2. The genus name always starts with a capital letter. The species name is written in small letters.

3. The two names should be underlined separately if handwritten. If typed, they should be in Italics.

Table 1.5 gives scientific names of some common organisms.

1.8 Dichotomous key

The term dichotomous comes from the word “dichotomy” which means divided into two parts. A dichotomous key is a set of instructions used to identify unknown organisms. To be able to identify organisms, observable features are used.This key uses two variations in description of a characteristic or feature for identification. The key is used to identify and place new or unknown organisms into specific taxonomic units or groups. The organisms can then be named.

Activity 1.16

In pairs

Materials:

• Hand lens

• Fish (in a bottle containing water)

• Grasshoppers (in a specimen bottle)

• Snails

• Millipedes

• Rabbits (caged)

• Chicken (caged)

• Pair of forceps

What to do

1. Observe each of the specimens provided carefully. Note down the unique structural observable features in a table like the one shown below. (Use a hand lens if you cannot see the features clearly.)

2. Use this figure to construct a dichotomous key as shown below.

3. Observe the leaves carefully. (You may also use a hand lens if there is need).

4. List down the major characteristics of all the five leaves. For instance:

• Type of leaf 

• Type of apex

• Type of venation  

• Arrangement of leaflets

5. Fill the gaps in the table below based on your observation.

6. Summarise each characteristic of the leaves into two variations as shown in the following table.

7. Starting with one characteristic of the leaves, group the leaves into two groups according to the two variations of that characteristic. Assign the number “1” to the first characteristic. For example, type of leaf: 1 – To the two variations, assign the letters (a) and (b). For example, simple leaf (a), compound leaf (b).

8. In the two groups of leaves obtained, further sort them out using a different characteristic. Then re group them into two new groups each according to variations in that characteristic. For example, sort out the leaves according to the arrangement of leaflets on the stalk i.e. those with digitate arrangement and those with pinnate arrangement. Assign the number 2 to the new characteristic. For example, arrangement of leaflets – 2

Did your dichotomous key in Activity 1.17 above look like this?

1. Compound or simple leaf

1. (a) Compound leaf (leaf divided into leaflets………….go to 2

(b) Simple leaf (leaf not divided into leaflets)…..…….go to 3

2. Arrangement of leaflets

2. (a) Digitate arrangement of leaflets (leaflets attached at the tip of the leaf)………..E (Cassava)

(b) Pinnate arrangement of leaflets (leaflets attached at several points on stalk)………D (Eucalyptus)

3. Arrangement of leaf veins

3. (a) Parallel venation (veins running from main stalk to the tip of the leaf (apex) i.e. parallel to each other off a central vein.…..B (Maize)

3. (b) Network venation (veins branched out from the midrib or central point…..….go to 4

4. Overall leaf shape

4. (a) Leaf round-shaped………..C ( Cabbage)

(b) Leaf not round - shaped………..….. A ( Orange)

Note

• The numbers 1, 2, 3 and 4 in the key represent the characteristic indicated in the tree.

• The letters (a) and (b) represent two variations in each characteristic.

• The phrase “go to” indicates the characteristic into which the specified leaves are to be further grouped. For example, in 1 (a) compound leaf …go to 2, indicates that the compound leaves are grouped further into two according to the arrangement of leaflets (2) i.e. digitate (a) and pinnate (b).