IGCSE Biology 0610 - 1.2 Concept and uses of classification systems

Objective: Construct and use dichotomous keys based on identifiable features.

What is a Classification System?

A classification system is a way of organizing living things into groups based on similarities and differences. This helps us understand the relationships between different organisms and makes it easier to identify them.

Why do we use Classification Systems?

Classification systems are useful for several reasons:

• Organization: They provide a logical way to organize the vast diversity of life.
• Identification: They help us identify unknown organisms by comparing their features to those in the key.
• Understanding Relationships: They reveal evolutionary relationships between organisms. Organisms in the same group are more closely related than those in different groups.
• Communication: They provide a common language for scientists to talk about organisms.

The Linnaean System

The most widely used classification system is the Linnaean system, developed by Carl Linnaeus. It uses a hierarchical system of categories:

1. Domain (the broadest category)
2. Kingdom
3. Phylum (for animals) / Division (for plants)
4. Class
5. Order
6. Family
7. Genus
8. Species (the most specific category)

Each organism is given a unique two-part name called a scientific name, which consists of its genus and species. This is written in italics, with the genus capitalized and the species in lowercase (e.g., Homo sapiens).

Dichotomous Keys

A dichotomous key is a tool used to identify organisms. It presents a series of paired statements (couplets). Each couplet describes a feature of an organism, and you choose the statement that best describes the organism you are trying to identify. This leads you to another couplet, and so on, until you reach a single species.

Constructing a Dichotomous Key

To construct a dichotomous key, you need to:

1. Identify key features: Choose features that are easily observable and show variation between organisms.
2. Pair statements: Create paired statements that present two mutually exclusive choices.
3. Number the statements: Assign a number to each statement.
4. Start at the beginning: Begin with the first couplet and follow the instructions until you reach the species name.

Example of a Dichotomous Key

Consider the following key for identifying some common garden plants:

No.	Statement	Species
1	Leaves are needle-like.	Pine Tree
2	Leaves are broad and flat.	Continue to 3
3	Leaves are toothed.	Holly
3	Leaves are smooth-edged.	Rowan Tree

How to use this key:

1. Examine a plant and observe its leaves.
2. If the leaves are needle-like, go to Pine Tree.
3. If the leaves are broad and flat, go to the next couplet.
4. If the leaves are toothed, go to Holly.
5. If the leaves are smooth-edged, go to Rowan Tree.

Practice

Question: Use the following dichotomous key to identify a plant with broad, flat leaves that are toothed.

No.	Statement	Species
1	Leaves are needle-like.	Pine Tree
2	Leaves are broad and flat.	Continue to 3
3	Leaves are toothed.	Holly
3	Leaves are smooth-edged.	Rowan Tree

Answer: Holly

Important Considerations

Dichotomous keys are only as good as the features they are based on. It's important to choose features that are reliable and easily observable. Also, keys are often specific to a particular group of organisms.