Construct and use dichotomous keys based on identifiable features.

IGCSE Biology 0610 – 1.2 Concept and Uses of Classification Systems

Why Do We Classify Living Things?

Classification helps scientists to:

• Organise the huge diversity of organisms. – e.g., grouping all flowering plants together makes it easier to study them.
• Predict characteristics of an unknown organism. – e.g., if a plant is placed in the family Fabaceae, you can predict it will have nitrogen‑fixing root nodules.
• Communicate clearly using a universal naming system. – e.g., the name Rosa canina is recognised worldwide, avoiding confusion with common names like “dog rose”.
• Study evolutionary relationships. – e.g., organisms that share a recent common ancestor are placed in the same genus.
• Group organisms because they share one or more traits. – shared features may be morphological (leaf shape), physiological (photosynthetic pathway), or genetic (DNA sequence similarity).

What Is a Species?

A species is a group of individuals that can inter‑breed in natural conditions and produce fertile offspring. Example (exam‑style): “*Homo sapiens* is a species because all humans can inter‑breed and produce fertile children, whereas *Homo sapiens* and *Pan troglodytes* (chimpanzee) cannot.”

Binomial Nomenclature – An Internationally Agreed Two‑Part Naming System

The scientific name of every species consists of two parts, both italicised (or underlined when handwritten):

1. Genus name – capitalised (e.g., Homo).
2. Specific epithet – lower‑case (e.g., sapiens).

Full example: Homo sapiens. The specific epithet is never capitalised, and the whole name is always written in italics.

Basic Taxonomic Hierarchy

Evolutionary Relationships in Modern Classification

Modern taxonomy groups organisms so that those sharing a more recent common ancestor are placed together, reflecting evolutionary relationships. DNA‑sequence similarity is now a primary tool for inferring these relationships, complementing traditional morphological traits.

Dichotomous Keys

A dichotomous key is a step‑by‑step identification tool that uses a series of two‑choice (binary) statements based on **observable, mutually exclusive traits**. By following the key a student can arrive at a single species name, and the key can also be used in reverse to identify an unknown specimen.

Steps to Construct a Dichotomous Key

1. Gather the set of organisms you need to identify.
2. Record clear, observable characteristics for each (leaf shape, presence of veins, flower colour, animal limb type, etc.).
3. Choose a characteristic that cleanly separates the whole group into two subsets.
4. Write the first pair of statements (1a and 1b) describing those subsets.
5. Repeat the process for each subset, creating new numbered pairs (2a/2b, 3a/3b, …) until every organism is isolated.
6. Number each new pair sequentially and ensure the wording is unambiguous.
7. Test the key by using it to identify a specimen; revise any ambiguous steps.

Example Key – Five Common Garden Plants

1. 1a. Plant has broad, flat leaves … go to step 2.
   1b. Plant has needle‑like or scale leaves … go to step 4.
2. 2a. Leaves are toothed … Rosa (rose).
   2b. Leaves are smooth … go to step 3.
3. 3a. Plant bears a single large flower … Rudbeckia (black‑eyed Susan).
   3b. Plant bears many small flowers in a dense cluster … Lavandula (lavender).
4. 4a. Leaves are arranged in whorls of three … Thuja (arborvitae).
   4b. Leaves are arranged singly along the stem … Pinus (pine).

Animal Example – Simple Key for Four Vertebrates

1. 1a. Animal has feathers … go to step 2.
   1b. Animal does not have feathers … go to step 3.
2. 2a. Animal can fly … Passer domesticus (house sparrow).
   2b. Animal cannot fly … Gallus gallus domesticus (domestic chicken).
3. 3a. Animal has scales and lives in water … Carassius auratus (goldfish).
   3b. Animal has smooth skin, four limbs and breathes air … go to step 4.
4. 4a. Animal has a long, muscular tail for jumping … Rana temporaria (common frog).
   4b. Animal has a backbone with a distinct neck and walks on land … Mus musculus (house mouse).

Practice Activity

Part A – Construct a key
Create a dichotomous key for the following five organisms. Record the observable feature you use at each step.

• Helianthus annuus – Sunflower
• Quercus robur – Oak tree
• Pteridium aquilinum – Fern
• Opuntia ficus‑indica – Cactus
• Poa pratensis – Grass

Part B – Use the key
You are given a sealed specimen (unknown to you). Use the key you have constructed to identify it. Record the step numbers you followed and the final species name.

Marking Checklist for the Teacher (AO‑aligned)

Key Take‑aways

• Classification simplifies the study of biodiversity and aids prediction of traits.
• A species is a group of inter‑breeding individuals that produce fertile offspring.
• Binomial nomenclature provides a single, universally accepted name for each species.
• Modern classification reflects evolutionary relationships; DNA similarity is a primary tool.
• Dichotomous keys rely on mutually exclusive, observable traits and develop logical, step‑wise thinking.
• Practising both the construction and the use of keys strengthens observation, reasoning and communication skills.