Concept and Uses of Classification SystemsCambridge IGCSE Biology: Revision notes
Section 1
What Makes a Species?
Living organisms are sorted into groups based on the features they share. The basic unit of classification is the species.
- A species is a group of organisms that can reproduce together to produce fertile offspring
- Organisms are classified into groups according to shared observable and genetic features
- Grouping by shared features allows scientists to organise the huge diversity of life in a logical way
Two animals that can mate are not necessarily the same species - the offspring must also be fertile. A mule (horse x donkey) is infertile, so horses and donkeys are different species.
Section 2
How Are Organisms Named? The Binomial System
Every species is given a unique two-part scientific name under the binomial system.
- The first part of the name shows the genus (always capitalised)
- The second part shows the species (always lower case)
- Example: Homo sapiens - Homo is the genus, sapiens is the species
- This system gives every organism one internationally recognised name, avoiding confusion between common names in different languages
Panthera leo (lion) and Panthera tigris (tiger) share the genus Panthera, showing they are closely related.
Section 3
How Do We Identify Unknown Organisms? Dichotomous Keys
A dichotomous key is constructed from a series of questions with two possible answers, each leading either to another question or to the identification of the organism.
- Start with a feature that splits the group into two
- Follow the branch that matches the organism being identified
- Repeat until only one organism remains
Keys are built using clearly identifiable features such as presence or absence of structures, number of legs, or body covering.
Section 4
Why Classify? Evolutionary Relationships and DNA Evidence
Modern classification systems aim to reflect evolutionary relationships between organisms, not just convenient similarities.
- Organisms that share a more recent common ancestor are grouped more closely together
- The sequence of bases in DNA is used as evidence for classification
- Groups that share a more recent ancestor have more similar DNA base sequences than groups that diverged longer ago
- DNA evidence can confirm or correct groupings originally based only on physical appearance
If asked why a classification changed for a group, think DNA evidence, not just appearance - exam answers often want 'more similar base sequences = more closely related'.
Section 5
The Five Kingdoms and Major Groups
All organisms can be placed into one of five kingdoms: animal, plant, fungus, prokaryote, protoctist.
| Kingdom | Feature used to place organisms |
|---|---|
| Animal | multicellular, no cell wall, feed on other organisms |
| Plant | multicellular, cell wall, photosynthesise |
| Fungus | cell wall (not cellulose), feed by decomposition |
| Prokaryote | single-celled, no true nucleus |
| Protoctist | mostly single-celled, has a nucleus |
Within the animal kingdom, organisms are further grouped:
- Vertebrates: mammals, birds, reptiles, amphibians, fish
- Arthropods: myriapods, insects, arachnids, crustaceans
Within the plant kingdom: ferns and flowering plants (split into dicotyledons and monocotyledons).
Viruses are not placed in any of the five kingdoms - they are classified separately, consisting only of a protein coat and genetic material.
Viruses are NOT classified in any of the five kingdoms - do not list them as a sixth kingdom, they sit outside the system entirely.
Must Know
- A species can interbreed to produce fertile offspring
- Binomial names have two parts: genus + species
- Dichotomous keys identify organisms using paired questions
- More similar DNA base sequences = more recent common ancestor = closer relationship
- The five kingdoms are animal, plant, fungus, prokaryote, protoctist
- Viruses are classified separately - just a protein coat and genetic material, not a kingdom
That's the notes covered.
Carry on to the next subtopic.