CIE iGCSE Biology-1.2 Concept and uses of classification systems- Study Notes- New Syllabus
CIE iGCSE Biology-1.2 Concept and uses of classification systems- Study Notes – New syllabus
CIE iGCSE Biology-1.2 Concept and uses of classification systems- Study Notes -CIE iGCSE Biology – per latest Syllabus.
Key Concepts:
Core
- State that organisms can be classified into groups by the features that they share
- Describe a species as a group of organisms that can reproduce to produce fertile offspring
- Describe the binomial system of naming species as an internationally agreed system in which the scientific name of an organism is made up of two parts showing the genus and species
- Construct and use dichotomous keys based on identifiable features
Supplement
- Explain that classification systems aim to reflect evolutionary relationships
- Explain that the sequences of bases in DNA are used as a means of classification
- Explain that groups of organisms which share a more recent ancestor (are more closely related) have base sequences in DNA that are more similar than those that share only a distant ancestor
Classification by Shared Features
Classification is the process of grouping living organisms based on the features they share. It helps scientists organize the vast diversity of life, study evolutionary relationships, and identify species more easily – like sorting books in a library.
🔍 Key Definition
Organisms can be classified into groups by the features they share, such as body structure, reproduction method, and cell type.
🧪 Why Is Classification Important?
| Purpose | Explanation |
|---|---|
| Makes study easier | Easier to study millions of species |
| Shows evolutionary links | Similar features may suggest common ancestry |
| Helps in conservation | Understand endangered species and protect them |
| Useful in medicine | Identify disease-causing organisms accurately |
| Aids communication | Scientists worldwide use shared terms |
📂 How Are Organisms Grouped?
| Type of Feature | Examples |
|---|---|
| Structural features | Backbone, limbs, body shape |
| Cellular features | Cell wall, chloroplasts, nucleus |
| Reproductive method | Egg laying or live birth |
| Feeding method | Autotrophic vs Heterotrophic |
| Genetics | DNA and protein similarities |
| Habitat | Aquatic, terrestrial, parasitic |
🧬 The Modern Classification System
| Rank | Description | Example (Human) |
|---|---|---|
| Kingdom | Broadest group | Animalia |
| Phylum | Body plan features | Chordata |
| Class | More specific traits | Mammalia |
| Order | Grouped families | Primates |
| Family | Closely related genera | Hominidae |
| Genus | Group of similar species | Homo |
| Species | Can reproduce fertile offspring | sapiens |
Mnemonic: King Philip Came Over For Good Soup
🧠 What Does “Shared Features” Mean?
- Similar body parts (e.g., wings, limbs, feathers)
- Similar embryonic development
- Similar biochemistry (enzymes, proteins)
- Genetic resemblance (e.g., DNA base sequences)
Final Recap
Classification = Grouping organisms by shared features. It brings order to life’s diversity, makes study easier, and shows how species are connected. From body structure to DNA, shared traits tell life’s evolutionary story.
What Is a Species?
A species is one of the most important concepts in biology. It defines which organisms belong to the same natural group and can reproduce successfully to continue life.
🔍 Key Definition
A species is a group of organisms that can reproduce with each other to produce fertile offspring.
“Fertile offspring” means the babies can also grow up and reproduce successfully. Different species cannot do this.
🧠 Why Is This Definition Important?
- Helps scientists tell species apart
- Shows genetic relationships
- Explains how species evolve or go extinct
🔍 Key Characteristics of a Species
| Characteristic | Description |
|---|---|
| Can reproduce with each other | Members of the same species can mate |
| Produce fertile offspring | Their young can also reproduce |
| Share similar genes | Same number of chromosomes, similar DNA |
| Often look alike | But differences like gender or age may exist |
❌ Different Species: Why Fertility Matters
Some animals from different species can reproduce, but their offspring are sterile (cannot reproduce). These offspring are called hybrids.
| Parents | Offspring | Fertile? | Same Species? |
|---|---|---|---|
| Horse + Horse | Foal | Yes | Yes |
| Zebra + Donkey | Zedonk | No | No |
| Lion + Tiger | Liger | No | No |
Note: Hybrids like Zedonk or ligers cannot have babies – so their parents are not the same species.
🌿 Does This Apply to Plants Too?
Yes! In plants, species can be defined by the ability to:
- Cross-pollinate with others of the same kind
- Produce fertile seeds that grow into healthy plants
🔁 Evolution and Species
Over time, a single species may split into two new species through a process called speciation. This happens when:
- Populations become geographically isolated
- They change genetically so much that they can no longer interbreed
Final Recap
A species is like a natural family – its members can mate, have babies, and continue the line. Fertility is the key that keeps the cycle of life going within that group.
Binomial System of Naming Species
The binomial system is a universal way of naming living organisms using two Latin names – the genus and species. It helps avoid confusion caused by local names and allows scientists across the world to communicate clearly.
🔍 Key Definition
The binomial system is an internationally agreed system in which the scientific name of an organism is made up of two parts: the genus name (first word) and the species name (second word).
🧠 Features of the Binomial System
| Rule | Example |
|---|---|
| Two words | Homo sapiens |
| Genus starts with capital letter | Homo |
| Species in lowercase | sapiens |
| Written in italics or underlined | Homo sapiens or Homo sapiens |
| Uses Latin or Latinized names | Universal format |
🧑🔬 Why Use This System?
| Reason | Benefit |
|---|---|
| Universal | Used by scientists everywhere |
| Avoids confusion | Same species may have many local names |
| Shows relatedness | Same genus = shared ancestry |
| Helps classification | Fits into taxonomic groups |
🌍 Examples of Binomial Names
| Common Name | Binomial Name | Notes |
|---|---|---|
| Human | Homo sapiens | Homo = genus, sapiens = species |
| Lion | Panthera leo | Same genus as tiger |
| Tiger | Panthera tigris | Close relative of lion |
| House cat | Felis catus | From genus Felis |
| Rice plant | Oryza sativa | From genus Oryza |
🌱 How Is It Used in Science?
- Naming new species
- Comparing evolutionary relationships
- Organizing databases, museums, herbariums
- Used in research papers and reports
Final Recap
The binomial system gives every species a unique scientific name, telling us both its group (genus) and identity (species). It keeps global biology precise, consistent, and connected.
Dichotomous Keys
A dichotomous key is a scientific tool used to identify organisms based on their observable features. “Dichotomous” means “divided into two parts” – each step gives two choices, and your decision leads to the next step or the name of the organism.
🔍 Key Definition
A dichotomous key is a series of two-choice questions (yes/no, present/absent) used to identify organisms by their physical characteristics.
🧠 Why Are Dichotomous Keys Useful?
| Purpose | Benefit |
|---|---|
| Helps identify unknown species | Great for insects, plants, etc. |
| Based on observable traits | No microscopes or DNA needed |
| Step-by-step method | Easy to follow in the field |
| Used in biology & taxonomy | Popular in labs and schools |
How a Dichotomous Key Works?
At each step, you are given two contrasting options. Choose the one that matches the organism – it leads you to the next step or to the organism’s identity.
📖 Example: Key for 4 Animals
| 1a. Has feathers → go to 2 |
| 1b. Does not have feathers → go to 3 |
| 2a. Can fly → Bird |
| 2b. Cannot fly → (not needed here) |
| 3a. Has dry, scaly skin → go to 4 |
| 3b. Has moist skin → Frog |
| 4a. Has legs → Turtle |
| 4b. Has no legs → Snake |
How to Make a Dichotomous Key
- Observe the organisms’ visible traits
- List key features (e.g. skin type, number of legs)
- Start with broad characteristics (e.g. “Has wings / No wings”)
- Use pairs of clear, contrasting statements
- Each path ends with a name or label
Final Recap
A dichotomous key is like a decision tree that helps you identify organisms using just visible features. By answering one yes/no question at a time, you can unlock the identity of even unfamiliar plants or animals.
Classification and Evolutionary Relationships
Modern classification systems group living organisms based not only on visible features but also on their evolutionary ancestry. This helps scientists understand how life has evolved over time.
🔍 Definition
Classification systems aim to reflect evolutionary relationships by grouping organisms that share a common ancestor and have similar genetic or structural traits inherited over time.
🧠 What Does That Mean?
- Species with similar DNA or body parts probably evolved from a common ancestor
- Modern classification helps trace evolutionary family trees
- Systems like cladistics use DNA evidence and shared traits
🧬 Example: Evolutionary Grouping
| Organism | Classification Link | Evolutionary Connection |
|---|---|---|
| Humans | Closely related to chimpanzees | Share ~98% DNA, common primate ancestor |
| Birds & Reptiles | Similar bone structures | Evolved from ancient reptiles |
| Whales & Hippos | Similar genes and skulls | Share a recent land mammal ancestor |
🧬 Old vs. Modern Classification
| System Type | Based On | Used Today? |
|---|---|---|
| Old (Linnaean) | Physical appearance only | Partly |
| Modern (Phylogenetic) | Evolutionary history + DNA | Yes |
🧬 How Scientists Find Evolutionary Relationships
| Evidence Type | What It Shows |
|---|---|
| Fossil records | How species changed over time |
| DNA comparisons | Genetic similarity = shared ancestry |
| Anatomical features | Similar body parts (e.g. limbs) |
| Embryology | Similar early development stages |
🌳 Phylogenetic Trees (Evolution Trees)
Final Recap
Classification today is all about ancestry. Modern systems group organisms based on how they evolved – not just how they look. Every group is a branch on life’s tree, revealing who is related to whom in the great story of evolution.
Using DNA Base Sequences for Classification
Scientists now use DNA sequences – the exact order of bases A, T, C, G – to classify organisms. This method reveals deep evolutionary connections, even between species that look very different.
🔍 Definition
The sequences of bases in DNA (adenine, thymine, cytosine, guanine) are used in classification because they can show how closely related organisms are, based on similarities in their genetic code.
🧠 Why Use DNA Sequences in Classification?
| Reason | Benefit |
|---|---|
| More accurate than physical features | Shows true genetic relationships, not just appearance |
| Detects close and distant relatives | Even when organisms look very different |
| Universal in all living things | All life uses the same DNA code |
| Reflects evolutionary history | Similar DNA means shared ancestry |
🧬 How It Works
- Scientists extract DNA from both organisms
- They compare the sequence of bases (A, T, C, G)
- The more base pairs that match, the more closely related they are
- This helps classify organisms into accurate groups
Example: DNA Similarity
| Organisms Compared | % Similarity in DNA | What It Suggests |
|---|---|---|
| Human and chimpanzee | ~98–99% | Very closely related |
| Human and mouse | ~85% | Distantly related |
| Human and corn plant | ~60% | Share some ancient genes |
🧪 DNA vs Physical Features
| Basis of Comparison | Can Be Misleading? | Example |
|---|---|---|
| Physical features only | Yes | Dolphins look like fish but are mammals |
| DNA base sequences | No | Shows true evolutionary link |
🔁 Linked to Evolution
Over time, DNA sequences change due to mutations and evolution. The fewer differences two species have, the more recently they shared a common ancestor. This is why DNA helps build accurate phylogenetic trees.
Final Recap
DNA is like life’s barcode the more similar the base sequence, the closer the species are related. It is now one of the most trusted tools in modern biology to classify organisms and understand their evolutionary history.
DNA Similarity and Evolutionary Relationships
Organisms that share a recent common ancestor have more similar DNA base sequences. This insight is central to modern classification and evolutionary biology.
🧠 Key Explanation
DNA is made up of four bases: A (adenine), T (thymine), C (cytosine), and G (guanine).
The specific order of these bases forms a genetic code unique to each organism.
If two species have similar DNA base sequences, it shows they:
Evolved from a common ancestor
Are more closely related evolutionarily
The more similar the DNA, the more recent the shared ancestor.
🔬 DNA Reveals Evolutionary Distance
| Relationship Type | DNA Base Similarity | Shared Ancestor? |
|---|---|---|
| Close relatives (e.g., human & chimpanzee) | Very high similarity (~98–99%) | Recent common ancestor |
| Distant relatives (e.g., human & bird) | Moderate similarity | Ancient shared ancestor |
| Unrelated organisms (e.g., human & bacteria) | Very different DNA | Extremely distant or unclear relation |
🌳 Linked to the Evolutionary Tree
In the tree of life:
- Species that split off recently share more DNA
- Species that diverged long ago show greater genetic differences
DNA mutations over time increase the difference between distant species.
🧬 DNA Similarity Examples
| Species A | Species B | DNA Similarity | Relationship |
|---|---|---|---|
| Human | Chimpanzee | 98–99% | Very closely related |
| Human | Mouse | ~85% | Related, but more distant |
| Human | Corn plant | ~60% | Distant common ancestor |
| Human | Bacteria | ~10–15% | Extremely distant |
🧪 Why It Matters in Classification
- DNA comparisons help group species more accurately
- More reliable than comparing visible traits alone
- Helps scientists build evolutionary trees based on divergence times
Final Recap
DNA acts like a genetic timeline – the closer the match, the closer the evolutionary relationship. More shared DNA = more recent ancestor = more closely related species.