How Do Biologists Decide Whether Two Organisms Belong to the Same Species?
- Species are the basic unit of all classification.
- Biologists need a clear rule to decide whether two organisms belong to the same species.
- The most widely accepted one is the biological species concept, based on producing fertile offspring.
- If offspring are sterile, the organisms have separate gene pools, so they are considered different species.
Definition
Species
A group of organisms that can interbreed to produce fertile offspring.
Tip
The simplest indicator of different species is a sterile hybrid, like a mule (horse × donkey).
Why Are Some Organisms Hard to Classify?
Definition
Hybridization
When individuals from different species mate and produce offspring.
- Nature doesn't always follow neat categories.
- Species that are very similar genetically can occasionally hybridise, blurring boundaries.
- Fossils can't be tested for reproduction at all.
- Bacteria reproduce asexually, so the biological species concept doesnn't apply.
- For these cases, scientists use DNA, behaviour, structure, and ecology to classify organisms.
Common Mistake
- Don't assume species boundaries are fixed.
- In reality, classification changes when new DNA evidence appears.
What Patterns Link All Life on Earth?
Definition
Shared characteristics
Features found across all living organisms.
- Classification works because all organisms share deep structural patterns.
- These include cells, DNA, energy use, and growth.
- They show that all living species evolved from a common ancestor.
- This is why classification forms a branching tree rather than random categories.
| Universal feature | Why it matters |
|---|---|
| Cells | All life is built from cells, allowing comparison across species. |
| DNA | Provides a shared genetic language used for classification. |
| Metabolism | Shows how organisms obtain and use energy. |
| Reproduction | Passes traits to the next generation, linking species over time. |
Analogy
Just as languages share grammar rules because of a shared origin, all organisms share cellular and genetic patterns because of common ancestry.
How Are Organisms Grouped Into Kingdoms?
- Kingdoms are one of the broadest levels in classification.
- They separate organisms based on features such as cell structure and how they obtain food.
- This helps biologists organise millions of species into meaningful groups.
| Kingdom | Cell Type | Nutrition | Structure |
|---|---|---|---|
| Animalia | Eukaryotic | Heterotrophic | Multicellular |
| Plantae | Eukaryotic | Autotrophic | Multicellular |
| Fungi | Eukaryotic | Heterotrophic (absorptive) | Mostly multicellular |
| Protista | Eukaryotic | Mixed | Mostly multicellular |
| Bacteria | Prokaryotic | Mixed | Unicellular |
Note
Modern classification aims to reflect evolutionary relationships, not just appearance.
What Is the Hierarchical System of Classification?
- Biologists organise living organisms using a hierarchy of categories.
- Each level becomes more specific as you move down the hierarchy.
- This structure helps scientists show how closely organisms are related.
- The hierarchy is fixed in order, making it easy to compare groups across different species.
- Species remain the most specific level, while domains and kingdoms sit at the top.
Tip
- A common memory trick for the hierarchy: “Dear King Philip Came Over For Good Soup.”
- Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species)
What Are the Main Groups of Flowering Plants?
- Flowering plants are divided into monocots and dicots.
- Their differences are structural, visible, and easy to test.
- These patterns allow fast classification in the field.
| Feature | Monocots | Dicots |
|---|---|---|
| Seed leaves (cotyledons) | One | Two |
| Leaf veins | Parallel | Branching |
| Vascular bundles | Scattered | In a ring |
| Examples | Grasses, lilies | Beans, roses |
Tip
Leaf vein patterns are the quickest visual clue for identifying monocots and dicots.
Self review
- Which universal features link all life, and how do they support the idea of common ancestry?
- What criteria are used to separate organisms into different kingdoms?
- What is the purpose of a hierarchical classification system?
- Which level in the hierarchy is the broadest, and which is the most specific?
- How can you quickly tell monocots and dicots apart using leaf patterns?
- Why is DNA evidence sometimes more reliable than appearance when classifying organisms?
