🧬 Species and Classification
Big idea: Life on Earth is organised into organisms and grouped into species so scientists can study, identify, and predict patterns in nature.
🐾 What is an organism?
An organism is a single living individual — one plant, one animal, one bacterium.
Examples of organisms: 🐕 One dog in a park 🌻 One sunflower in a garden 🦠 One bacterium in your gut
Each of these is one organism.
- All organisms carry out the functions of life (eating, breathing, growing, reproducing, etc.)
- Organisms can be huge (blue whale) or tiny (bacteria you can't see)
Organism = ONE individual living thing, not a group!
❌ "A herd of elephants" is NOT one organism ✅ "One elephant" IS one organism
🐱 What is a species?
A species is a group of organisms that can have babies together — and those babies can also have babies.
Same species example: 🐕 A Labrador and a Poodle look very different, but they can breed and produce fertile puppies (Labradoodles that can also have puppies).
✅ Same species: Canis familiaris
Different species example: 🦁 A lion and 🐯 a tiger can breed and produce a "liger" — but ligers are usually infertile (can't have babies).
❌ Different species!
- Same species = can interbreed
- Offspring must be fertile (can have their own babies)
- Different species = cannot normally produce fertile offspring
Key test for a species: Can they breed AND produce fertile offspring? If not, they're different species!
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📚 Why do we need classification?
There are over 8 million species on Earth! Scientists use classification to organise this huge variety.
Think of it like a library: 📚 Imagine a library with millions of books but no organisation — chaos!
Classification is like putting books into sections (fiction, science, history), then shelves, then alphabetical order.
It helps you find what you need quickly.
- Identify unknown organisms ("What did I just find?")
- Predict characteristics ("If it's a mammal, it probably has fur")
- Communicate worldwide (scientists everywhere use the same names)
🏷️ Scientific names (binomial naming)
Every species gets a binomial name — like a first name and surname, but backwards!
How it works: Humans: Homo sapiens - Homo = genus (like a surname for related species) - sapiens = species (like a first name)
Domestic dog: Canis familiaris - Canis = genus (includes wolves, dogs, coyotes) - familiaris = species (the pet dog)
- First word = genus (capital letter)
- Second word = species name (lower case)
- Always written in italics or underlined
Exam formatting rule: ✅ Homo sapiens (italics, capital H, lowercase s) ❌ Homo Sapiens (wrong capitalisation) ❌ Homo sapiens (not italicised)
🪜 Taxonomy: the classification ladder
Taxonomy organises life into levels — from very specific (species) to very broad (domain).
Classification of humans: Domain: Eukarya (cells with nuclei) Kingdom: Animalia (animals) Phylum: Chordata (backbone) Class: Mammalia (mammals) Order: Primates (apes, monkeys) Family: Hominidae (great apes) Genus: Homo Species: sapiens
- Domain (biggest group)
- Kingdom
- Phylum
- Class
- Order
- Family
- Genus
- Species (smallest group)
Memory trick: Dear King Philip Came Over For Good Soup
(Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species)
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⚠️ Limits of classification
Classification is helpful, but nature doesn't always follow our neat categories!
Real-world problems: 🦆 Platypus: Has fur (mammal), lays eggs (reptile/bird), has a bill (bird). Where does it fit?
🧬 Genetic surprises: DNA tests have shown some species we thought were related actually aren't — and vice versa!
- Some organisms share features across groups
- New genetic evidence can change classifications
- New discoveries may redefine species relationships
IB loves this idea: Classification systems change as scientific knowledge improves.
This shows science is dynamic, not fixed!
IB-style question — Reading a graph — identify & explain species diversity change
A graph shows estimated species diversity on Earth over the past 400 million years. (a) Identify the time period when diversity was at its lowest. [1] (b) Explain the sharp drop in diversity approximately 66 million years ago, naming the event and one cause. [2] (c) Outline how natural selection can lead to new species and increase global diversity over time. [2]
How to answer it, step by step
- Read the graph / name the event
• (a) Find the lowest point on the curve and give the time in Mya — the unit is required for the mark.
• (b) Name the event (e.g. end-Cretaceous extinction) + one cause (asteroid impact, volcanic activity, or rapid cooling). - Chain to speciation
• Mutation creates variation → natural selection favours better-adapted traits → isolated populations diverge → reproductive isolation → new species.
• You must reach 'reproductive isolation' — stopping at 'natural selection improves fitness' scores zero.
Final answer
Always write the unit (Mya) in part (a). In part (c), the key link is reproductive isolation leading to speciation — without it, no marks.
IB-style question — Explain speciation from tectonic / geographic events [7-mark essay]
Explain how tectonic plate movements, including volcanic activity and continental drift, can give rise to new species over time. [7]
How to answer it, step by step
- Physical barrier → isolated populations
• Volcanic islands, continental drift, or mountain uplift split one population into two geographically separated groups.
• Each group now faces different climate, food, and predator conditions. - Divergence → new species
• Different selection pressures favour different traits in each group; mutations accumulate over generations.
• Eventually the groups can no longer interbreed (reproductive isolation) → they are separate species.
• Add a named example (e.g. Darwin's finches on isolated volcanic islands) for extra credit.
Final answer
You need the full chain — new barrier → isolated populations → different selection → inherited divergence → reproductive isolation → new species. Missing reproductive isolation is the most common reason students score 4–5 instead of 7.