Organisms and species
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. Organisms can be very large (like trees or whales) or extremely small (like bacteria).
- All organisms carry out the functions of life
- An organism can exist on its own, even though it usually interacts with others
Exam tip: Organism = one individual living thing, not a group.
What is a species?
A species is a biological grouping based on reproduction, not appearance alone.
- Members of the same species can interbreed
- Their offspring must be fertile
- Different species cannot normally produce fertile offspring
Lions and tigers can produce hybrids, but because the offspring are usually infertile, they are considered different species.
Why classification is needed
There are millions of species on Earth. Scientists use classification to make studying life manageable.
- Helps identify unknown organisms
- Allows prediction of characteristics
- Creates a shared scientific language worldwide
Binomial naming (scientific names)
Each species is given a binomial name so it can be recognised globally.
- First word = genus (capital letter)
- Second word = species name (lower case)
- Names are written in italics or underlined
Example: Humans have the scientific name Homo sapiens.
Exam rule: Genus is capitalised, species is not. Both words are italicised.
Taxonomy: grouping life
Taxonomy arranges organisms into a hierarchy from very specific to very broad groups.
- Species
- Genus
- Family
- Order
- Class
- Phylum
- Kingdom
- Domain
Memory trick: Dear King Philip Came Over For Good Soup.
Limits of classification
Classification is useful, but not perfect. Nature does not always fit neatly into human-made categories.
- Some organisms share features across groups
- Genetic evidence can change classifications
- New discoveries may redefine species relationships
IB likes this idea: classification systems change as scientific knowledge improves.
Quick check (exam-ready)
- An organism is one living individual
- A species is a reproductive group
- Scientific names use binomial name
- Classification helps identification, prediction, and communication
How organisms are identified
Big idea: Scientists need reliable ways to identify organisms so they can study biodiversity, monitor ecosystems, and detect environmental change.
Why identification matters
- Correct identification allows accurate biodiversity measurement
- It helps track population changes over time
- It is essential for conservation decisions
- Misidentification can lead to wrong conclusions about ecosystems
If you don’t know what species you are observing, you cannot accurately study population size, distribution, or change.
How organisms are identified
Organisms are usually identified using observable physical characteristics, such as shape, size, colour, number of limbs, or leaf structure.
- Body shape and size
- Number of legs, wings, or petals
- Leaf shape and arrangement
- Presence or absence of key features
Identification in ESS focuses on visible features, not genetic testing.
Dichotomous keys
A dichotomous key works by giving two contrasting options at each step.
Each choice removes some possibilities until only one organism fits all the answers.
- Step 1: Choose between two opposite characteristics
- Step 2: Follow the instruction to the next pair of choices
- Step 3: Repeat until a name is reached
- Final step: Organism is identified
Dichotomous = two choices only at each step.
Example of how a dichotomous key works
- Does the organism have wings? → Yes / No
- Does it have six legs? → Yes / No
- Does it have scales or feathers?
- Each answer narrows down the options
Always read both choices carefully before deciding — rushing leads to mistakes.
Strengths of dichotomous keys
- Simple and quick to use
- Low cost — no equipment needed
- Works well for fieldwork
- Useful for beginners and students
Limitations of dichotomous keys
- Damaged organisms may be hard to identify
- Young organisms may not show adult features
- Closely related species may look very similar
- Keys depend on the user choosing correctly
In exam answers, always mention one strength AND one limitation.
Big exam takeaways
- Identifying organisms is essential for studying ecosystems
- Dichotomous keys use paired choices
- Identification is based on observable features
- Keys are useful but not perfect
- Errors increase when organisms are damaged or immature