The big idea: Members of the same species can breed together and produce offspring that are themselves fertile (able to have young of their own).
This is the biological species concept — the main way biologists decide where one species ends and another begins.
Two organisms that cannot produce fertile offspring together belong to different species.
Same species
- Can interbreed in nature
- Offspring are fertile — they can breed too
- Example: a husky and a poodle are both the dog species
Different species
- Usually cannot interbreed
- If they do, the offspring are often sterile
- Example: a horse × a donkey gives a mule, which is sterile
The two-part test: For two organisms to count as the same species, both must be true:
1. they can interbreed, and
2. their offspring are fertile.
Fail either part and they are different species.
The two-part test as a decision tree: interbreed? then fertile offspring? Fail either step and the organisms are different species — like a horse × donkey, whose mule is sterile.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
It is not enough for two organisms simply to mate or to produce any young.
The offspring must be fertile — able to reproduce themselves. If the cross produces sterile young, the two parents are still counted as different species.
- Species
- A group of organisms that can interbreed and produce fertile offspring.
- Biological species concept
- Defining a species by the ability to interbreed and produce fertile offspring.
- Fertile offspring
- Young that are themselves able to reproduce.
- Hybrid
- The offspring of a cross between two different species.
- Sterile
- Unable to reproduce (cannot produce offspring of its own).
The classic example: the mule: A horse and a donkey are different species. They can mate and produce a mule.
But a mule is almost always sterile — it cannot have young of its own.
Because the offspring is not fertile, the horse and donkey stay separate species. The mule is a hybrid.
The biological species concept also has limits. It is hard to apply to organisms that do not reproduce sexually (such as many bacteria), or to fossils, where no breeding can ever be observed.
Stop wasting time on topics you know
Our AI identifies your weak areas and focuses your study time where it matters. No more overstudying easy topics.
How this is tested: On Paper 2 a 1-mark Define question often asks for the term species — you must give both parts (interbreed and fertile offspring) to score.
A linked Describe question can ask how karyotyping (comparing chromosomes) shows that two organisms are the same species.
On Paper 1A an MCQ may ask which criterion confirms two organisms share a species.
IB-style question — define and apply 'species'
Two breeds of pigeon that look very different are bred together by a keeper. The young birds grow up and successfully raise chicks of their own. State whether the two breeds belong to the same species, and define the term species. [2]
How to score both marks
- Read the clue. The young birds 'successfully raise chicks of their own' — so the offspring are fertile.
- Apply the rule. The two breeds interbreed AND give fertile offspring, so they are the same species (just different breeds, like dog breeds).
- Answer the command term (Define). A species is a group of organisms that can interbreed and produce fertile offspring — give both parts for the mark.
Final answer
Same species. A species is a group of organisms that can interbreed and produce fertile offspring.
✓ Mark-scheme check: Mark 1: states same species.
Mark 2: definition includes both 'interbreed' and 'fertile offspring'. Giving only one half (just 'can breed') would not score the definition mark.