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).
The biological species concept
- 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.
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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.
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The big idea: The breeding test is the main definition of a species, but it cannot always be used, so other definitions exist.
The points to remember
- Fossils: extinct organisms can never be tested for breeding.
- Asexual organisms, such as many bacteria, do not breed with each other at all.
- Populations far apart may never meet, so nobody can see whether they would interbreed.
- Some hybrids are fertile, so two species can pass the breeding test.
- So competing definitions exist: by shared visible traits (Linnaeus) or by DNA similarity.
Remember it as: No breeding to watch? Then look at the body or the DNA.
Fossils
No living individuals, so no breeding to test.
Asexual organisms
They reproduce without a partner.
Separated populations
They never meet in the wild.
Fertile hybrids
Two species that still give fertile young.
Real example: grizzly and polar bears: Grizzly bears and polar bears are counted as two species, yet in the wild in Canada they have mated and produced hybrids that went on to breed. The breeding test alone would call them one species.
In an answer, the breeding test has two parts, and both are needed:
'Can interbreed' is not enough: Many different species can mate. Say they interbreed AND produce fertile offspring.
How this is tested: Expect a 1-mark Define, a linked Describe question and sometimes an MCQ.
Where it comes up
- Paper 2: a 1-mark Define question often asks for the term species — you must give both parts (interbreed and fertile offspring) to score.
- Paper 2: a linked Describe question can ask how karyotyping (comparing chromosomes) shows that two organisms are the same species.
- Paper 1A: an MCQ may ask which criterion confirms two organisms share a species.
Exam-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.
Grove snails in two valleys have different shell colours. Suggest how evidence could be gathered to test whether the two populations belong to the same species.
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