Organisms and species at Higher Level: The same species concept as SL. At HL you use it again when you classify species into clades and study their life cycles.
Practise this as you read
- Apply the fertile-offspring test to each cross.
- Ignore looks: judge by the young.
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One organism, one species: An organism is a single individual. Each individual organism is a member of a species.
The points to remember
- An organism is one individual living thing.
- Every organism is a member of a species.
- A species is a group of organisms that can interbreed...
- ...and produce fertile offspring: young that can breed in turn.
- This is the biological species concept.
Remember it as: Same species: they breed, and their babies breed too.
Real example: a tiny Shetland pony and a tall Arabian horse can breed, and their foals are fertile. So both are one species, the horse, Equus caballus.
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The test is what happens to the young. Read the last column of the table.
Using the test
- Mating is not enough: the young must be fertile.
- Young that cannot breed show the parents are different species.
- Looks do not decide it: very different-looking animals can be one species.
- Similar-looking animals can be two species if they do not interbreed.
Say 'fertile': 'They can have babies' is not the definition. Say they interbreed and produce fertile offspring.
Real example: a zebra and a horse can mate, but their young, the zorse, cannot breed. So zebras and horses are two species.
How this comes up: A short Paper 2 part: define a species, or use it to explain a cross [1-2].
A zoo in Mexico keeps a zorse, the young of a zebra and a horse. Zorses cannot have young.
Explain why zebras and horses are classified as different species.
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