Populations (AQA A Level Biology): Revision Note
Exam code: 7402
Species exist in populations
The definition of a species
A species can be defined as:
A group of similar organisms that can reproduce to give fertile offspring
Organisms of the same species have the same number of chromosomes in their cells
E.g., Humans have 46 chromosomes (with the exception of some chromosome abnormalities, e.g. Turner's syndrome)
Two organisms from different species can’t produce fertile offspring because their cells have different numbers of chromosomes
A horse has 64 chromosomes, and a donkey has 62 chromosomes
Their gametes have 32 and 31 chromosomes
When they combine, the offspring has 63 chromosomes
An odd number of chromosomes means they can’t pair up properly in meiosis, so the offspring can’t make gametes and is infertile
Members of a species do not live alone
Instead, they live in populations
The definition of a population
A population can be defined as:
A group of organisms of the same species occupying a particular space at a particular time that can potentially interbreed
Examiner Tips and Tricks
Defining a species can be difficult because:
Physical differences may exist within a species (variation)
Some different-looking organisms (e.g. hawthorn trees) can still interbreed successfully
Fertility of offspring is often the key test, but not always practical to observe
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