Populations (AQA A Level Biology): Revision Note

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

  • Humans have 46 chromosomes

• 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