Variation: state that there is usually extensive genetic variation within a population of a species

The idea

A population is the group of individuals of one species living in a particular area. Within that group, individuals differ in many inherited features because they carry different combinations of alleles. This is genetic variation. For example, in a population of banded snails, shell colour and banding pattern vary because different alleles for shell-pattern genes are present. Variation arises through mutation and is reshuffled by sexual reproduction, so populations usually contain a wide range of genotypes and phenotypes rather than being uniform. The word extensive signals that the variation is normally large, not slight or occasional. Recognising this matters because natural selection acts on the variation already present: if a population lacks variation, it cannot adapt so easily when the environment changes.

What earns the marks

• Defines a population as the individuals of one species living together in the same area at the same time.
• States that members of a population usually differ genetically from one another.
• Uses the term genetic variation to describe differences in alleles or genotypes within the population.
• Explains that the variation is extensive, meaning many individuals differ and differences are common.
• Links genetic variation to differences in phenotype, such as shell colour or banding pattern in snails.
• Recognises that sexual reproduction and mutation are sources that maintain variation in a population.

Common mistakes and how to put them right

• Confusing a population with a species.

  Put it right: Define population as one species in one area.

• Assuming all members of a population are genetically identical apart from a few mutants.

  Put it right: State that extensive variation is usual because many alleles and allele combinations exist.

• Treating variation as only environmental, such as a plant growing taller because of more light.

  Put it right: Genetic variation means inherited differences in alleles, although phenotype can also be affected by environment.

Exam tips

• Use the exact phrase genetic variation within a population when answering, rather than writing that animals are simply different.
• Give a named example, such as banded snails or human blood groups, to show the variation is inherited and not just due to age or diet.
• If asked why variation matters, link it to natural selection: a population with extensive variation is more likely to contain individuals suited to a changed environment.