Genetic diversity is a factor enabling natural selection to occur.

The idea

Natural selection can only act on variation that already exists. In a population where every individual carries identical alleles, no individual has a genetic advantage when the environment changes, meaning allele frequencies cannot shift through selection. Genetic diversity supplies the raw material: many different alleles produce many different phenotypes. When a new selection pressure arises, such as a novel disease, this phenotypic variation results in differences in survival and reproductive success. The advantageous alleles are then passed on. Without genetic diversity, a population may contain no alleles conferring resistance or adaptation, increasing the risk of extinction.

What earns the marks

• Stating that natural selection requires genetic variation within the population.
• Explaining that different alleles result in different phenotypes.
• Linking advantageous phenotypes to increased chances of survival and reproductive success.
• Explaining that low genetic diversity means a population may lack advantageous alleles when the environment changes.

Common mistakes and how to put them right

• Writing that organisms adapt or mutate in response to a selection pressure, rather than selection acting on existing mutations.

• Saying individuals evolve, when it is populations whose allele frequencies change over time.

• Treating all variation as heritable, when only genetic variation can be passed to offspring.

Exam tips

• Open natural selection answers by stating that there are different alleles in the population, originally produced by random mutation.
• Avoid words like 'want', 'need', or 'try' to survive, as examiners may penalise Lamarckian phrasing.