As a result, over many generations, the new allele increases in frequency…

The idea

Allele frequency is the proportion of all copies of a gene in a population's gene pool that are of one particular allele. It is a proportion, not a count. If an allele gives a reproductive advantage, individuals carrying it tend to leave more offspring, so its share of the gene pool rises over generations while other alleles' shares fall. The change is gradual: a small advantage may take many generations, whereas intense selection, such as heavy antibiotic use, can produce a measurable rise in very few. Bacteria can change fastest because their generation time is short and reproduction is asexual, so a favourable allele is copied into every descendant. In humans, alleles protecting against malaria reached high frequencies in Africa over thousands of years, mainly where malaria is present. Chance also alters allele frequencies, especially in small populations, through genetic drift.

What earns the marks

• State that the frequency, proportion or percentage of the advantageous allele increases in the population.
• State that the change occurs over many generations.
• State that the frequency of the alternative allele or alleles decreases.
• Relate the rate of change to the strength of the selection pressure or the generation time of the organism.
• Make clear that the change occurs in the population, not within individuals.
• Use the term frequency, proportion or percentage rather than number of alleles when describing the change.

Common mistakes and how to put them right

• Writing that the number of alleles increases rather than the frequency or proportion.

  Put it right: Mark schemes require frequency/proportion and may reject 'number'.

• Saying individuals change or adapt during their lifetime.

  Put it right: Selection acts on existing variation, and populations change across generations.

• Expecting the change to be complete within one generation.

  Put it right: Allele frequency shifts gradually over many generations.

• Claiming the disadvantageous allele disappears at once, when heterozygotes can carry it unexpressed and keep it in the gene pool.

• Forgetting that in small populations allele frequency also changes by chance through genetic drift, not only by selection.

Exam tips

• Use the words frequency, proportion or percentage, together with the phrase 'over many generations'.
• Where a graph is given, quote the starting and finishing values with units and the time taken.
• Remember a recessive allele persists in heterozygotes, so its frequency falls slowly.