A-Level Biology October/November 2024 Q2(a): Outline the processes that may affect allele frequencies in wildlife populations.

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  Allele frequencies in wildlife populations can be affected by several key processes:

  • Mutation: This is the ultimate source of all new alleles. A random change in the base sequence of DNA can create a new allele, thereby altering the allele frequency within the population's gene pool.

  • Natural Selection: Environmental factors and selection pressures lead to differential survival and reproduction. Individuals with advantageous alleles are more likely to survive, reproduce, and pass on these alleles to their offspring. This causes the frequency of advantageous alleles to increase over generations. This can be, for example, directional or disruptive selection.

  • Genetic Drift: This involves random, chance fluctuations in allele frequencies between generations. Its effects are most pronounced in small populations, where chance events can lead to a particular allele becoming more or less common, or even being lost entirely, independent of any selective advantage.

  • Gene Flow (Migration): The movement of individuals into (immigration) or out of (emigration) a population results in the transfer of alleles. This can introduce new alleles to a gene pool or change the frequencies of existing alleles, reducing genetic differences between populations.

  • Population Bottleneck: This is a form of genetic drift where a population's size is drastically reduced by a catastrophic event (e.g., disease, natural disaster). The surviving individuals may have a gene pool with allele frequencies that are not representative of the original population, simply by chance.

  • Founder Effect: This occurs when a small number of individuals from a larger population colonise a new area, founding a new population. The allele frequencies of this new population may be different from the source population because the founders are unlikely to be a representative genetic sample.

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  How the marks are awarded

  • B1 — Awarded for identifying mutation as a process that creates new alleles and affects their frequency. (Mark scheme point 1)
  • B1 — Awarded for outlining natural selection (including types like directional/disruptive) where advantageous alleles increase in frequency. (Mark scheme point 5)
  • B1 — Awarded for identifying genetic drift as a random change in allele frequencies, especially in small populations. (Mark scheme points 2 & 8)
  • B1 — Awarded for describing gene flow or migration as the movement of alleles between populations. (Mark scheme point 6)
  • B1 — Awarded for describing the bottleneck effect, where a large fall in population size alters allele frequencies. (Mark scheme point 4)
  • B1 — Awarded for describing the founder effect, where a new population is established by a small group of individuals. (Mark scheme point 3)

  Common mistakes

  • Confusing genetic drift with natural selection; drift is due to random chance, whereas selection is non-random and related to adaptive advantage.
  • Mixing up the bottleneck effect and the founder effect. A bottleneck is a drastic reduction of an existing population, while the founder effect involves a small group establishing a new, separate population.
  • Forgetting to mention mutation as the ultimate source of new alleles, focusing only on processes that change the frequencies of existing alleles.
  • Providing vague descriptions, such as 'the environment changes things', without linking the selection pressure directly to differential survival/reproduction and the resulting change in allele frequency.

  Examiner tip: For 'outline' questions that ask for multiple 'processes' or 'factors', use a separate bullet point for each distinct concept to structure your answer clearly and maximise the number of marks you can earn.

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