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A2 · Practice questions · Natural selection, drift and resistance

Selection and chance.

Six original Cambridge-style questions on how natural selection and chance change allele frequencies in populations.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Variation, selection, inheritance.

01
Explain
[4 marks]

Explain how natural selection can lead to a change in the frequency of an allele in a population.

  • Populations produce more offspring than the environment can support, so there is competition for resources (a struggle for existence). ✓
  • There is variation; some individuals have an allele that gives a phenotype better adapted to the conditions (selective advantage). ✓
  • These individuals are more likely to survive and reproduce. ✓
  • They pass on the advantageous allele to their offspring, so its frequency increases over generations. ✓
02
Explain
[3 marks]

Explain how stabilising and directional selection differ in their effect on the distribution of a continuous feature.

  • Stabilising: intermediate phenotypes are favoured and both extremes selected against. ✓
  • The mean stays the same and the range / standard deviation decreases. ✓
  • Directional: one extreme is favoured, often after an environmental change, so the mean shifts in that direction over generations. ✓
03
Explain
[4 marks]

Outline how a population of bacteria can become resistant to an antibiotic.

  • A random mutation produces an allele that gives resistance, for example coding for an enzyme that breaks down the antibiotic. ✓
  • When the antibiotic is used, susceptible bacteria are killed; it acts as a selection pressure. ✓
  • Resistant bacteria survive and reproduce by binary fission, passing on the allele, so the proportion of resistant bacteria increases. ✓
  • Resistance genes on plasmids can be transferred to other bacteria by conjugation. ✓
04
Recall
[3 marks]

Distinguish between the founder effect and the bottleneck effect.

  • Founder effect: a few individuals leave and start a new, isolated population. ✓
  • Bottleneck effect: a large population is drastically reduced in size, for example by a disaster, disease or hunting. ✓
  • In both, the surviving alleles are a chance sample of the original gene pool, so allele frequencies change and genetic diversity is reduced. ✓
05
Explain
[3 marks]

Explain why genetic drift has a greater effect on allele frequencies in small populations than in large populations.

  • Which individuals reproduce, and which alleles are passed on, is partly due to chance. ✓
  • In a small population the alleles passed on are a small sample, so chance differences form a large proportion of the gene pool. ✓
  • Alleles can be lost or fixed within a few generations; in large populations random changes tend to cancel out. ✓
06
Data
[3 marks]

In a hospital, the percentage of infections caused by penicillin-resistant bacteria rose from 8% to 46% over 12 years, during which penicillin use increased. Suggest an explanation for this change.

  • Resistant bacteria were present at the start (8%) because of earlier random mutation. ✓
  • Increased penicillin use increased the selection pressure: susceptible bacteria were killed while resistant bacteria survived and reproduced. ✓
  • The resistance allele was passed on (and spread by plasmids), so resistant bacteria became a larger proportion, rising by 38 percentage points. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.