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3.7.3 Evolution and speciation

AQA A-level Biology 3.7.3 practice on Evolution and speciation, with free MCQs, clues and worked explanations drawn from the Genetics, populations, evolution and ecosystems section of specification 7402.

Specification route
3.7.3
Question bank
28 questions
Course stage
Year 13 / A-level only

Sample questions

QUESTION 1 · 3.7.3 · LEVEL 2

What is the primary source of all new genetic variation within a species?

  • Random mutation
  • Crossing over during meiosis
  • Independent segregation
  • Random fertilisation of gametes
Show clue

Meiosis only shuffles existing genes. What process actually creates a brand new allele?

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Answer: Random mutation

The primary source of genetic variation is mutation. While meiosis and fertilisation produce further variation by combining alleles, mutations create entirely new alleles.

QUESTION 2 · 3.7.3 · LEVEL 2

How does natural selection drive evolution in a population?

  • Organisms with phenotypes providing selective advantages are more likely to survive, reproduce, and pass on their favourable alleles, increasing those allele frequencies over generations.
  • Organisms actively mutate their own DNA in order to adapt to new environmental challenges.
  • The environment physically forces the weaker organisms to emigrate to a new habitat.
  • Organisms learn new behaviours to survive and pass these acquired skills down through their DNA.
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Evolution relies on differential reproductive success based on existing genetic traits.

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Answer: Organisms with phenotypes providing selective advantages are more likely to survive, reproduce, and pass on their favourable alleles, increasing those allele frequencies over generations.

Those organisms with phenotypes providing selective advantages are likely to produce more offspring and pass on their favourable alleles to the next generation. This causes the advantageous allele to increase in frequency.

QUESTION 3 · 3.7.3 · LEVEL 3

Which type of selection favours extreme phenotypes at both ends of a range, while selecting against the mean phenotype?

  • Disruptive selection
  • Stabilising selection
  • Directional selection
  • Artificial selection
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This type of selection 'tears' the population in two and can eventually lead to sympatric speciation.

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Answer: Disruptive selection

The effects of stabilising, directional and disruptive selection. Disruptive selection favours individuals at both extremes of a phenotypic range, potentially splitting the population.

QUESTION 4 · 3.7.3 · LEVEL 1

How is evolution most accurately defined in terms of genetics?

  • A change in the allele frequencies in a population over time.
  • The sudden appearance of an entirely new species in a single generation.
  • The gradual elimination of all recessive alleles from a gene pool.
  • The increase in physical size and complexity of organisms in an ecosystem.
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It involves the shifting proportions of genetic variants within the group.

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Answer: A change in the allele frequencies in a population over time.

Evolution as a change in the allele frequencies in a population.

QUESTION 5 · 3.7.3 · LEVEL 2

What is the key initiating event in allopatric speciation?

  • The geographical separation of two populations, preventing gene flow between them.
  • A mutation that prevents the formation of fertile gametes within a single population.
  • A change in courtship behaviour that splits a population in the same habitat.
  • The sudden introduction of a novel predator into a single environment.
Show clue

The prefix 'allo-' means different, and '-patric' means country or homeland.

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Answer: The geographical separation of two populations, preventing gene flow between them.

Allopatric speciation occurs when reproductive separation of two populations results from a geographical barrier, leading to the accumulation of differences in their gene pools.

QUESTION 6 · 3.7.3 · LEVEL 3

Which of the following scenarios is an example of sympatric speciation?

  • A mutation alters the flowering time of some plants in a field, reproductively isolating them from the rest of the population sharing the same habitat.
  • A river changes its course, dividing a forest and separating a squirrel population into two groups that evolve differently.
  • A continental drift slowly separates a single species of flightless bird onto two different continents.
  • A small group of finches is blown to a remote island, where they evolve differently from the mainland population.
Show clue

Sympatric means 'same homeland'. The barrier to reproduction must be biological or behavioural, not geographical.

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Answer: A mutation alters the flowering time of some plants in a field, reproductively isolating them from the rest of the population sharing the same habitat.

Allopatric and sympatric speciation. Sympatric speciation occurs without geographical isolation; populations in the same area become reproductively isolated due to behavioural, temporal, or genetic (e.g., polyploidy) changes.

QUESTION 7 · 3.7.3 · LEVEL 3

Why is genetic drift a more significant evolutionary force in small populations?

  • In small populations, chance events can cause massive, rapid fluctuations in allele frequencies, heavily impacting the smaller gene pool.
  • Small populations have a much higher rate of spontaneous mutation than large populations.
  • Natural selection cannot operate at all in small populations.
  • Small populations always consist of identical clones, making them susceptible to drift.
Show clue

Flipping a coin 4 times vs 4000 times: which is more likely to give you a highly skewed ratio of heads to tails purely by chance?

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Answer: In small populations, chance events can cause massive, rapid fluctuations in allele frequencies, heavily impacting the smaller gene pool.

The importance of genetic drift in causing changes in allele frequency in small populations. Chance dictates which alleles are passed on; in a small gene pool, the random loss of a few individuals can drastically alter allele frequencies.

QUESTION 8 · 3.7.3 · LEVEL 2

Why do many spontaneous mutations have no effect on the phenotype of an organism?

  • The genetic code is degenerate, meaning the mutated triplet may still code for the exact same amino acid.
  • Mutations always occur in introns, which are removed before translation.
  • DNA polymerase immediately repairs all errors after transcription.
  • Mutations only ever change recessive alleles, which are masked by dominant alleles.
Show clue

Think about silent mutations and how multiple codes can mean the same thing.

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Answer: The genetic code is degenerate, meaning the mutated triplet may still code for the exact same amino acid.

Many mutations are harmless. Due to the degenerate nature of the genetic code, a base substitution might not alter the amino acid sequence encoded, leaving the protein and phenotype unchanged.

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