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A2 · Practice questions · Evolution and speciation

Isolate, diverge, separate.

Six original Cambridge-style questions on evolution, molecular evidence for relationships and how new species form.

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

No gene flow, then new species.

01
Recall
[2 marks]

Outline the theory of evolution.

  • New species are formed from pre-existing species over (long periods of) time. ✓
  • This happens through changes in the gene pool / allele frequencies of populations from generation to generation (by mutation, natural selection and genetic drift). ✓
02
Data
[4 marks]

A section of a gene was sequenced in four species of bird. The numbers of base differences were: A-B 3, A-C 14, A-D 15, B-C 13, B-D 14, C-D 4. Use the data to suggest the evolutionary relationships between the four species and explain your reasoning.

  • A and B differ by only 3 bases, so they share a recent common ancestor. ✓
  • C and D differ by only 4 bases, so they also share a recent common ancestor. ✓
  • The A/B pair differs from the C/D pair by 13 to 15 bases, so the two pairs diverged from a common ancestor much longer ago. ✓
  • Mutations accumulate at a roughly constant rate, so the number of differences reflects the time since divergence. ✓
03
Explain
[4 marks]

Explain how allopatric speciation can occur.

  • A population is split by a geographical barrier such as a river, mountain range or sea. ✓
  • There is no gene flow between the two populations. ✓
  • Different mutations occur, and different selection pressures (and genetic drift) act on each, so allele frequencies / gene pools change differently. ✓
  • Eventually the populations cannot interbreed to produce fertile offspring even if they meet: they are reproductively isolated, separate species. ✓
04
Explain
[3 marks]

Explain how sympatric speciation can occur without a geographical barrier.

  • Individuals in the same area become separated ecologically (for example using different host plants, habitats or food) or behaviourally (different courtship or breeding times). ✓
  • So they mate mainly within their own group, reducing gene flow between groups. ✓
  • Different selection pressures and mutations change each gene pool until the groups can no longer interbreed to produce fertile offspring. ✓
05
Explain
[2 marks]

Suggest why comparing DNA sequences can give more reliable evidence of evolutionary relationships than comparing the appearance of organisms.

  • Similar appearance can evolve separately in unrelated organisms living in similar environments (convergent evolution), whereas DNA differences reflect shared ancestry. ✓
  • DNA data are quantitative and objective: the number of base differences can be counted and related to time since divergence. ✓
06
Analysis
[3 marks]

Two populations of fish in a lake look different and rarely mate in the wild, but in an aquarium they mate and produce fertile offspring. Discuss whether they should be classified as separate species.

  • They produce fertile offspring when mated, which suggests they are still one species (not reproductively isolated). ✓
  • However, in the wild there is little gene flow between them because of behavioural or ecological separation, so they may be in the process of sympatric speciation. ✓
  • Further divergence of their gene pools could lead to full reproductive isolation; DNA comparison could show how different the gene pools are. ✓

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.