(e) the evidence for the theory of evolution by natural selection

The idea

Evolution by natural selection explains how heritable variation plus differential survival and reproduction changes allele frequencies in populations over generations. The evidence is varied and each type supports a different part of the theory. Fossil records show change over time and transitional forms, though the record is incomplete. Comparative anatomy reveals homologous structures with shared ancestry and analogous structures from convergent evolution. Comparative biochemistry and DNA sequencing show closer relatedness between species with more similar sequences. Observation of contemporary populations, such as antibiotic resistance in bacteria or industrial melanism in moths, shows selection acting in real time. Selective breeding demonstrates that heritable traits can be changed by artificial selection.

What earns the marks

• Fossils provide evidence of change over geological time and of transitional forms linking ancestral and descendant groups, although the fossil record is incomplete.
• Homologous structures indicate shared ancestry because they share a common basic structure despite different functions; analogous structures arise from convergent evolution and do not indicate close relatedness.
• Molecular evidence such as DNA base sequences and protein amino acid sequences shows that species with more similar sequences are more closely related.
• Observations of present-day populations, such as antibiotic resistance in bacteria or melanism in moths, show natural selection changing allele frequencies within populations.
• Selective breeding shows that heritable variation exists and that selection by humans can change a population's characteristics over generations.
• Evolution is a change in allele frequency in a population over generations, not an individual organism changing during its lifetime.

Common mistakes and how to put them right

• Confusing homologous and analogous structures.

  Put it right: Homology reflects common ancestry while analogy reflects similar selection pressures acting on unrelated lineages.

• Treating an individual's acclimatisation or acquired characteristic as evolution.

  Put it right: Evolution is a change in allele frequencies in a population across generations.

• Assuming the fossil record is complete and ordered perfectly.

  Put it right: Gaps and missing intermediates limit but do not eliminate its evidential value.

• Claiming molecular similarity proves no relationship or that any similarity proves close relatedness.

  Put it right: Degree of sequence similarity is compared quantitatively to infer relatedness.

Exam tips

• Read the stem to identify which claim the evidence must support, then eliminate options that describe the wrong type of evidence.
• Check whether an option describes a population-level change over generations rather than a single organism's response.
• Distinguish homology from analogy by asking whether the structures share a common basic plan or merely a similar function.