Students should be able to describe the evidence for evolution including…

The idea

Evolution is the change in inherited characteristics of a population over many generations, driven by natural selection. Fossils provide direct evidence: remains, impressions or preserved traces show organisms that lived long ago, and sequences of fossils in rock layers reveal gradual change over time. For example, fossil horse ancestors show increasing body size and changing teeth. Antibiotic resistance in bacteria provides evidence that can be observed in real time. When bacteria are exposed to an antibiotic, most die, but random mutations give a few individuals resistance. These survivors reproduce, passing on the resistance allele, so the resistant proportion of the population rises. This is natural selection in action, and it shows that populations, not individuals, evolve.

What earns the marks

• Define a fossil as the preserved remains or traces of an organism that lived long ago, found in rocks or other preserved material.
• Explain that fossils show changes in organisms over time, and that older rock layers generally contain older, simpler fossils.
• Describe antibiotic resistance as arising from random mutation, with resistant bacteria surviving and reproducing when the antibiotic is present.
• Explain that the resistant bacteria pass on the resistance allele, so the proportion of resistant bacteria in the population increases.
• Link both examples to natural selection: variation exists, selection pressures act, and advantageous alleles become more common over generations.

Common mistakes and how to put them right

• Saying bacteria choose to become resistant.

  Put it right by stating that resistance arises from random mutation before exposure to the antibiotic.

• Claiming an individual bacterium evolves.

  Put it right by explaining that natural selection changes the allele frequencies in a population over generations.

• Treating fossils as only bones.

  Put it right by including footprints, burrows, impressions and preserved traces as fossil evidence.

Exam tips

• Use the full chain mutation, variation, selection, reproduction, inheritance when explaining antibiotic resistance.
• Refer to rock layers or strata when describing how fossils show change over time.
• State clearly that evidence supports evolution rather than proving a particular mechanism, and keep examples specific.