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What causes variation, and how does evolution by natural selection happen?

The causes of variation (genetic and environmental), the theory of evolution by natural selection, the role of mutation, evidence for evolution including fossils and antibiotic resistance, and how new species form.

A focused answer to AQA GCSE Biology 4.6.2 to 4.6.3, covering the causes of variation, the theory of evolution by natural selection, the role of mutation, the evidence for evolution, and the formation of new species.

Generated by Claude Opus 4.811 min answer

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  1. What this dot point is asking
  2. Causes of variation
  3. Evolution by natural selection
  4. Evidence for evolution
  5. Speciation
  6. Try this

What this dot point is asking

AQA wants you to describe the causes of variation, explain Darwin's theory of evolution by natural selection, describe the role of mutation, give the evidence for evolution (fossils and antibiotic-resistant bacteria), and explain how new species form by speciation.

Causes of variation

Most variation is caused by a number of different genes working together; only rarely is a single gene responsible for a characteristic. Mutations (random changes to DNA) occur continually and are the source of new alleles, and so of new variation. Most mutations have little or no effect on the phenotype, some have a small effect, and very rarely a mutation has a large effect, which may be harmful or, occasionally, beneficial.

Evolution by natural selection

If a mutation gives an individual a useful new characteristic (for example resistance to a poison), natural selection makes the allele for it spread through the population over time. It is important to be clear that organisms do not choose to change; selection acts on variation that already exists by chance.

Evidence for evolution

  • Fossils: the remains or traces of organisms from millions of years ago, found in rocks. Arranged in age order, they show how organisms have changed over time. The early fossil record is incomplete because the earliest organisms were soft-bodied and rarely fossilised, and many fossils have been destroyed by geological activity. This is one reason scientists cannot be certain about exactly how life began.
  • Antibiotic-resistant bacteria: bacteria reproduce very quickly. A chance mutation can make a bacterium resistant to an antibiotic. When the antibiotic is used, the non-resistant bacteria are killed but the resistant ones survive and reproduce, so the resistant strain spreads. This is evolution by natural selection happening fast enough to observe directly (for example MRSA).

Speciation

Try this

Q1. State two causes of variation in a population. [2 marks]

  • Cue. Genetic (inherited alleles) and environmental factors.

Q2. Explain how a population of bacteria becomes resistant to an antibiotic. [3 marks]

  • Cue. A random mutation makes one bacterium resistant; it survives the antibiotic and reproduces; the resistant strain spreads.

Exam-style practice questions

Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

AQA 20186 marksExplain how a population of dark-coloured moths could come to replace a population of light-coloured moths in an area where tree bark has become darkened by pollution, using the theory of evolution by natural selection.
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A 6-mark explain question rewards a complete natural-selection sequence applied to the example.

There is genetic variation in the moth population, caused by mutation, so both light and dark moths exist. On dark, polluted bark the dark moths are better camouflaged, so they are less likely to be seen and eaten by predators (birds), while light moths are easily seen and eaten. The dark moths are therefore more likely to survive and reproduce, passing on the alleles for dark colour to their offspring. Over many generations the allele for dark colour becomes more common and the allele for light colour rarer, so the population becomes mostly dark.

Markers reward variation from mutation, the dark moths having a survival advantage (camouflage from predators), survivors reproducing and passing on the allele, and the allele frequency changing over generations.

AQA 20214 marksDescribe two pieces of evidence that support the theory of evolution, and explain how each provides support.
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A 4-mark question rewards two distinct pieces of evidence each with an explanation.

Fossils are the remains or traces of organisms preserved in rock over millions of years. By arranging fossils in age order, scientists can see how organisms have gradually changed over time, which supports evolution; the early record is incomplete because soft-bodied organisms rarely fossilise.

Antibiotic-resistant bacteria provide evidence we can observe now: bacteria reproduce quickly, a chance mutation makes some resistant, the antibiotic kills the non-resistant ones, and the resistant bacteria survive and reproduce so the resistant strain spreads. This shows natural selection happening over a short time.

Markers reward fossils showing gradual change over time and antibiotic resistance as observed natural selection, each correctly explained.

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