Variation, evolution and selection — AQA GCSE Biology revision

Natural selection

• Variation exists in a population due to genes and environment.
• Individuals with useful alleles survive, reproduce and pass those alleles on.
• Over many generations the population changes — that's evolution.

Selective breeding and GM

• Selective breeding: humans choose parents with desired traits; risk of inbreeding and disease.
• Genetic engineering: a useful gene is cut out and inserted into another organism, e.g. insulin-producing bacteria.

Key terms

Mutation

A random change in DNA; most have no effect.

Speciation

When populations become so different they can no longer interbreed.

Selective breeding

Choosing parent organisms with desired characteristics.

Variation

Differences between individuals of the same species, caused by genes and/or environment.

Selection pressure

An environmental factor that affects survival chances, e.g. predators, disease, competition.

Homologous structure

Similar body structures in different species suggesting a shared ancestor.

Common exam traps

• Saying organisms 'choose' or 'want' to adapt — selection is not deliberate.
• Forgetting the phrase 'over many generations'.
• Saying organisms 'try' or 'choose' to evolve — selection acts on existing random variation, it isn't goal-directed.

Practice questions with answers

1. 1. Explain how antibiotic-resistant bacteria arise.

   Answer: A random mutation makes some bacteria resistant; the antibiotic kills the rest, the resistant ones survive, reproduce and pass on the allele.

   Mutation → survival → reproduction → spread.

2. 2. Which is a disadvantage of selective breeding?

   • • Reduced genetic variation and more inherited disease
   • • It is illegal
   • • It slows plant growth
   • • It causes mutations in humans

   Answer: Reduced genetic variation and more inherited disease

   Inbreeding reduces the gene pool.

3. 3. Give one benefit of genetic engineering in medicine.

   Answer: Bacteria can be modified to produce human insulin in large quantities.

   Use a specific example.

4. 4. Charles Darwin's theory of evolution is based mainly on:

   • • Natural selection
   • • Genetic engineering
   • • Selective breeding
   • • Cloning

   Answer: Natural selection

   Darwin proposed that organisms best suited to their environment survive and reproduce.

5. 5. Explain why fossils provide evidence for evolution.

   Answer: Fossils show how organisms have changed gradually over long periods of time, showing a sequence of gradual change linking earlier and later species.

   The fossil record shows change over geological time.

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