Edexcel GCSE Combined Science CB4 Natural selection and genetic modification: a complete overview of evolution, selective breeding and genetic engineering

What CB4 actually demands

Natural selection and genetic modification explains how species change naturally and how humans deliberately change living things. The examiners love the six-mark natural-selection explanation applied to a new context (often antibiotic resistance), and they reward a clear distinction between selective breeding and genetic engineering.

This guide walks through the two halves of the topic and ties together the matching dot-point pages, each with its own practice questions.

Evolution by natural selection

Evolution is the change in a population's inherited characteristics over time. Darwin's natural selection follows a fixed chain:

1. Variation in the population, much of it from mutations.
2. Survival of the best-adapted individuals competing for resources.
3. Reproduction, passing on the advantageous alleles.
4. Inheritance over generations, so the population changes.

Evidence comes from fossils (simpler forms in older rocks) and from antibiotic-resistant bacteria (evolution observed in real time). When populations become isolated and change so much they can no longer interbreed, a new species forms.

Selective breeding

Selective breeding chooses organisms with useful characteristics and breeds them over many generations: select, breed, select, repeat. It produces high-yield cattle, disease-resistant crops and many dog breeds, but reduces variation and can make harmful recessive alleles more common through inbreeding.

Genetic engineering

Genetic engineering transfers a gene into another organism's genome using a vector such as a plasmid. Examples include bacteria making human insulin and crops resistant to pests. GM offers higher yields and better nutrition, but raises concerns about biodiversity, gene spread to wild plants, and long-term effects.

How CB4 is examined

• Extended explanation. The six-mark natural-selection answer applied to bacteria, moths or other organisms.
• Evidence. Describing fossils and antibiotic resistance as support for evolution.
• Method. Describing selective breeding repeated over generations, and the steps of genetic engineering.
• Evaluation. Weighing the benefits and concerns of GM technology.

Check your knowledge

A mix of recall and explanation questions covering CB4. Attempt them under timed conditions, then check against the solutions.

1. State the source of new alleles for natural selection. (1 mark)
2. Give the four steps of natural selection in order. (2 marks)
3. Name two pieces of evidence for evolution. (2 marks)
4. Define selective breeding. (2 marks)
5. State one risk of selective breeding. (1 mark)
6. Name a useful product made by genetically modified bacteria. (1 mark)
7. State one benefit of GM crops. (1 mark)
8. State one concern about GM crops. (1 mark)