Natural selection and selective breeding
How adaptation arises naturally, and how humans direct it.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 18.1.1Describe variation and mutation as the source of genetic variation.
- 18.1.2Explain natural selection and compare it with selective breeding.
6 minute read
Natural selection and selective breeding
Variation and its source
Variation is the differences between individuals of the same species. Continuous variation gives a range of values with no distinct categories (height, mass); discontinuous variation gives distinct categories with nothing in between (blood group, tongue rolling). The ultimate source of new genetic variation is mutation — a random change in the base sequence of DNA. Most mutations are neutral or harmful, but occasionally one is beneficial, and that is what natural selection can act on.
Natural selection
The same four-step sequence explains resistance in bacteria, peppered moths and Darwin's finches:
- There is variation within a population, caused by mutation.
- There is competition for limited resources, and more offspring are produced than can survive.
- Individuals with advantageous characteristics are more likely to survive and reproduce — "survival of the fittest".
- They pass the advantageous alleles to their offspring, so over many generations the allele becomes more common in the population.
Selective breeding (artificial selection)
Humans do deliberately what the environment does blindly:
- Choose the individuals with the desired characteristic.
- Breed them together.
- Select the best of the offspring.
- Repeat over many generations.
This produces high-yield crops, cows producing more milk, and domestic dog breeds.
The key difference
In natural selection the environment determines who reproduces; in selective breeding humans do. Selective breeding is much faster, but it reduces the gene pool, leaving populations with less variation and so more vulnerable to disease.
Think of it like this
Natural selection is a filter the environment holds; selective breeding is the same filter held by a farmer. The mechanism is identical — only the hand doing the choosing changes.
Worked examples
Method, step by step
Explain how a population of insects can become resistant to a pesticide.
- 1Random mutation means a few insects in the population are already resistant.
- 2When the pesticide is applied, non-resistant insects are killed while resistant ones survive.
- 3The survivors reproduce and pass the resistance allele to their offspring.
- 4Over many generations the proportion of resistant insects increases until the pesticide is ineffective.
Variation from mutation means some insects are resistant. The pesticide kills the rest, so resistant insects survive, reproduce, and pass on the allele — increasing its frequency over generations.
Common misconceptions
- Saying organisms "adapt" during their lifetime to pass on the change. Individuals do not adapt; populations change as the proportion of alleles shifts over generations.
- Thinking mutations occur *because* they are needed. Mutation is random and happens regardless of usefulness.
- Describing "the fittest" as the strongest. It means best suited to the environment — which may mean best camouflaged or most disease-resistant.
In the exam
- Use the four-step structure for every natural-selection question: variation → competition → survival and reproduction of the advantaged → allele frequency increases over generations.
- Include the phrase "over many generations" — questions frequently reserve a mark for showing the change is not immediate.