Natural and artificial selection
Cambridge A-Level Biology (9700) · Unit 17: Selection and evolution · 10 flashcards
Natural and artificial selection is topic 17.2 in the Cambridge A-Level Biology (9700) syllabus , positioned in Unit 17 — Selection and evolution , alongside Evolution. In one line: The founder effect occurs when a small group of individuals establishes a new population, carrying only a subset of the original population's genetic diversity. This can lead to altered allele frequencies in the new population compared to the original, potentially reducing genetic variation.
Marked as A2 Level: examined at A Level in Paper 4 (A Level Structured Questions) and Paper 5 (Planning, Analysis and Evaluation). It is not tested on the AS-only papers (Papers 1, 2 and 3).
The deck below contains 10 flashcards — 2 definitions and 8 key concepts — covering the precise wording mark schemes reward. Use the 2 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
The founder effect and explain its impact on allele frequencies
The founder effect occurs when a small group of individuals establishes a new population, carrying only a subset of the original population's genetic diversity. This can lead to altered allele frequencies in the new population compared to the original, potentially reducing genetic variation.
What the Cambridge 9700 syllabus says
2025-2027 syllabus, in our words · A2 LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Explain that natural selection happens because populations can produce many offspring, which compete for resources. In this 'struggle for existence', the best-adapted individuals are the most likely to survive, reproduce and pass their alleles on to the next generation.
- Explain how environmental factors may act as directional, stabilising or disruptive forces of natural selection.
- Explain how allele frequencies in populations may be affected by genetic drift (including the bottleneck effect), the founder effect and selection.
- Outline how bacteria develop resistance to antibiotics, as one example of natural selection.
- Calculate genotype and allele frequencies in populations using the Hardy-Weinberg principle, and state the conditions under which the principle applies (both Hardy-Weinberg equations will be provided, as set out in the Mathematical requirements).
- Describe the principles of artificial selection (selective breeding).
- Outline these examples of selective breeding: introducing disease resistance into wheat and rice varieties; producing uniform, vigorous maize varieties by inbreeding and hybridisation; increasing how much milk dairy cattle yield.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9700 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Natural and artificial selection
- › If you know the frequency of the homozygous recessive phenotype (q²), square-root it to find q, then use p + q = 1.
- › Clearly state that mutations are random; selection pressures only determine which existing alleles have a selective advantage in the environment.
- › Describe low genetic diversity as few different alleles of a gene being present in a population.
Explain how a population's capacity to produce many offspring contributes to natural selection.
Overproduction leads to competition for limited resources. This 'struggle for existence' results in individuals with advantageous adaptations being more likely to survive, reproduce, and pass on their alleles, driving natural selection.
Describe how directional selection affects allele frequencies in a population.
Directional selection favors one extreme phenotype, causing a shift in the population's allele frequencies over time. Alleles contributing to the favored phenotype become more common, while those linked to the opposite extreme become less frequent.
Define the founder effect and explain its impact on allele frequencies.
The founder effect occurs when a small group of individuals establishes a new population, carrying only a subset of the original population's genetic diversity. This can lead to altered allele frequencies in the new population compared to the original, potentially reducing genetic variation.
Outline how genetic drift, specifically the bottleneck effect, can alter allele frequencies in a population.
The bottleneck effect occurs when a population undergoes a drastic reduction in size, often due to a catastrophic event. The surviving individuals may not represent the original population's genetic diversity, leading to random changes in allele frequencies and reduced genetic variation.
Describe the mechanism by which bacteria become resistant to antibiotics.
Random mutations can arise in bacteria, some conferring resistance to antibiotics. In the presence of antibiotics, susceptible bacteria die, while resistant bacteria survive and reproduce. This leads to a rapid increase in the frequency of antibiotic resistance alleles within the bacterial population due to natural selection.
State the conditions under which the Hardy-Weinberg principle can be applied.
The Hardy-Weinberg principle applies when: 1) There is no mutation, 2) There is random mating, 3) There is no gene flow, 4) There is no natural selection, and 5) The population size is large (no genetic drift).
Describe the principles of selective breeding (artificial selection).
Selective breeding involves humans selecting individuals with desirable traits to breed, aiming to enhance these traits in subsequent generations. This process can lead to significant changes in a population's characteristics over time, but reduces genetic diversity and can lead to inbreeding.
Give an example of selective breeding for disease resistance in plants.
Varieties of wheat and rice have been selectively bred to introduce genes conferring resistance to specific fungal or viral diseases. This process involves crossing plants with resistance genes with high-yielding varieties, selecting for offspring that combine both traits.
Explain how inbreeding and hybridization are used in maize breeding.
Inbreeding is used to create homozygous lines in maize, leading to uniform traits but reduced vigor. Hybridization then crosses these inbred lines to create F1 hybrids, which exhibit hybrid vigor (increased yield and robustness) due to the masking of deleterious recessive alleles.
Outline the process of selective breeding to improve milk yield in dairy cattle.
Cows with high milk yields are chosen for breeding. Bulls cannot be judged directly, so they are chosen by progeny testing: the milk yield of their daughters (and their mothers) is measured. Semen from the best bulls is used to artificially inseminate many high-yielding cows, and the best offspring are selected again over many generations.
Review the material
Read full revision notes on Natural and artificial selection — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 17 — Selection and evolution
Natural and artificial selection sits alongside these A-Level Biology decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Natural and artificial selection deck
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