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
Official 2025-2027 spec · A2 LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- explain that natural selection occurs because populations have the capacity to produce many offspring that compete for resources; in the ‘struggle for existence’, individuals that are best adapted are most likely to survive to reproduce and pass on their alleles to the next generation
- explain how environmental factors can act as stabilising, disruptive and directional forces of natural selection
- explain how selection, the founder effect and genetic drift, including the bottleneck effect, may affect allele frequencies in populations
- outline how bacteria become resistant to antibiotics as an example of natural selection
- use the Hardy–Weinberg principle to calculate allele and genotype frequencies in populations and state the conditions when this principle can be applied (the two equations for the Hardy–Weinberg principle will be provided, as shown in the Mathematical requirements)
- describe the principles of selective breeding (artificial selection)
- outline the following examples of selective breeding: • the introduction of disease resistance to varieties of wheat and rice • inbreeding and hybridisation to produce vigorous, uniform varieties of maize • improving the milk yield of dairy cattle
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
- › Refer to them as 'populations of the same species' until reproductive isolation is established and they can no longer interbreed.
- › Identify speciation occurring within the same geographic area as 'sympatric speciation', often caused by behavioral or temporal isolation.
- › Clearly state that mutations are random; selection pressures only determine which existing alleles have a selective advantage in the environment.
- › Read the context carefully; for stem elongation, focus on how active GA leads to the breakdown of DELLA proteins, allowing PIF to bind to promoters.
- › Define low genetic variation as a small number of alleles existing at a gene locus within a population, regardless of their frequencies.
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.
Dairy cattle with high milk yields are selectively bred, and their offspring are evaluated for milk production. Artificial insemination using sperm from high-yielding bulls accelerates genetic improvement. This process focuses on heritability of milk production traits and can be aided by genetic screening.
More 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
Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.
How to study this Natural and artificial selection deck
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