17.217-18 · Selection, Evolution, Classification and Conservation

Natural & Artificial Selection — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

10 min read 27 parts Lesson 6 of 36 5 quizzes

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision

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Lesson summary

17.2 Natural and Artificial Selection — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Explain how natural selection changes allele frequencies, and compare natural with artificial selection.Natural selection — the process by which individuals with a fitter phenotype…

Genetic drift
Founder effect
Bottleneck effect
Artificial selection (selective breeding)
Wheat and rice
Racehorses

Part 1 of 7Natural selection

Natural selection

Natural selection the process by which individuals with a fitter phenotype are more likely to survive and pass on their alleles,
so advantageous alleles increase in frequency over generations.

All organisms can produce more offspring than survive, so environmental factors limit population size.

Biotic factorspredation, competition for resources and disease.
Abiotic factorslight, water supply and soil pH.

Variation exists in the population, so individuals have different phenotypes.

Environmental factors act as selection pressures that raise the survival chance of some phenotypes.

Fitness

Fitness an organism's ability to survive and pass on its alleles to offspring.
Survival of the fitter is likely, not guaranteed.
Selection pressures can change over time, so a different phenotype may become fitter.

Part 2 of 7Types of selection

Type	What it does	Effect on the population
Stabilising	Favours intermediate phenotypes and acts against both extremes	Allele frequencies stay constant while the environment is stable; e.g. human birth mass
Directional	Favours one extreme phenotype	Gradual change in allele frequencies over generations when the environment changes; e.g. smaller fish body size in warmer seas
Disruptive	Favours both extremes and acts against intermediates	Maintains two sets of alleles (polymorphism); e.g. small and large beaks in Galapagos birds

Part 3 of 7Changes in allele frequency by chance

Genetic drift

Genetic drift random change in allele frequencies, caused by chance rather than selection.
It is much more likely in small populations, where an allele can be lost or fixed by chance.

Founder effect

Founder effect a few individuals start a new population and carry only part of the gene pool.
The small new population is highly susceptible to genetic drift.

Bottleneck effect

Bottleneck effect a large population suddenly falls in size, so alleles are lost and survivors inbreed.
Cheetahs show low genetic variation after a bottleneck at the end of the Ice Age.

Low genetic variation reduces the ability of a species to respond to environmental change.

Part 4 of 7Antibiotic resistance

A random mutation in one bacterium produces an allele for resistance; the antibiotic does not cause it.

The antibiotic acts as a selection pressure and kills non-resistant bacteria.

Resistant bacteria survive, reproduce and pass on the allele.

Bacteria have one copy of each gene,

so a new allele is expressed immediately, and rapid division makes the population resistant within a few generations.

Alleles on plasmids also spread between bacteria, even of different species (horizontal gene transfer).

Part 5 of 7Hardy-Weinberg principle

Hardy-Weinberg principle

Predicts allele and genotype frequencies in a population that is not evolving.

Conditionsno natural selection, no migration, random mating, large population, no mutation.

p = frequency of the dominant allele and q = frequency of the recessive allele.

p + q = 1 for alleles.
p² + 2pq + q² = 1 for genotypes.
p² is homozygous dominant, 2pq heterozygous, q² homozygous recessive.

Start from the part of the equation you are given; the recessive phenotype frequency (q²) is the one genotype that can be read directly.

Part 6 of 7Artificial selection

1Artificial selection (selective breeding)

Artificial selection (selective breeding) humans choose organisms with desirable traits and breed them together over many generations.
Individuals are selected by phenotype, so knowledge of the alleles is not needed.

Steps in selective breeding:

Choose a population showing phenotypic variation.
Select two individuals with the desired phenotype that are not closely related.
Breed them and test the mature offspring.
Breed from those showing the trait most strongly and repeat for many generations.

Linked traits may be enhanced accidentally, sometimes harming the health of the organism.

There are biological limits to how extreme a trait can become.

Part 7 of 7Examples of artificial selection

Cattle

Cattle breeding for increased milk yield, but selected cows have more mastitis, milk fever and lameness.

Wheat and rice

Wheat and rice introducing alleles for resistance to fungal and bacterial diseases to raise yield.

Maize

Maize inbreeding causes inbreeding depression, and outbreeding gives hybrid vigour.

Racehorses

Racehorses breeding the fastest sprinters together.

Unlike natural selection, humans apply the selection pressure for a desired outcome rather than for survival in the environment.

Key terms

Term	Meaning
Natural selection	Process by which fitter phenotypes survive and reproduce, so their alleles increase in frequency.
Fitness	Ability of an organism to survive and pass on its alleles to its offspring.
Selection pressure	An environmental factor that affects the chance of survival of an organism.
Genetic drift	Change in allele frequency due to chance, most important in small populations.
Founder effect	Loss of genetic variation when a few individuals found a new population.
Bottleneck effect	Loss of genetic variation after a sudden fall in population size.
Hardy-Weinberg principle	Equations predicting allele and genotype frequencies in a non-evolving population.
Artificial selection	Selective breeding of organisms with desirable traits by humans.

Avoid these mistakes

Saying organisms change their alleles to adapt — selection changes allele frequencies in the population over generations.

Saying the antibiotic causes the resistance mutation — the mutation arises randomly and the antibiotic only selects resistant bacteria.

Confusing genetic drift with natural selection — drift is chance, strongest in small populations, and not linked to advantage.

Assuming survival of the fittest is certain — fitter organisms only have a higher chance of surviving and reproducing.

Saying selective breeding considers the organism's survival — it selects traits useful to humans, which may harm health.

Last updated: 18 October 2026