Ch 2 · 17.2 Natural and artificial selection · AS & A Level Biology 9700
Topic 2 of 3 in Selection and evolution — AS & A Level Biology 9700 — 69 lessons.
Introduction
Natural selection changes the frequency of alleles in a population over time; chance events can change them too, especially in small populations. Artificial selection lets humans steer the same process towards characteristics we want. (Syllabus section 17.2.)
Key ideas — How natural selection works
The environmental factor that makes some phenotypes more likely to survive is the selection pressure (e.g. a predator, a disease, a change in climate).
🧑🏫 Teacher's explanation
Imagine a crowded buffet where there are only 10 slices of pizza but 50 hungry students. Not everyone can eat; they must compete for the limited food. In nature, this 'struggle for existence' is constant because populations produce far more offspring than the environment can support.
📖 Definitions
🔍 Worked example — Easy
In a population of 100 wildflowers, 16 have the genotype AA (red), 48 have Aa (pink), and 36 have aa (white). Calculate the frequency of the recessive allele a.
🔍 Worked example — Standard
A population of bacteria is treated with an antibiotic. Initially, 98% of the bacteria are susceptible (sensitive) and 2% are resistant. If the population size is 1 × 10^9 and the resistance allele is passed to 50% of the offspring, calculate the new frequency of the resistance allele in the next generation.
🔍 Worked example — Exam-style
In a population in Hardy-Weinberg equilibrium, the frequency of the dominant allele A is 0.7. Calculate the frequency of the heterozygous genotype Aa.
⚠️ Common errors
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🎯 Exam question
(a) Explain how the frequency of the brown allele will change over several generations. [3] (b) State one condition required for a population to be in Hardy-Weinberg equilibrium. [1] [4 marks]
Key ideas — Stabilising, disruptive and directional selection
Think of a graph of the number of individuals against a continuous characteristic:
🧑🏫 Teacher's explanation
Imagine a population of birds where beak size determines how well they eat. If the environment is steady and only medium seeds are available, birds with medium beaks thrive while the very small and very large ones struggle; this is like a crowd of people all wearing average-sized shirts because that's what's in stock. This is stabilising selection.
📖 Definitions
🔍 Worked example — Easy
A population of beetles has a continuous distribution of body length. If the environment changes such that only very small and very large beetles can hide from predators, describe the effect on the population's phenotype distribution.
🔍 Worked example — Standard
In a population of bacteria, 99\% of individuals are susceptible to an antibiotic, while 1\% have a mutation providing resistance. If the antibiotic is applied, explain the change in allele frequency over generations.
🔍 Worked example — Exam-style
A researcher observes a population of moths. Initially, the mean wing color is light grey. Due to industrial pollution, trees become dark. Predict the shift in the mean phenotype and name the type of selection.
⚠️ Common errors
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🎯 Exam question
(a) Name the type of natural selection occurring. [1] (b) Describe the effect this will have on the distribution of flower height in the population over several generations. [2] [3 marks]
Key ideas — Founder effect and genetic drift
Allele frequencies can also change by chance, without selection:
🧑🏫 Teacher's explanation
Imagine you have a huge jar of multicolored jellybeans. If you reach in and grab a handful to start a new jar, you might accidentally grab only red and blue ones, even if the original jar was mostly green. That's the founder effect.
📖 Definitions
🔍 Worked example — Easy
A small group of 5 lizards colonizes a new island from a mainland population. On the mainland, the frequency of allele A is 0.5. On the island, the frequency of allele A is 0.8. Explain why this difference occurred.
🔍 Worked example — Standard
A population of 100 beetles has 20 individuals with the genotype aa. If a sudden flood kills 95% of the population randomly, leaving only 5 beetles, explain the impact on the gene pool.
🔍 Worked example — Exam-style
In a small population of 10 insects, an allele b has a frequency of 0.1. If, by pure chance, none of the individuals carrying allele b reproduce, what is the new frequency of allele b in the next generation?
⚠️ Common errors
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🎯 Exam question
(a) Name the phenomenon that describes this change in allele frequency. [1] (b) Explain why the genetic diversity of the surviving population is likely to be lower than the original population. [2] [3 marks]
Key ideas — Antibiotic resistance: natural selection in action
🧑🏫 Teacher's explanation
Imagine a crowded room where everyone is trying to grab the last few slices of pizza. Not everyone gets a slice, and the ones who are faster or better at reaching them are the ones who stay full and energetic enough to go home and have children. In biology, this 'struggle for existence' is driven by limited resources like food, space, or mates.
📖 Definitions
🔍 Worked example — Easy
A population of bacteria contains a single bacterium with a mutation for antibiotic resistance. After exposure to penicillin, the population size drops from 10^6 to 10^2, but all 10^2 survivors are resistant. Describe the process that leads to the entire population becoming resistant.
🔍 Worked example — Standard
In a population of bacteria, the frequency of a resistance allele (R) is 0.02. Calculate the frequency of the susceptible allele (r).
🔍 Worked example — Exam-style
In a population of bacteria, the frequency of the resistant allele (R) is 0.3. Calculate the frequency of the homozygous susceptible genotype (rr) assuming the population is in Hardy-Weinberg equilibrium.
⚠️ Common errors
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🎯 Exam question
Key ideas — The Hardy–Weinberg principle
The Hardy–Weinberg principle lets you calculate allele and genotype frequencies in a population. If p is the frequency of the dominant allele and q the frequency of the recessive allele:
Key ideas — Selective breeding
In selective breeding (artificial selection), humans choose which individuals breed:
🧑🏫 Teacher's explanation
Imagine you are a professional dog breeder. If you only want Poodles with curly hair, you don't care if they are good at hunting or surviving in the wild; you only care about that one specific trait. You pick the curliest Poodles, mate them, and repeat this for years. Eventually, almost every Poodle in your line will have curly hair. This is selective breeding.
📖 Definitions
🔍 Worked example — Easy
A farmer wants to increase milk yield in dairy cattle. Outline the steps the farmer should take to achieve this through selective breeding.
🔍 Worked example — Standard
In a population of maize, the frequency of the allele for high yield (A) is 0.7. Calculate the frequency of the recessive allele (a) and the frequency of the homozygous recessive genotype (aa).
🔍 Worked example — Exam-style
A population of wheat is being bred for disease resistance. If the frequency of the resistant allele is 0.8, what is the frequency of the heterozygous genotype (Aa)?
⚠️ Common errors
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🎯 Exam question
(a) Describe the process the farmer should use to increase the milk yield in the herd. [3] (b) Explain why intense selective breeding might lead to a decrease in the overall genetic variation of the cattle population. [2] [5 marks]
Key ideas — Examples of selective breeding
🧑🏫 Teacher's explanation
Imagine you are a professional chef trying to create the perfect sourdough bread. You don't just pick any flour; you pick the best flour, bake it, and then take the best loaves from that batch to make the next batch. This is the essence of selective breeding: humans acting as the 'environment' to choose specific traits.
📖 Definitions
🔍 Worked example — Easy
A farmer wants to increase milk yield in a dairy herd. Describe the process he should use to achieve this over several generations.
🔍 Worked example — Standard
In maize production, explain why farmers must buy new hybrid seeds every single year rather than saving seeds from their harvest.
🔍 Worked example — Exam-style
A scientist is breeding wheat to be resistant to stem rust. Describe the method used to introduce this resistance into a high-yielding variety.
⚠️ Common errors
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🎯 Exam question
(a) Describe how the farmer can select the best bulls for breeding. [2] (b) Explain why the farmer might use artificial insemination rather than natural mating. [1] [3 marks]
Worked example
Question. Cystic fibrosis is caused by a recessive allele. In a population, the disease occurs in 1 in 2500 people. (a) Calculate the frequency of the recessive allele in this population. [2] (b) Calculate the frequency of people who are heterozygous carriers of the allele. [3]
🔍 Worked example — Easy
In a population of wildflowers, the frequency of the recessive allele for white petals is q^2 = 0.09. Calculate the frequency of the dominant allele (p).
🔍 Worked example — Standard
In a population of 500 insects, 20 individuals show a recessive phenotype. Calculate the frequency of the heterozygous genotype.
🔍 Worked example — Exam-style
Cystic fibrosis is caused by a recessive allele. In a population, the disease occurs in 1 in 2500 people. (a) Calculate the frequency of the recessive allele in this population. (b) Calculate the frequency of people who are heterozygous carriers of the allele.
⚠️ Common errors
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Practice
Answers