Ch 3 · Selection · O Level Biology 5090
Topic 3 of 3 in Variation and Selection — O Level Biology 5090 — 48 lessons.
Introduction
Why do organisms fit their environments so well? Darwin's answer, natural selection, is one of the most powerful ideas in biology and one of the most predictable exam questions: whatever the context, the marking points are the same six-step argument. Humans also direct selection themselves, producing high-yield wheat and dairy cows by selective breeding.
Key ideas — Natural selection
The Cambridge description has a fixed sequence. Learn it as a chain and adapt each step to the organism in the question.
🧑🏫 Teacher's explanation
Think of natural selection like a high-stakes survival game where the rules are set by the environment. Imagine a population of beetles living on green leaves. If some beetles are born bright green and others are bright red due to random genetic differences, the green ones have a 'cheat code'—they are harder for birds to see.
📖 Definitions
🔍 Worked example — Easy
A population of bacteria is exposed to an antibiotic. Describe the process of how a strain of antibiotic-resistant bacteria develops. Use the correct sequence of steps.
🔍 Worked example — Standard
A species of bird lives on an island where only large, hard seeds are available. Explain how the average beak size of the population might change over many generations.
🔍 Worked example — Exam-style
Calculate the percentage increase in the frequency of a beneficial allele if it increases from 0.15 to 0.45 in a population.
⚠️ Common errors
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🎯 Exam question
Key ideas — Adaptation and antibiotic resistance
Adaptation is a process. Cambridge defines adaptation as the process, resulting from natural selection, by which populations become more suited to their environment over many generations. Individuals do not adapt during their lifetime; populations change. A change in the adaptive features of a population over time is evolution.
🧑🏫 Teacher's explanation
Imagine you are playing a video game where you have to survive a level. If you find a 'power-up' (like a shield) that makes you harder to kill, you are more likely to reach the next level and play again. In biology, that 'power-up' is a genetic mutation.
📖 Definitions
🔍 Worked example — Easy
A population of bacteria initially contains 99\% non-resistant bacteria and 1\% resistant bacteria. After exposure to an antibiotic, the non-resistant bacteria die, and the resistant bacteria multiply. Calculate the percentage increase in the proportion of resistant bacteria in the population.
🔍 Worked example — Standard
Describe the process by which a population of bacteria becomes resistant to an antibiotic.
🔍 Worked example — Exam-style
Compare natural selection and selective breeding (artificial selection) in terms of the 'selection pressure' and 'variation'.
⚠️ Common errors
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🎯 Exam question
(a) Explain how the proportion of resistant bacteria increases in the population over time. [4] (b) State one difference between natural selection and selective breeding. [1] [5 marks]
Key ideas — Selective breeding
Selective breeding is selection carried out by humans, again as a repeating sequence:
🧑🏫 Teacher's explanation
Imagine you are a professional baker trying to create the ultimate sourdough bread. You don't just take any random yeast; you look for the specific yeast cultures that make the fluffiest, most aromatic dough. You take those 'best' cultures, mix them together, and see which new batch produces the best bread. You keep only the best from that batch and repeat the process for years until every single loaf you bake is perfect. This is selective breeding.
📖 Definitions
🔍 Worked example — Easy
A farmer wants to breed sheep with thicker wool. Describe the first three steps the farmer should take to achieve this using selective breeding.
🔍 Worked example — Standard
Compare natural selection and selective breeding in terms of the 'selection pressure'.
🔍 Worked example — Exam-style
A scientist is breeding a new strain of rice that is resistant to a specific fungus. If the scientist only selects the top 10\% of resistant plants to breed, explain why this process must be repeated over many generations to be successful.
⚠️ Common errors
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🎯 Exam question
(a) Describe how the farmer should use selective breeding to achieve this goal. [3] (b) State one disadvantage of using selective breeding in livestock. [1] [4 marks]
Key ideas — Artificial selection in agriculture
Improving crops and livestock, and comparing the two kinds of selection. In crop plants, breeders cross varieties carrying different useful features, such as high yield and disease resistance, grow the seed, keep only offspring showing both features, and cross them again. Artificial selection has produced almost all economically important plants and animals: food crops, dairy and beef cattle, sheep and poultry. Over many generations this gives varieties with higher yield, disease resistance, short stems or drought tolerance. In domesticated animals, breeders choose parents with the highest milk yield, fastest growth or best wool, breed them, keep the best offspring and repeat.
🧑🏫 Teacher's explanation
Imagine you are a chef trying to create the perfect chocolate chip cookie. You don't just bake one cookie and stop; you bake a batch, pick the ones with the best texture and most chocolate, and use those as your 'parents' for the next batch. Eventually, you have a recipe that is much better than the original. This is exactly what humans do with crops and animals.
📖 Definitions
🔍 Worked example — Easy
A farmer wants to increase the milk yield in his dairy herd. Describe the process he should follow to achieve this through selective breeding.
🔍 Worked example — Standard
Compare natural selection and artificial selection in terms of the 'selector' and the 'speed of change'.
🔍 Worked example — Exam-style
A breeder is working on a new strain of wheat. After 5 generations of selective breeding, the yield has increased from 2.5 tonnes/hectare to 4.0 tonnes/hectare. Calculate the percentage increase in yield.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) Describe the process the farmer should use to achieve this. [3] (b) State one disadvantage of this process for the sheep population. [1] [4 marks]
Worked example
Question. A farmer treated a bacterial infection in a herd of cattle with an antibiotic. After several years the antibiotic no longer cured the infection. Explain, using ideas about natural selection, how the population of bacteria became resistant. [5]
🔍 Worked example — Easy
A population of beetles has a range of colors due to genetic variation. A predator arrives that can easily see the green beetles but struggles to see the brown beetles. Describe the immediate effect on the population's phenotype distribution.
🔍 Worked example — Standard
A scientist observes that in a population of 500 bacteria, 10 individuals possess a mutation for antibiotic resistance. After exposure to the antibiotic, the population grows to 2,000 individuals, but only 1,800 are resistant. Calculate the percentage increase in the proportion of resistant bacteria.
🔍 Worked example — Exam-style
Explain how the use of antibiotics leads to the evolution of antibiotic-resistant bacteria. [5 marks]
⚠️ Common errors
Common mistakes
Practice
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