Study Companion
Topic 02 | Biological sciences

Evolution by natural selection

Year 10 (Levels 9-10 band): variation, selection pressure and adaptation; evidence from fossils, biogeography, comparative anatomy and molecular data; speciation and relatedness.

55-75 min Printable practice Answer key Challenge included
VC2S10U05
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What you will learn

Worked example 0 Real-world example: antibiotic resistance in a hospital

A hospital ward uses a new antibiotic. Before treatment, roughly 1 in 10910^9 bacteria in a patient carry a resistance allele by chance mutation. Explain how a resistant strain can dominate within weeks.

  1. Variation: random mutations produce rare resistant bacteria.
  2. Selection pressure: the antibiotic kills non-resistant bacteria.
  3. Survival and reproduction: resistant bacteria survive and divide every ∼20\sim 20 minutes.
  4. Inheritance: resistance is encoded in DNA, so daughter cells inherit it.
  5. After 2424 hours, a resistant lineage that started from one cell can exceed 102010^{20} cells.

Key idea: natural selection does not create the mutation — it merely amplifies any variant that already survives and reproduces better in the current environment.

1. The four conditions for natural selection

Variation(mutations)Selection(pressure)Survive &reproduceInherit(heritable)
The four ingredients of natural selection.
  1. Variation within a population (individuals differ).
  2. Selection pressure from the environment (food, predators, climate, pathogens).
  3. Differential survival and reproduction (fitter variants leave more offspring).
  4. Heritability (the advantageous trait is coded in DNA and passed on).

Over many generations, advantageous traits accumulate. This is adaptation.

Worked example 1 Peppered moth, England, 1800s

Before the Industrial Revolution, most peppered moths in England were light-coloured, camouflaged on pale lichen-covered trees. As factories blackened the trees with soot, the frequency of dark moths rose from below 1%1\% to over 95%95\% within about 5050 years. Explain using the four conditions.

  1. Variation: both light and dark forms existed (genetic polymorphism).
  2. Selection pressure: birds preyed more on the now-conspicuous light moths against sooty bark.
  3. Reproduction: dark moths survived to lay more eggs.
  4. Heritability: colour is genetic, so offspring were also dark.

When clean-air laws reversed the pollution, the light form recovered — a second wave of selection in the opposite direction.

2. Evidence for evolution

Worked example 2 Homologous vs analogous

Compare the wings of a bat and the wings of a butterfly. Are they homologous or analogous? What does this tell us about evolution?

  1. Bat wings have the same bone layout as a human arm (humerus, radius, ulna, fingers) — evidence of shared ancestry. They are homologous to human arms.
  2. Butterfly wings are made of chitin with no bones. They evolved independently for the same function (flight).
  3. Bat and butterfly wings are analogous: similar function, different origin. This is an example of convergent evolution.

3. Speciation

A species is a group whose members can interbreed to produce fertile offspring. Speciation is the formation of a new species.

Allopatric speciation (“other homeland”): a population is split by a geographic barrier — a mountain range, a widening river, an island forming. The two groups can no longer interbreed. Different mutations accumulate, different selection pressures apply, and over many generations the groups become reproductively isolated. If they later meet, they cannot interbreed — they are now separate species.

Worked example 3 Darwin's finches

On the Galapagos islands, one ancestral finch species radiated into ∼15\sim 15 species with different beaks. Explain this using allopatric speciation.

  1. A small founding population reached different islands.
  2. Each island had different food sources (large seeds, insects, cactus flowers).
  3. Isolation limited gene flow between islands.
  4. Natural selection favoured different beak shapes on each island.
  5. Over thousands of generations the populations became distinct species — ground finches, cactus finches, tree finches, warbler finches.

Key idea: geographic isolation plus different selection pressures drives species to diverge from a common ancestor.

4. Relatedness and the tree of life

All living organisms share a common ancestor. The fraction of shared DNA measures how recently two species diverged. Scientists build phylogenetic trees where branch points are common ancestors.

Worked example 4 Reading a phylogenetic tree

Humans and chimpanzees share a common ancestor about 66 million years ago; humans and gorillas about 88 million; humans and orangutans about 1414 million. Which species is most closely related to humans, and how does DNA confirm this?

  1. The most recent common ancestor is with chimpanzees.
  2. DNA similarity with chimps is about 98.8%98.8\%, with gorillas ∼98%\sim 98\%, with orangutans ∼97%\sim 97\%.
  3. More DNA similarity ↔\leftrightarrow more recent common ancestor.

Practice: Year 10

Fluency

Concepts and definitions

    1. State the four conditions for natural selection.
    2. Define: adaptation, fitness, selection pressure.
    3. Explain the difference between homologous and analogous structures, with one example of each.
    4. What is a vestigial structure? Give one human example.
    5. Define a species.
    6. Describe allopatric speciation in one sentence.
Fluency

Evidence

    1. List four lines of evidence for evolution.
    2. Why are transitional fossils (e.g. Archaeopteryx) important?
    3. Humans share about ___%\_\_\_\% of their DNA with chimpanzees. Fill in the blank.
    4. Why do Australia and South America each have unique mammal faunas (marsupials and placentals, respectively)?
    5. True or false: natural selection causes mutations to arise. Justify.
Reasoning

Case studies

    1. Explain, step by step, how a population of bacteria becomes resistant to an antibiotic after exposure over several weeks.
    2. Dark peppered moths rose from <1%<1\% to >95%>95\% of the population as British cities industrialised. After air-quality laws cleaned the atmosphere, the trend reversed. Explain in terms of natural selection.
    3. In a dry year on Daphne Major island, large hard seeds dominate. Finches with larger, stronger beaks survive better. In a wet year, small seeds are plentiful and smaller beaks do better. Explain why the average beak size oscillates.
    4. A cheetah can run at over 100100 km/h. Explain, using variation and selection pressure, how this adaptation evolved.
    5. Pesticide-resistant insects can emerge within a few growing seasons. Explain, and suggest two farming strategies that slow this process.
Problem solving

Analysis and argument

    1. An island population of lizards shows a wide range of body sizes. After a hurricane, the survivors are mostly larger individuals. Is this evidence of natural selection? What else would you need to see to be confident?
    2. Two bird populations on neighbouring islands were once the same species. They are still anatomically similar, but their songs are so different they no longer recognise each other as mates. Have they become separate species? Justify.
    3. Whale embryos show temporary hind limbs, and adult whales have vestigial pelvic bones. How do these facts support the claim that whales evolved from land mammals?
    4. A creationist argues that “the eye is too complex to have evolved.” Give two evolutionary responses: one from direct evidence and one from the logic of small incremental steps.

Challenge

Reasoning

Harder reasoning

    1. Antibiotic resistance is sometimes called “evolution in real time.” Design a simple school experiment (using non-pathogenic bacteria) that would let students watch the frequency of a resistance allele change. Identify the selection pressure, the heritable variation, and how you would measure fitness.
    2. The horse evolutionary lineage shows a progression from 5050-million-year-old Hyracotherium (dog-sized, four-toed) to modern Equus (single hoof, large body). Why is it misleading to call this a “ladder of progress”? What does the branching bush of horse fossils actually show?
    3. A population of 10 00010\,000 moths has an allele frequency of q=0.1q = 0.1 for a recessive dark allele. Assuming Hardy-Weinberg equilibrium, what fraction of the population expresses the dark phenotype? If a sudden selection pressure removes all light moths (p2+2pqp^2 + 2pq combined) in one generation, what is the new qq?
    4. Human activity — habitat destruction, climate change, introduced species — is now a major evolutionary force. Give two examples of modern species evolving rapidly in response to human-caused selection pressures.
Answers

Answer key

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Year 10 answers

Fluency

Concepts and definitions

    1. Variation; selection pressure; differential survival and reproduction; heritability.
    2. Adaptation — an inherited trait that improves survival or reproduction in a given environment. Fitness — the number of viable offspring an individual contributes. Selection pressure — an environmental factor (predator, climate, disease, food supply) that favours some variants over others.
    3. Homologous: same underlying structure, different function (human arm and whale flipper share bones). Analogous: same function, different origin (butterfly wing and bat wing).
    4. A structure that no longer serves its original function (e.g. human appendix, wisdom teeth, tailbone).
    5. A group whose members can interbreed in nature to produce fertile offspring.
    6. A physical barrier splits a population; the separated groups diverge until they can no longer interbreed.
Fluency

Evidence

    1. Fossil record, biogeography, comparative anatomy (homologous structures), molecular (DNA/protein) similarity. (Also embryology, direct observation.)
    2. They show intermediate features between ancestral and modern groups, matching the predictions of common descent.
    3. About 98.8%98.8\%.
    4. The continents were isolated for tens of millions of years; mammals evolved along separate lines on each land mass.
    5. False. Mutations arise randomly, independent of the environment. Selection then acts on the variation already present.
Reasoning

Case studies

    1. Mutations produce rare resistant cells; the antibiotic kills susceptible cells; resistant cells survive, reproduce rapidly (every ∼20\sim 20 min), and pass resistance to daughter cells; within weeks resistant strains dominate.
    2. When soot darkened the trees, birds preyed on conspicuous light moths; dark moths survived and reproduced more, raising the frequency of the dark allele. After clean-air laws pale bark returned, reversing the advantage.
    3. Selection pressure changes with rainfall; the trait that maximises fitness changes each year, so the mean beak size tracks the current food supply.
    4. Prey speed is a selection pressure; faster cheetahs caught more prey and raised more cubs; the fastest heritable traits (long legs, flexible spine, large heart and lungs) accumulated over many generations.
    5. Mutation produces rare resistant individuals; pesticide kills susceptible ones; survivors breed; resistance spreads. Strategies: rotate between pesticides with different modes of action; leave “refuge” areas untreated so susceptible genes persist in the population.
Problem solving

Analysis and argument

    1. Consistent with selection, but a one-off survival event could be chance. Confidence requires heritability (do offspring of survivors also tend to be larger?) and repeatability across generations or populations.
    2. Yes, effectively. Behavioural isolation (mate recognition) is a reproductive barrier; gene flow has stopped. They are on their way to being recognised as distinct species even if still anatomically similar.
    3. Vestigial limbs and embryonic hind-limb buds are expected if whales descended from four-legged ancestors. Genetic programs for limbs persist but are no longer expressed fully.
    4. Direct: transitional forms (light-sensitive patches in flatworms, cup eyes in snails, pinhole eyes in nautiluses, full lens eyes in vertebrates) show a gradient of working intermediate eyes. Logic: each small improvement in light detection confers survival advantage, so intermediate eyes are favoured — not disadvantageous.
Reasoning

Challenge

    1. Grow a large bacterial culture; plate onto agar with and without low-dose antibiotic. Selection pressure: the antibiotic. Heritable variation: resistance alleles from spontaneous mutation. Measure fitness by counting colonies; track allele frequency by repeated subculturing.
    2. Evolution is not directional. The horse fossil record is a branching bush with many extinct side lineages — not a straight march to Equus. Modern horses are one surviving branch; most lineages went extinct.
    3. Fraction expressing dark phenotype =q2=0.01= q^2 = 0.01 (1%). If only q2q^2 survives, the new allele frequency is q=1q = 1 (the dark allele fixes at 100%100\%) since only homozygous dark individuals remain.
    4. Urban birds changing songs to cut through city noise; tuskless elephants becoming more common under ivory poaching pressure; peppered moth; antibiotic-resistant bacteria; herbicide-resistant weeds; salmon reaching sexual maturity earlier under heavy fishing of large fish. (Any two.)

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