Biology • Year 12 • Module 6 • Lesson 19

Variation and Allele Frequency

Lock in the vocabulary of variation sources, allele frequency, and the selection-versus-drift distinction, and map how mutation, selection and drift shape a gene pool.

Build · Vocab & Recall

1. Term–definition match

The ten definitions below are shuffled. In the right-hand column write the matching term from this list: mutation, genetic recombination, gene flow, gene pool, allele frequency, natural selection, genetic drift, selection pressure, Hardy-Weinberg equilibrium, fitness. 10 marks

#Definition (shuffled)Matching term
1.1A permanent change in the DNA sequence; the ultimate source of all new alleles.
1.2The shuffling of existing alleles into new combinations during meiosis and sexual reproduction.
1.3The transfer of alleles between populations by the migration of individuals.
1.4The total set of alleles in a population.
1.5The proportion of a specific allele in a gene pool, often written as a decimal or percentage.
1.6A non-random change in allele frequency caused by the environment consistently favouring certain variants.
1.7Random change in allele frequency, especially strong in small populations, with no adaptive reason.
1.8An environmental factor (e.g. a pathogen or pesticide) that favours some variants over others.
1.9A theoretical state in which allele frequencies remain constant from generation to generation.
1.10An individual's relative ability to survive and reproduce in a given environment.
Stuck? Revisit lesson Cards 1 (variation), 2 (allele frequency) and 3 (selection vs drift), and the Key Terms card.

2. True or false, with correction

For each statement, circle T or F. If the statement is false, write the corrected version on the line provided. 10 marks (1 T/F + 1 correction where needed)

2.1 Mutation is the ultimate source of all new alleles because it changes the DNA sequence itself.    T  /  F

2.2 Genetic recombination creates brand-new alleles that did not previously exist in the population.    T  /  F

2.3 Evolution at the population level is best described as a change in allele frequencies across generations.    T  /  F

2.4 A change in allele frequency always means natural selection is occurring.    T  /  F

2.5 Genetic drift has its strongest effect in small populations.    T  /  F

Stuck? Revisit the Misconception card and Cards 1–3.

3. Cloze paragraph, fill the blanks

Complete the paragraph using terms from the word bank below. Each term is used once. 10 marks

Word bank: alleles • chance • drift • fitness • frequency • gene flow • generations • mutation • populations • recombination

Variation enters a population through three processes. changes the DNA sequence and creates new ; shuffles existing alleles into new combinations during meiosis; and moves alleles between populations by migration. Allele is the proportion of a specific allele in the gene pool, and evolution is defined as a change in this value over . If an allele confers a advantage, carriers leave more offspring and the allele increases, this is natural selection. By contrast, genetic changes allele frequency by rather than for an adaptive reason. Either way, it is , not individuals, that evolve.

Stuck? Cards 1–3 and the "Copy into your books" card contain all the clues.

4. Function recall

Answer each in 1–2 sentences using precise terms from the lesson. 8 marks (2 each)

4.1 Why is mutation, rather than recombination, called the ultimate source of variation?

4.2 What does allele frequency measure, and why is it useful for describing evolution?

4.3 How does a selection pressure change the frequency of a favoured allele over generations?

4.4 Why do populations, rather than individuals, evolve?

Stuck? Revisit Cards 1–3 and the common-misconception callouts.

5. Build a concept map

Draw labelled arrows between the six terms below to show how they connect. Each arrow must carry a linking phrase (e.g. "creates", "shifts", "is measured by", "by chance"). Aim for at least 6 labelled arrows. 6 marks

Supplied terms: mutation • new alleles • gene pool • allele frequency • natural selection • genetic drift.

mutation
new alleles
gene pool
allele frequency
natural selection
genetic drift
Hint: consider chains such as mutation → creates → new alleles; and natural selection → shifts → allele frequency.
Answers, Do not peek before attempting

Q1, Term–definition matches (10 marks)

1.1 mutation • 1.2 genetic recombination • 1.3 gene flow • 1.4 gene pool • 1.5 allele frequency • 1.6 natural selection • 1.7 genetic drift • 1.8 selection pressure • 1.9 Hardy-Weinberg equilibrium • 1.10 fitness.

1 mark per correct match.

Q2, True / false with correction (10 marks)

2.1 True.

2.2 False. Correction: recombination does not create new alleles, it only shuffles existing alleles into new combinations during meiosis. Mutation is what creates new alleles.

2.3 True.

2.4 False. Correction: allele frequencies can change through natural selection, genetic drift, mutation or gene flow; in small populations, drift can cause large random changes with no selective advantage, so a frequency change alone does not prove selection.

2.5 True.

1 mark for each correct T/F; 1 mark for each correct correction (false statements only).

Q3, Cloze paragraph (10 marks)

In order: mutation / alleles / recombination / gene flow / frequency / generations / fitness / drift / chance / populations.

1 mark per correct blank.

Q4.1, Why mutation is the ultimate source (2 marks)

Mutation is the ultimate source because it changes the DNA sequence itself, producing alleles that did not previously exist [1]. Recombination only rearranges existing alleles into new combinations; it cannot invent a new allele, so the raw material it shuffles must first have come from mutation [1].

Q4.2, What allele frequency measures (2 marks)

Allele frequency is the proportion of a specific allele in a population's gene pool [1]. It is useful because evolution at the population level is defined as a change in allele frequency over generations, giving a measurable, quantitative way to describe evolutionary change [1].

Q4.3, How selection pressure changes frequency (2 marks)

If an allele gives carriers a fitness advantage under a selection pressure, those individuals survive and reproduce more successfully and leave more offspring [1]. Because the allele is inherited, more of the next generation carry it, so its frequency rises in the gene pool over successive generations [1].

Q4.4, Why populations evolve, not individuals (2 marks)

Evolution is defined as a change in allele frequencies across a population over generations [1]. An individual's genotype is fixed for life, it survives or dies, so it cannot change its own allele frequencies; only the population's composition shifts across generations, which is why populations evolve and individuals do not [1].

Q5, Sample concept map (6 marks)

A correct map should include arrows such as:

  • mutationcreates→ new alleles
  • new allelesenter→ gene pool
  • gene poolis measured by→ allele frequency
  • natural selectionshifts (non-randomly)→ allele frequency
  • genetic driftshifts (by chance)→ allele frequency
  • genetic driftacts most strongly on a small→ gene pool

Any biologically valid linking phrases are accepted. Award 1 mark per correctly labelled, directionally appropriate arrow, up to 6 marks.

HSCScience • Biology • Year 12 • Module 6 • Lesson 19 • Worksheet 1 (Build)