Fertilisation, Meiosis and Mutation as Causes of Genetic Variation
Peter and Rosemary Grant have studied Geospiza fortis finches on Daphne Major island since 1977. Their data show beak depth heritability of 0.79, and three independent variation sources: mutation rate, crossing over frequency, and independent assortment. After the severe 1977 drought eliminated 85% of the population, mean beak depth increased 17% in just one generation, all from pre-existing heritable variation, with no new mutations required. Meiosis and mutation do different jobs.
Connect sibling variation to genetic processes
Explain difference without assuming a new mutation.
Three quick questions from earlier lessons. Pulling old material back to mind before you learn something new makes the new material stick better, so this is not busywork.
Practise this lesson
Four printable worksheets that build from the foundations up to exam-style questions, start at whatever level suits you.
Normal meiosis produces gametes with one copy of each chromosome; nondisjunction produces aneuploid gametes that cause trisomy or monosomy conditions such as Down and Turner syndrome.
Two siblings look different from each other, but neither parent has experienced any new mutation in the relevant genes.
Explain how that can still happen. Then add one sentence explaining what mutation contributes that meiosis and fertilisation usually do not.
Explain three sources of variation
Separate mutation, meiosis and fertilisation.
Know
- Mutation, meiosis and fertilisation all contribute to variation.
- Mutation creates new alleles.
- Meiosis and fertilisation mainly reshuffle existing alleles.
Understand
- These processes have different roles, not interchangeable roles.
- Sibling difference can arise without new mutation.
- Population change depends on new alleles entering and then being combined or spread.
Apply
- Compare sources of variation in exam language.
- Explain why meiosis/fertilisation are not the source of all new alleles.
- Link Module 5 heredity to Module 6 population change.
Core Content
Main comparison · new alleles vs new combinations
After the 1977 drought on Daphne Major, surviving Geospiza fortis finches produced offspring with deeper beaks on average than the pre-drought generation. The observed shift did not require a new mutation: selection acted mainly on standing heritable variation, while meiosis and fertilisation generated new combinations of existing alleles.
| Process | What it does | Creates new alleles? | Main variation role |
|---|---|---|---|
| Mutation | Changes DNA sequence | Yes | Introduces new alleles into the gene pool |
| Meiosis | Independent assortment and crossing over | Not usually | Reshuffles existing alleles into different gametes |
| Fertilisation | Fusion of gametes | No | Combines gametes randomly to create new allele combinations in offspring |
Mutation creates new alleles by changing DNA sequence; meiosis reshuffles existing alleles into different gametes through independent assortment and crossing over; fertilisation combines gametes randomly to form new allele combinations in offspring.
Pause, copy the highlighted comparison into your book before moving on.
Mutation creates new alleles; meiosis and fertilisation create new _____ of existing alleles.
Represent meiotic reshuffling
Trace crossing over and independent assortment.
Module 5 link · independent assortment + crossing over
We just saw that mutation, meiosis and fertilisation each play a distinct role in variation. That raises a question: exactly how does meiosis generate variation before fertilisation even occurs? This card answers it → independent assortment and crossing over.
Meiosis contributes to variation in two major ways. First, homologous chromosomes assort independently, so different combinations of maternal and paternal chromosomes enter different gametes. Second, crossing over exchanges segments between homologous chromosomes, producing recombinant chromatids.
This means one parent can produce many genetically different gametes even without any new mutation. Module 5 established this as the basis of inheritance variation, and Module 6 now places it beside mutation so students do not confuse reshuffling with new allele creation.
Meiosis contributes to variation through independent assortment (different chromosomes in different gametes) and crossing over (recombinant chromatids), one parent can produce many genetically different gametes without any new mutation.
Add the highlighted point to your notes before the check below.
Which two meiosis processes generate genetic variation?
Random combination · selection acts on phenotypes
We just saw that meiosis generates variation through reshuffling before fertilisation. That raises a question: how does fertilisation add an additional layer of variation? This card answers it → random gamete combination.
Even after meiosis has produced varied gametes, fertilisation creates additional variation because which sperm meets which egg is largely random. This combines one set of maternal alleles with one set of paternal alleles in a new genotype.
Fertilisation therefore does not create new alleles by itself, but it creates new combinations of alleles in offspring. That matters biologically because natural selection acts on whole phenotypes produced by these combinations, not just on isolated alleles.
Fertilisation creates new allele combinations by randomly fusing one gamete from each parent; it does not create new alleles but adds another layer of variation because natural selection acts on whole phenotypes produced by these combinations.
Pause, write the highlighted distinction into your book.
Fertilisation by itself creates brand-new alleles.
Random fertilisation contributes to genetic variation by combining gametes from two genetically different parents.
Independent assortment creates new alleles that were not present in either parent.
Combine the three processes
Distinguish new alleles from new combinations.
Variation framework · distinct jobs
We just saw that fertilisation adds random gamete combination on top of meiotic variation. That raises a question: how do all three processes fit together as a single system? This card answers it → the three-step variation framework.
1. Mutation
New allele enters the gene pool when DNA sequence changes.
2. Meiosis
Existing alleles are reshuffled into genetically different gametes.
3. Fertilisation
Random gamete fusion creates new allele combinations in offspring.
Population consequence
Mutation supplies novelty; meiosis and fertilisation spread combinations, together generating the variation selection acts on.
If students say "genetic variation is caused by mutation, meiosis and fertilisation", that is correct but incomplete. High-quality HSC answers explain how each process contributes and clearly separate "new allele" from "new combination".
Genetic variation is best understood as a three-step system: mutation supplies new alleles, meiosis reshuffles them into varied gametes, and fertilisation creates new genotypes, mutation provides novelty while meiosis and fertilisation spread combinations.
Pause, copy the highlighted framework into your notes before continuing.
Which process supplies genuinely new genetic novelty (new alleles)?
Match outcomes to processes
Apply and repair new-allele language.
Activities
Match the Process to the Role
Name the process (mutation, meiosis or fertilisation) responsible for each effect.
- Different maternal and paternal chromosomes enter different gametes.
- A new allele appears because the DNA sequence changed.
- One sperm fuses with one egg out of many possible combinations.
- Homologous chromosomes exchange segments by crossing over.
Explain Sibling Difference
Explain why siblings can differ genetically without any new mutation, then add why mutation still matters in the long term.
Synthesise sources of variation
Capture the long-term role of mutation.
Core biological claim
- Mutation, meiosis and fertilisation all contribute to variation, but they contribute in different ways.
Mechanism or process
- Mutation creates new alleles, meiosis reshuffles existing alleles into gametes, and fertilisation combines gametes randomly into new genotypes.
Common exam error
- Saying meiosis or fertilisation creates all new alleles.
Evaluative sentence starter
- "Although meiosis and fertilisation generate extensive genetic variation, mutation remains essential because it is the source of genuinely new alleles."
Practise sources-of-variation reasoning
Answer fresh questions and compare model responses.
A fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
UnderstandBand 3(3 marks) 1. Explain how meiosis contributes to genetic variation.
AnalyseBand 4(4 marks) 2. Compare the roles of mutation, meiosis and fertilisation in producing genetic variation.
EvaluateBand 5–6(5 marks) 3. Evaluate the claim: "Meiosis and fertilisation are enough to explain all genetic variation, so mutation is not very important."
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Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Activity 1, Match the process to the role
1. Meiosis.
2. Mutation.
3. Fertilisation.
4. Meiosis.
Activity 2, Explain sibling difference
Siblings differ genetically because each parent produces genetically different gametes through meiosis, including independent assortment and crossing over. Fertilisation then combines one gamete from each parent at random, producing different allele combinations in each child. Mutation still matters in the long term because it introduces genuinely new alleles into the population.
Short Answer Model Responses
Q1 (3 marks): Meiosis contributes to genetic variation by producing genetically different gametes [1]. It does this through independent assortment of homologous chromosomes [1] and crossing over between homologous chromosomes [1].
Q2 (4 marks): Mutation creates new alleles by changing DNA sequence [1]. Meiosis contributes to variation by reshuffling existing alleles through independent assortment and crossing over [1]. Fertilisation contributes by combining gametes randomly [1]. Therefore mutation provides new genetic novelty, while meiosis and fertilisation mainly create new combinations of alleles already present [1].
Q3 (5 marks): The claim is incomplete because meiosis and fertilisation do explain much of the variation seen between siblings [1]. Meiosis reshuffles existing alleles into different gametes [1]. Fertilisation combines those gametes randomly in offspring [1]. However, mutation is still essential because it introduces genuinely new alleles into the gene pool [1]. Therefore meiosis and fertilisation are major sources of variation, but mutation remains critical for long-term genetic change in populations [1].
Review and reflect
Retrieve the distinctions and revisit sibling variation.
Mutation
Source of genuinely new alleles.
Meiosis
Reshuffles existing alleles into different gametes.
Fertilisation
Combines gametes randomly into new genotypes.
Exam trap
Using "variation" as if all sources do the same job.
Rapid-fire questions on mutation, meiosis, fertilisation and new alleles vs new combinations. Beat the boss to bank a tier, gold (perfect + fast), silver (80%+), or bronze (cleared).
Return to the Daphne Major finch study. You should now be able to explain that the observed shift in mean beak depth did not require a new mutation: selection acted mainly on pre-existing heritable variation, while meiosis and fertilisation reshuffled existing alleles. Mutation still matters in the long term because it is the ultimate source of new alleles.