Fertilisation, Meiosis and Mutation as Causes of Genetic Variation
You'll be able to compare sources of variation in exam language.
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."
Analyse the relative contributions of mutation, meiosis and fertilisation to the genetic variation present in a population.
The dot point instructs you to investigate the causes of genetic variation, which scopes content rather than setting a written task, so it is examined with Analyse — investigate → analyse. The word relative means the marks are for how the three compare, not for three descriptions.
Plan: identify the components, state what constrains them, relate them at population scale, then draw the implication and the limits.
The three processes do not contribute the same kind of variation. Mutation creates alleles that did not exist; meiosis, through independent assortment and crossing over, creates new combinations of existing alleles within a gamete; fertilisation creates new combinations by uniting two such gametes.
They are linked by dependence: the two combination processes can only rearrange what mutation has already supplied. A population in which mutation stopped entirely would continue to generate novel genotypes through meiosis and fertilisation for many generations, but the pool of alleles being shuffled would be fixed and would slowly erode through drift and selection.
Quantitatively the balance is lopsided. Mutation rates are of the order of one base change per hundred million per generation, so mutation adds almost nothing to a single generation's variation. Meiosis and fertilisation, by contrast, generate an astronomically large number of genotypes each generation — more than eight million chromosome combinations per human gamete before crossing over, squared at fertilisation. In any one generation nearly all observable variation comes from recombination, and over evolutionary time all of it originates in mutation.
The limits show where the analysis stops. Asexually reproducing organisms have neither meiotic recombination nor fertilisation, so mutation is their sole source of new variation — which is why bacterial populations depend on high mutation rates and on horizontal gene transfer instead. And none of the three explains how variation is distributed: gene flow and genetic drift redistribute alleles between and within populations without generating any.
It follows that the three are not competing sources but a sequence: mutation supplies the alleles, meiosis and fertilisation deploy them into combinations on which selection can act, and the rate-limiting step for adaptation is usually the supply rather than the shuffling. This is why a small, long-isolated population can be recombining vigorously and still have little capacity to adapt.
The three processes do not contribute the same kind of variation: mutation creates alleles that did not exist, meiosis creates new combinations of existing alleles within a gamete through independent assortment and crossing over, and fertilisation creates further combinations by uniting two such gametes. They are linked by dependence, since the combination processes can only rearrange what mutation has supplied — a population in which mutation ceased would keep producing novel genotypes for many generations, but from a fixed pool of alleles that drift and selection would slowly erode. Quantitatively the balance is lopsided: mutation rates of the order of one base change per hundred million per generation add almost nothing in any single generation, whereas meiosis and fertilisation generate more than eight million chromosome combinations per human gamete before crossing over, squared at fertilisation. So nearly all variation observable in one generation comes from recombination, while over evolutionary time all of it originates in mutation. The analysis has limits: asexual organisms have neither recombination nor fertilisation, so mutation is their only source of new variation, which is why bacterial populations rely on high mutation rates and horizontal gene transfer; and none of the three explains how variation is distributed, since gene flow and drift redistribute alleles without generating any. The three are therefore not competing sources but a sequence — mutation supplies the alleles and the other two deploy them into combinations selection can act on — and because the supply rather than the shuffling is usually rate-limiting, a small isolated population can recombine vigorously and still have little capacity to adapt.
Evaluate the claim that mutation is the most important source of genetic variation in a population.
Evaluate demands a judgement against criteria and a position that commits. The claim is neither simply true nor simply false, so an answer that only lists points on both sides has not evaluated it.
Plan: define the criteria the word “important” must be judged on, put the case for, put the case against, reach a verdict, then qualify it.
Importance will be judged on three criteria: how much variation each source contributes per generation, whether the variation it contributes is of a kind the others cannot produce, and whether a population could persist and adapt without it.
On the second and third criteria the claim holds firmly. Mutation is the only source of genuinely new alleles, so every allele that meiosis and fertilisation shuffle originated in a mutation at some point, and no amount of recombination can produce a version of a gene that has never existed. A population that lost the capacity to mutate could not adapt to a genuinely novel selection pressure once its existing variation was exhausted, so mutation is irreplaceable in a way the other processes are not.
On the first criterion the claim fails badly. Mutation rates are around one base change per hundred million bases per generation, so in any single generation mutation contributes a negligible fraction of the variation present, while meiosis and fertilisation together produce an effectively unlimited number of genotypes each generation. If importance is measured by contribution to the variation a population actually displays now, mutation is by far the least important of the three.
The claim is therefore correct over evolutionary timescales and incorrect over generational ones, and the disagreement is about the timescale rather than about the biology. Mutation is the ultimate source of variation and recombination the immediate one, so the claim should be accepted with that qualification made explicit rather than accepted or rejected outright.
One qualification limits even that. In asexually reproducing populations there is no meiotic recombination or fertilisation at all, so mutation is not merely the ultimate source of variation but the only one, and the claim is true without qualification. The claim's accuracy therefore depends on the reproductive mode of the population it is made about — which is a reminder that a general statement in biology usually carries an unstated organism in it.
Importance will be judged on how much variation each source contributes per generation, whether that variation is of a kind the others cannot produce, and whether a population could adapt without it. On the second and third criteria the claim holds firmly: mutation is the only source of genuinely new alleles, so every allele that meiosis and fertilisation shuffle originated in a mutation, no amount of recombination can produce a version of a gene that has never existed, and a population unable to mutate could not adapt to a novel selection pressure once its existing variation was exhausted. On the first criterion the claim fails badly, since mutation rates of about one base change per hundred million per generation contribute a negligible fraction of the variation present in any one generation, while meiosis and fertilisation together generate an effectively unlimited number of genotypes each generation — so measured by the variation a population actually displays, mutation is the least important of the three. The claim is therefore correct over evolutionary timescales and incorrect over generational ones, and the disagreement is about timescale rather than biology: mutation is the ultimate source and recombination the immediate one, so the claim should be accepted with that qualification stated rather than accepted or rejected outright. One further qualification limits even that, because asexual populations have no recombination or fertilisation at all, making mutation not merely the ultimate source of their variation but the only one — so the claim's accuracy depends on the reproductive mode of the population it is made about.
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."
Show all answers
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.