Meiosis and fertilisation get all the attention for creating variation, but neither of them can invent anything. They deal the cards; mutation is what printed them in the first place. That distinction is worth a lot of marks.
📘 What you need to know
Variation is the differences between organisms of the same species, and it comes from small differences in DNA base sequences.
Three sources of variation: mutation, meiosis and random fertilisation.
Mutation is the original source of variation, because it is the only one that makes a genuinely new allele.
A new allele may be advantageous, disadvantageous, or have no apparent effect.
Only mutations in the dividing cells of the sex organs reach the gametes, so only those affect natural selection.
In asexually reproducing species, mutation is the only source of variation.
What variation is
No two members of a species are identical. Coat colour differs between mammals, body length differs between fish of the same species, flower colour differs between plants of the same kind. All of these differences are called variation.
Underneath every one of them are small differences in the base sequence of the DNA carried by individual organisms in a population. Different base sequence, different allele, different characteristic.
Where the differences come from
Meiosis and fertilisation are brilliant at making new combinations. Neither can produce an allele that did not already exist somewhere.
Think of a deck of cards. Meiosis shuffles the deck, and fertilisation deals two random hands together. Both produce combinations nobody has seen before — but the cards themselves were printed by mutation. Every allele in every organism alive today started as a mutation at some point in the past.
If a question asks for the original source of variation, the answer is mutation, and nothing else. If it just asks for sources of variation, give all three.
What happens to a new allele
A mutation produces a new allele, and that allele can turn out to be useful, harmful, or neither. Nothing about the mutation decides this — the environment does.
The same allele can sit in different boxes in different places. Sickle cell is a good reminder of that.
An advantageous allele makes an organism more likely to survive and reproduce, so it is more likely to be passed on. Over many generations it becomes more common.
A disadvantageous allele makes survival and reproduction less likely, so it tends to die out.
An allele with no apparent effect is neither favoured nor removed.
Over long periods this is what drives evolution by natural selection. Without mutation supplying new alleles, natural selection would eventually have nothing left to select between.
Which mutations count for evolution? Only those in the dividing cells of the sex organs, because only those end up in gametes. A mutation in a body cell is never passed on, so it has no effect on natural selection at all.
Asexual reproduction: mutation is all there is
Meiosis and random fertilisation both belong to sexual reproduction. An organism that reproduces asexually has neither, so its offspring are genetically identical to the parent — unless a mutation occurs. For those species, mutation is the only source of variation.
This is also why new strains of bacteria and viruses keep appearing. They reproduce quickly and in huge numbers, so even a low mutation rate produces plenty of new variants.
Nature of science: commercial genetic testing
Genetic tests can tell you about your risk of future disease. There are two very different routes to getting one, and the difference matters.
Feature
Clinical (medical) testing
Commercial testing
Who arranges it
A healthcare provider — doctor, nurse practitioner or genetic counsellor
The customer, by buying a kit online or in a shop
Choosing the test
The provider decides which test is appropriate and orders it from a laboratory
The customer decides, with no professional advice
Getting the results
Explained by the provider, who checks the person understands what they mean
Sent straight back from the company, often with no interpretation
Support afterwards
Counselling and follow-up available
Frequently none at all
The concern with commercial testing is not the technology; it is what happens after the envelope is opened:
Some commercially available tests are not scientifically validated and can give inaccurate results.
Unexpected findings about health, ancestry or family relationships can be upsetting, and arrive with nobody there to talk them through.
People may make serious decisions about treatment based on results they have misunderstood.
Genetic counselling is often not provided.
Nature of science questions want a balanced answer, not an opinion. Give the benefit (access to your own genetic information without a gatekeeper) and then the risks (accuracy, distress, misinterpretation, no counselling).
Worked examples
WORKED EXAMPLE
Explain why mutation is the original source
Meiosis and random fertilisation both produce variation. Explain why mutation is still described as the original source of genetic variation.
Point 1: What meiosis and fertilisation do
They produce new combinations of alleles that already exist.
Point 2: What mutation does
It changes the base sequence, which creates a new allele that did not exist before.
Point 3: Put them together
Without mutation there would be no new alleles for the other two processes to rearrange.
Only mutation creates new allelesThe word to get in there is “new”. Combinations are not the same as new alleles.
WORKED EXAMPLE
Variation in a bacterial population
Bacteria reproduce asexually. Explain how variation still arises in a population of bacteria.
Step 1: Rule out the other two sources
No meiosis and no fertilisation, so no shuffling of alleles.
Step 2: Identify what is leftMutation is the only source of variationStep 3: Explain why it is enough
Bacteria reproduce rapidly and in vast numbers, so even a low mutation rate produces many new variants — which is how new strains appear.
Mutation aloneThis is the standard route into antibiotic resistance questions, so it is worth knowing well.
WORKED EXAMPLE
Evaluate a home genetic test
A person buys a genetic test kit online and is told they carry an allele linked to a serious disease. Outline two problems with relying on this result.
Problem 1: Accuracy
Commercial tests are not always scientifically validated, so the result may be inaccurate.
Problem 2: Interpretation
There is usually no professional to explain what carrying the allele actually means for their risk.
Why it matters
The person may be distressed, or make an important treatment decision based on a misunderstanding.
Unvalidated result, no counselling“Outline two problems” means exactly two, each with a short explanation. Do not write four.
💡 Exam tip
Learn the one-line distinction: mutation makes new alleles, meiosis and fertilisation make new combinations.
If the question says original source, answer mutation and explain why the other two do not qualify.
Use the three-way classification for a new allele: advantageous, disadvantageous, no apparent effect.
Mention that only mutations in cells of the sex organs can affect the next generation.
For asexual species, say plainly that mutation is the only source of variation.
In nature of science answers, give benefits and risks. One-sided answers rarely get full marks.
⚠ Common mix-up
Saying meiosis creates new alleles. It creates new combinations of existing ones.
Thinking useful mutations happen because they are needed. Mutations are random; the environment selects afterwards.
Forgetting that somatic mutations do not affect evolution. They are never passed on.
Assuming all variation is genetic. Environment shapes many characteristics too, though that is not a source of new alleles.
Believing asexual species cannot vary. They can — through mutation alone.
Treating a commercial test result as a diagnosis. It is a risk indicator, and not always a reliable one.
Up next: Cell Division — how one cell becomes two, and why the nucleus always has to divide before the cytoplasm does.
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