Variation & Evolution

Lesson 6.06 · Spec 4.6.2.1–4.6.2.2 · Paper 2

A natural selection describe question — three marks — recently had only one third of students scoring full marks, and 15% scoring zero. The mark scheme listed four creditworthy steps. Most students got three of them right. The step they missed was the last one: instead of writing "pass on alleles," they wrote "pass on characteristics." That single word cost the mark. In the next 10 minutes, you will know exactly what to write.

By the End of This Lesson…

You will be able to:

Define variation and state its three causes (genetic, environmental, combination)

Recall the three-tier spec statement about mutations and phenotype

Explain how sexual reproduction produces genetic variation (meiosis + fertilisation)

Distinguish continuous from discontinuous variation and link each to the correct graph type

Define evolution using the exact spec wording

Describe natural selection in four steps using mark-scheme language — specifically "alleles," not "traits"

Define speciation in terms of fertile offspring

Spec Refs

4.6.2.1 — Variation (causes, mutations, continuous/discontinuous)

4.6.2.2 — Evolution (natural selection mechanism, speciation)

Paper 2 · Higher Tier · ⭐ Priority topic

What Is Variation?

Definition

Variation — differences in the characteristics of individuals in a population. These differences may be due to the genes they have inherited, the conditions in which they have developed, or a combination of both.

Spec opening statement

The spec requires you to describe simply how the genome and its interaction with the environment influence the development of the phenotype of an organism. In plain terms: your genes set the possible range; the environment determines what is actually expressed.

Genetic causes

Differences in the genes (alleles) inherited from parents.

e.g. blood group — determined entirely by alleles

Environmental causes

Differences in the conditions in which an organism develops.

e.g. nutrition, climate, UV exposure, exercise

A combination

Most characteristics are influenced by both genes and environment.

e.g. height — genes set the range; nutrition determines the outcome

Spec note

The spec states there is usually extensive genetic variation within a population of a species. This is the raw material for natural selection — without variation, evolution cannot occur.

Mutations: The Source of All Variants

Definition

Mutation — a random change in DNA. Mutations occur continuously in all organisms. All variants in a population arise from mutations.

The spec requires you to recall this three-tier statement about what mutations do to the phenotype:

MOST

Have no effect on the phenotype — the protein produced is unchanged.

SOME

Influence the phenotype — the protein is altered but the organism survives.

VERY FEW

Determine the phenotype — the mutation alone controls the characteristic (e.g. inherited disorders).

Key spec statement

Very rarely, a mutation will lead to a new phenotype. If that new phenotype is suited to an environmental change, it can lead to a relatively rapid change in the species. This is the link between mutation and evolution.

Common error

"All mutations are harmful." — Wrong. Most have no effect. Very rarely, a mutation is beneficial. Harmful mutations are the minority, not the rule.

Sexual Reproduction & Genetic Variation

Sexual reproduction produces genetic variation through two mechanisms. Both must be named for full marks on a 2-mark explain question.

MechanismWhat happensEffect on variation

1. MeiosisChromosomes are randomly mixed as gametes form — each gamete receives a different combination of parental allelesEvery gamete is genetically unique

2. Random fertilisationAny sperm can combine with any egg — an additional layer of random mixingEach offspring has a unique combination of alleles from both parents

2-mark answer template

"During meiosis, chromosomes are randomly mixed so each gamete contains a different combination of parental alleles. At fertilisation, any sperm can combine with any egg, producing further variation. This is why siblings with the same parents can look very different."

Mark scheme note

Mark schemes award 1 mark for meiosis / random mixing and 1 mark for random fertilisation — these are two separate points. Mentioning only one caps you at 1 mark.

Continuous vs Discontinuous Variation

Continuous Variation

A range of values with no distinct groups

Caused by multiple genes + environment

Plotted on a histogram (normal distribution / bell curve)

Examples: height, weight, leaf length, blood pressure

Discontinuous Variation

A limited number of distinct categories

Caused by one or a few genes , little environmental influence

Plotted on a bar chart (bars do not touch)

Examples: blood group (A, B, AB, O), tongue rolling, sex

Graph type matters

Continuous → histogram (bars touch; x-axis has continuous scale). Discontinuous → bar chart (bars do not touch; x-axis has named categories). Using the wrong chart type costs marks on data-handling questions.

Evolution: The Spec Definition

Spec Definition — memorise this exactly

Evolution — a change in the inherited characteristics of a population over time through a process of natural selection, which may result in the formation of a new species.

Three essential components

Inherited characteristics — the change must be heritable, not just environmentally acquired

Population over time — evolution acts on populations (not individuals), across generations

Natural selection — the mechanism; variants better suited to the environment reproduce more

The big picture — spec wording

The theory of evolution by natural selection states that all species of living things have evolved from simple life forms that first developed more than three billion years ago.

Individuals don't evolve

"The giraffe evolved a longer neck." — Wrong framing. Individual organisms do not evolve. Populations evolve over many generations as allele frequencies change. The giraffe with a longer-neck allele survived and reproduced more — over time the allele became more common in the population.

Natural Selection: The 4-Step Chain

This is the mark-scheme sequence. All four steps can be credited. Learn them in order — and learn the exact language at step 4.

1

VariationWithin a species, individuals show variation in their characteristics — arising from different alleles.

2

Better adapted surviveIn a given environment, individuals with phenotypes best suited to their environment (the exact spec phrase) are more likely to survive. "Better adapted" and "survival of the fittest" are both accepted in mark schemes.

3

Survivors reproduceThe better-adapted individuals survive to reproduce. Less-adapted individuals are more likely to die before reaching reproductive age.

4

Pass on alleles / genes / mutations⚠ The favourable alleles are passed to offspring. Over many generations, these alleles become more common in the population. NOT "traits" or "characteristics" — these words do not score.

The Language Trap: Alleles, Not Traits

Step 4 of natural selection is where the most marks are dropped. The mark scheme is explicit: certain words are credited, others are not — regardless of how close they seem.

✗ Not credited

"passes on traits"

"passes on characteristics"

"passes on genetic material"

"passes on chromosomes"

These are all explicitly rejected in AQA mark schemes for natural selection questions.

✓ Credited

"passes on alleles"

"passes on genes"

"passes on (favourable) mutations"

Any of these three words earns the mark. "Alleles" is the most precise and always safest.

Also note — step 1

The mark scheme for variation (step 1) says "ignore mutation" as the creditworthy answer for that step. Variation is the effect; mutation is the cause. At step 1, write "variation between individuals" — not "mutation."

Speciation

Spec Definition

A species is a group of organisms that can interbreed to produce fertile offspring. If two populations of one species become so different in phenotype that they can no longer interbreed to produce fertile offspring, they have formed two new species.

How it happens — the sequence:

Population divided

A geographic or other barrier separates a population into two isolated groups

Independent selection

Different environments mean different alleles are favoured in each group

New species

Groups diverge until they can no longer interbreed to produce fertile offspring

Key phrase — both words matter

The definition requires "fertile offspring" — not just "offspring." Two different species may be able to mate and produce offspring (e.g. horse × donkey → mule), but if those offspring are sterile, the parents are still separate species.

Worked Example

Natural selection — 3 marks3 marks

A population of moths lives in a woodland. The moths vary in wing colour — some are pale, some are dark. Over several decades, the proportion of dark moths increased significantly. Using the theory of natural selection, explain how this change came about.

1

There is variation in wing colour between individual moths in the population — some have alleles for pale wings, some for dark wings.

2

Dark-winged moths are better adapted to their environment (e.g. darker tree bark) and are less likely to be seen by predators, so they are more likely to survive.

3

The surviving dark moths reproduce and pass on their dark-wing alleles to offspring. Over many generations, the dark-wing allele becomes more common in the population.

Step 1 = variation ✓ · Step 2 = better adapted survive ✓ · Step 3 = reproduce + pass on alleles ✓ (not "traits"). Three from three. Writing "pass on dark-wing characteristics" at step 3 scores only 2/3.

Examiner Alerts

Alert 1 — Step 4 language: 15% scored zero

In a 3-mark natural selection question, 15% of students scored zero and only one third scored full marks. The single most cited error: writing "passes on characteristics / traits" instead of "passes on alleles / genes." The mark scheme explicitly rejects "characteristics" and "traits" for this step. This is the highest-impact single fix you can make.

Alert 2 — Step 1: "ignore mutation"

When describing step 1 of natural selection, writing only "mutation causes variation" is not credited for the variation step — the mark scheme says "ignore mutation" as the standalone answer here. Write "variation between members of the species" — mutation is the cause of variation, but variation itself is the creditworthy point.

Alert 3 — Mutation language: three-tier rule

The spec requires the precise three-tier statement: most → no effect; some → influence phenotype; very few → determine phenotype. Writing "most mutations are harmful" contradicts the spec. "Most have no effect" is the correct answer. If asked to recall the spec statement, use all three tiers.

Common Errors

✗ Wrong

"The giraffe evolved a longer neck" / "individuals evolve"

✓ Right

"Populations evolve over many generations as allele frequencies change"

✗ Wrong

"Passes on characteristics / traits / genetic material"

✓ Right

"Passes on alleles / genes / mutations" — the exact word scores the mark

✗ Wrong

"Height is discontinuous variation"

✓ Right

"Height is continuous variation — it shows a smooth range, influenced by multiple genes and environment"

✗ Wrong

"Two species can't breed at all"

✓ Right

"Two species cannot interbreed to produce fertile offspring — the offspring may exist but be sterile"

Recap: Three Takeaways

1

Variation: caused by genetic factors, environmental factors, or both. All variants arise from mutations — most have no effect on phenotype; some influence it; very few determine it. Continuous variation (histogram, many genes + environment) vs discontinuous (bar chart, few genes).

2

Natural selection — four steps: variation → better adapted survive → reproduce → pass on alleles (NOT traits or characteristics). Evolution = change in inherited characteristics of a population over time through natural selection. All species evolved from simple life forms >3 billion years ago.

3

Speciation: when two populations of one species become so different they can no longer interbreed to produce fertile offspring, they form two new species. The word "fertile" is essential in the definition — sterile offspring don't count.

Checkpoint

You should now be able to:

✓ State the three causes of variation (genetic, environmental, combination) using spec language

✓ Recall the three-tier mutation statement: most → no effect; some → influence phenotype; very few → determine phenotype

✓ Explain why siblings vary: random meiosis AND random fertilisation (both required for 2 marks)

✓ Distinguish continuous from discontinuous variation and select the correct graph type for each

✓ Write the spec definition of evolution word-for-word

✓ Describe natural selection in four steps, using "alleles" not "traits" at step 4

✓ Define speciation using "fertile offspring" — not just "offspring"

Next lesson

Lesson 6.07 — Selective Breeding & Genetic Engineering: how humans apply the principles of variation and inheritance to deliberately modify species — and the ethical debates that follow.