4.6.2 Variation and evolution

Why this matters

Charles Darwin's 1859 idea — that all life on Earth descended from common ancestors by a process of natural selection — is one of the most powerful explanations in science. At GCSE you don't need to read Darwin, but you do need the mechanism. There are three pieces. First, individuals in any population differ from each other — some red, some white, some tall, some short. Second, those differences are partly inherited from parents (genes / alleles) and partly caused by the environment (food, exercise, sunlight) and usually a mix of both. Third, when the environment changes — a new predator, a new disease, an antibiotic added to a bacterial colony — individuals whose inherited traits happen to suit the new conditions are more likely to survive long enough to reproduce. They pass their advantageous alleles to the next generation. Over many generations, the proportion of individuals carrying those alleles rises and the population evolves. The same logic explains antibiotic resistance in bacteria (a real-world emergency happening right now), the peppered moth changing colour during the industrial revolution, and the different beak shapes Darwin saw in finches on the Galapagos islands. Humans use the same mechanism on purpose — we call it selective breeding or artificial selection — to produce big-grained wheat, fast racehorses, friendly dogs and high-yield dairy cows. The price is a smaller gene pool, which leaves the breed vulnerable to a single new disease.

How to learn this topic

Build on what you already know

• GCSE 4.6.1: DNA, genes, alleles, chromosomes; sexual vs asexual reproduction.
• GCSE 4.1.2: mitosis produces identical cells; meiosis produces genetically different gametes.
• GCSE 4.3.1: antibiotic resistance in bacteria; pathogens.
• KS3: idea that animals and plants are adapted to their environment.

1. Define variation and distinguish genetic, environmental and combined causes.
2. Explain mutations — random changes in DNA, mostly silent, very rarely large.
3. Build natural selection step by step: variation → selection pressure → survival → reproduction → inheritance.
4. Worked example: antibiotic resistance in bacteria.
5. Other classic examples: peppered moth, Darwin's finches.
6. Selective breeding — humans as the selector; examples; the inbreeding problem.
7. Evidence for evolution: fossils, antibiotic resistance, extinction.
8. Practise mark-scheme answers using the four-line natural selection skeleton.

Key terms

variation

Differences in the phenotypes of individuals in a population. Caused by genes, environment, or a combination of both. (Examiners reward 'differences in characteristics between individuals'. Only genetic variation is heritable.)

mutation

A random change in the DNA sequence of a cell. Most mutations have no effect on the phenotype; some have a small effect; very rare mutations have a large effect and may produce new alleles. (Always describe mutation as RANDOM. AQA marking phrase: 'a random change in DNA'. Mentioning 'non-coding DNA' picks up the mark for why most are silent.)

allele

A version of a gene. Different alleles code for slightly different versions of the same protein and so produce different phenotypes.

natural selection

The process by which individuals with characteristics best suited to their environment are more likely to survive and reproduce, passing on their advantageous alleles. Over generations the population evolves. (Four-line skeleton: variation → selection pressure → survival of the fittest → inheritance. Hit every step.)

evolution

A change in the inherited characteristics of a population over time, through the process of natural selection. May result in the formation of a new species. (Evolution is a property of POPULATIONS, not individuals. A single bacterium cannot evolve.)

selection pressure

A factor in the environment (e.g. predator, disease, antibiotic, climate) that affects an organism's chances of surviving and reproducing. (Naming the specific pressure (antibiotic, predator, food shortage) makes answers concrete and gains marks.)

selective breeding

The process by which humans breed plants and animals for particular genetic characteristics. Individuals showing the desired trait are chosen and bred, repeated over many generations. (Always state 'repeated over many generations' — single-generation answers lose marks.)

inbreeding

Breeding between closely related individuals, which reduces genetic variation. Common in selectively bred populations because the gene pool has been narrowed deliberately. (Link inbreeding to consequences: vulnerability to disease, inherited disorders.)

gene pool

The total collection of alleles present in a population. A large gene pool means lots of variation; a small gene pool means little. (Selective breeding reduces the gene pool. This is the route into the 'susceptible to disease' marking point.)

antibiotic resistance

The ability of some bacteria to survive an antibiotic that would normally kill them. Arises by random mutation and spreads by natural selection when antibiotics are used. (Resistance arises by RANDOM mutation; antibiotics SELECT — they do not CAUSE — resistance.)

fitness

In biology, how well an individual is suited to surviving and reproducing in its environment — not 'physical fitness' in the gym sense. (AQA examiners accept 'best suited to the environment' as the marking phrase.)

extinction

When all members of a species die out. Often caused by an environmental change the species could not adapt to fast enough. (Extinction is the failure-case of evolution and a piece of evidence FOR evolution.)

Notes

What is variation?

Variation means the differences between individuals in a population. Some are tall, some are short; some have brown eyes, some have blue; some plants survive drought, some die. Variation has three origins.

• Genetic — caused by the alleles inherited from parents. Examples: blood group, natural eye colour, flower colour in plants, sickle-cell trait.
• Environmental — caused by the conditions an individual develops in. Examples: a scar, a sun-tan, an accent, a plant kept in low light that grows tall and spindly.
• Combined (genes + environment) — most real traits. Height, body mass, plant yield and intelligence all depend on both the alleles you inherited and the environment you grew up in.

Only genetic variation can be passed on to offspring, so only genetic variation feeds evolution. Sun-tans don't get inherited; alleles do.

Where genetic variation comes from

Two sources at GCSE.

• Mutation — a random change in the DNA sequence. Mutations happen all the time in every cell. They are usually caused by errors in DNA replication or by environmental factors like UV light, ionising radiation or some chemicals.
• Sexual reproduction — fusion of gametes from two parents brings together a brand-new combination of alleles. Each gamete is itself genetically unique (meiosis shuffles the alleles). This is why brothers and sisters look different despite sharing parents. Asexual reproduction, by contrast, makes genetically identical offspring (clones) — fast and reliable, but with no new variation.

What mutations actually do

A mutation may or may not change the protein the gene codes for. AQA wants three outcomes memorised:

• Most mutations have no effect on the phenotype — they happen in non-coding DNA, or change a codon to one that still codes for the same amino acid, or are silenced by the rest of the genome.
• Some mutations have a small effect — they alter one amino acid in a protein, which may slightly change how the protein works.
• Very rare mutations have a large effect — they produce a noticeably different phenotype. If that new phenotype happens to suit the current environment, it can give an advantage; if not, it is a disadvantage. New phenotypes that do fit the environment can spread quickly through a population.

Mutations are random. The environment does not cause useful mutations on demand — it simply selects from whatever variation already exists.

Natural selection — the four-step mechanism

Evolution by natural selection follows the same logic in every example. Learn the four steps as a writing skeleton.

1. Variation — mutation and sexual reproduction produce a population of individuals with different alleles and different phenotypes.
2. Selection pressure — the environment changes or a challenge appears (a predator, a disease, a drought, an antibiotic). Some individuals happen to be better suited to the new conditions than others.
3. Survival of the fittest — individuals with advantageous traits survive long enough to breed; those without them are more likely to die before reproducing.
4. Inheritance — survivors pass their advantageous alleles to their offspring. Over many generations, the frequency of advantageous alleles in the population increases. The population has evolved.

Evolution is therefore defined as a change in the inherited characteristics of a population over time through this process. Given enough time, evolution can produce new species.

Worked example — antibiotic resistance in bacteria

This is the example AQA loves and a real-world problem.

1. A bacterial population reproduces rapidly. Random mutations occasionally produce a bacterium with a new allele that makes it resistant to a particular antibiotic.
2. A patient is treated with the antibiotic. The non-resistant bacteria are killed; the rare resistant one survives.
3. The resistant bacterium reproduces, passing the resistance allele to its daughter cells (bacteria can also pass plasmids carrying resistance genes between species).
4. Over time, the bacterial population becomes mostly resistant. The antibiotic stops working. MRSA is the textbook example. Slowing antibiotic resistance needs: using antibiotics only when needed, finishing every course, developing new antibiotics, and good hygiene to limit spread.

Other classic examples

• Peppered moth — pale moths were camouflaged on lichen-covered trees before the industrial revolution. Soot blackened the trees; pale moths were eaten by birds, dark mutants survived. Dark moths spread, then declined again as the air cleaned up.
• Darwin's finches — different islands had different food sources. Birds with beak shapes suited to local food survived and bred. Over generations, the islands ended up with different beak-shape species.

Selective breeding (artificial selection)

Humans use the same mechanism deliberately. Selective breeding is the process by which we choose which individuals breed, picking those with the desired characteristic in every generation. Repeated over many generations, the desired trait becomes more and more common.

Familiar examples:

• Crops — wheat with larger grains; disease-resistant varieties.
• Livestock — dairy cows that produce more milk; sheep with better wool.
• Pets — dogs bred for temperament, size, or appearance.
• Garden flowers — particular colours or scents.

### The problem: a smaller gene pool

Because only individuals with the desired trait are allowed to breed, the genetic variation in the population falls. This is sometimes called inbreeding. Two consequences:

• Disease vulnerability — the whole crop or breed shares similar genes. A new pathogen that can infect one plant can sweep through the entire field. AQA wheat questions ask exactly this.
• Genetic disorders — recessive harmful alleles become more likely to pair up. Pedigree dogs often have inherited health problems for this reason.

Evidence for evolution

• Fossil record — a long timeline of progressively more modern forms.
• Antibiotic resistance — observed evolution happening within years.
• Extinction — when the environment changes and a population cannot evolve fast enough, it dies out. Extinction is the flip side of evolution.

Exam tips

• Write natural selection as a FOUR-step sequence every time: variation → selection pressure → survival → inheritance. Examiners mark each step.
• State mutations are 'random changes in DNA' that 'may produce new alleles'. Both phrases score marks.
• Always say 'over many generations' or 'over time' — selective breeding and natural selection are not one-generation events.
• Name the selection pressure (antibiotic, predator, drought) and the advantageous trait explicitly. Vague answers like 'the strong survive' do not gain marks.
• On selective breeding questions, link 'reduced variation' to a concrete consequence: 'more susceptible to disease' or 'inherited disorders'.
• Distinguish artificial selection (humans choose) from natural selection (environment chooses). The mechanism is the same but the selector is different.
• When asked about antibiotic resistance, mention mutation FIRST, then selection. Never write that bacteria 'try' to become resistant — they don't try.

Mark-scheme phrasing

Common misconceptions

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Worked example

Question:

Answer:

Frequently asked questions

Related topics in AQA GCSE Biology (8461)

• 4.6.1 Reproduction
• 4.6.3 The development of understanding of genetics and evolution
• 4.6.4 Classification of living organisms