4.1 Evolution by natural selection

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) because of random mutations in DNA. Third, when the environment changes — a new predator, a new disease, a colder winter, 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, the different beak shapes Darwin saw in finches on the Galapagos islands, and the snowshoe hare's seasonal white winter coat. Edexcel keeps the focus tight: just the mechanism and how to apply it to named examples.

How to learn this topic

Build on what you already know

• Edexcel Topic 3: DNA, genes, alleles, mutations; sexual vs asexual reproduction.
• Edexcel Topic 1: cells reproduce by mitosis; gametes are produced by meiosis.
• Edexcel Topic 6: pathogens and the use of antibiotics against bacteria.
• KS3: idea that animals and plants are adapted to their environment.

1. State the five steps of natural selection as a fixed writing skeleton.
2. Explain where variation comes from: random mutations in DNA.
3. Define selection pressure and give examples (predator, food, climate, disease, antibiotic).
4. Worked example: antibiotic-resistant bacteria (MRSA).
5. Apply the skeleton to peppered moths and Darwin's finches.
6. Apply the skeleton to snowshoe hares and rock pocket mice.
7. Practise five-step mark-scheme answers on fresh, unfamiliar scenarios.

Key terms

natural selection

The process by which individuals with traits best suited to their environment are more likely to survive and reproduce, passing on their advantageous alleles. Over many generations the population evolves. (Edexcel five-line skeleton: variation → selection pressure → survival of the fittest → inheritance → change over many generations. Every step scores.)

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.)

variation

Differences in the phenotypes of individuals in a population. At Edexcel 4.1 the source you must cite is random mutation in DNA. (Edexcel reward 'variation exists in the population due to random mutation'. State this first line every time.)

mutation

A random change in the DNA sequence of a cell. Most mutations are neutral, some are harmful, very rarely a mutation is advantageous and produces a new allele. (Always say mutation is RANDOM. Edexcel marking phrase: 'random mutation produces a new allele'.)

allele

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

selection pressure

A feature of the environment (predator, food shortage, climate, disease, antibiotic) that means not all individuals will survive. (Name the specific pressure (antibiotic, predator, food shortage) in your answer — vague answers lose marks.)

survival of the fittest

Individuals with traits better suited to their environment are more likely to survive long enough to reproduce. 'Fittest' means best suited to the environment, not physically strongest. (Edexcel accept 'best suited to the environment' as the marking phrase. Avoid 'strongest'.)

advantageous allele

An allele that gives its carrier a better chance of surviving and reproducing in the current environment. Advantageous alleles increase in frequency over generations. (Use the phrase 'pass advantageous alleles to offspring' — it picks up the inheritance mark.)

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.)

population

A group of individuals of the same species living in the same area. Evolution acts on populations, not on individuals. (Edexcel mark schemes always say 'over many generations the proportion of __ in the population increases'.)

species

A group of organisms that can interbreed to produce fertile offspring. Over enough time, natural selection can produce new species from a single ancestor. (Speciation is mentioned in Edexcel 4.1 only as the long-term outcome of evolution.)

Notes

The Edexcel five-step mechanism

Edexcel 1BI0 4.1 treats natural selection as a five-step process. Memorise the skeleton — every answer on this topic is a variation on the same five lines.

1. Variation — members of a species differ from each other because of random mutations in DNA. Most mutations are neutral, some are harmful, very occasionally one is advantageous.
2. Selection pressure — a feature of the environment (predators, food shortage, climate, disease, antibiotics) means not every individual will survive.
3. Survival of the fittest — individuals whose traits are better suited to the environment are more likely to survive long enough to reproduce.
4. Inheritance — those survivors pass their advantageous alleles to their offspring.
5. Over many generations — the proportion of individuals carrying the advantageous allele increases. The population has evolved.

Evolution is therefore a change in the inherited characteristics of a population over time through natural selection. Note: populations evolve, individuals do not.

Where variation comes from

At Edexcel level you only need one source: random mutation in DNA. Mutations happen all the time as cells copy their DNA, and can be triggered by UV light, ionising radiation or some chemicals. Sexual reproduction then shuffles those alleles between parents so that every offspring is a fresh combination. Three outcomes for any mutation:

• Most mutations are neutral — they sit in non-coding DNA, or change a codon to one that still codes for the same amino acid, or leave a protein still working.
• Some mutations are harmful — they damage a protein and reduce the individual's chance of surviving.
• Very rarely, a mutation is advantageous — it produces a new phenotype that happens to suit the current environment.

Crucially, mutations are random. The environment does not cause useful mutations to appear on demand — it merely selects from the variation that already exists.

Selection pressure

A selection pressure is any environmental factor that affects who survives long enough to breed. The Edexcel-named pressures are:

• Predators — visible prey get eaten first.
• Food shortage — only individuals who can find or digest available food survive.
• Climate — cold winters favour thicker fur; hot summers favour heat tolerance.
• Disease — only individuals with effective immunity survive an epidemic.
• Antibiotics (for bacteria) — only resistant bacteria survive treatment.

Naming the specific pressure in your answer scores marks. Vague phrases like "the strong survive" do not.

Worked example — antibiotic-resistant bacteria (MRSA)

This is Edexcel's flagship example.

1. A bacterial population reproduces rapidly. Random mutation occasionally produces a bacterium with an allele that makes it resistant to a particular antibiotic.
2. A patient is treated with the antibiotic. The antibiotic is the selection pressure: non-resistant bacteria are killed; the rare resistant bacterium survives.
3. The resistant bacterium reproduces, passing the resistance allele to its daughter cells. (Bacteria can also share plasmids carrying resistance genes.)
4. Over many generations the proportion of resistant bacteria in the population increases. Eventually the antibiotic stops working. MRSA — methicillin-resistant Staphylococcus aureus — is the textbook case.

Slowing the spread: only prescribe antibiotics when needed, finish every course, develop new antibiotics, limit use in farming, and maintain hospital hygiene.

Peppered moths

Before the industrial revolution, tree bark in Britain was pale and lichen-covered. Pale peppered moths were camouflaged; rare dark mutants were eaten by birds.

1. Variation exists in moth wing colour — pale and dark forms — produced by mutation.
2. As coal soot blackened tree trunks, dark moths were better camouflaged and pale moths became visible to birds (the selection pressure).
3. Dark moths survived and reproduced, passing the dark allele to offspring.
4. Over many generations the frequency of dark colouration increased in industrial areas. After clean-air laws the trend reversed.

Snowshoe hares

Snowshoe hares grow a white winter coat that camouflages them against snow.

1. Variation exists in the original hare population — some have thicker, whiter winter coats than others, because of random mutations.
2. White fur provides camouflage in snow, hiding hares from predators such as lynx; thin-coated hares freeze.
3. Hares with advantageous traits survive to reproduce, passing the alleles for white, thick winter fur to their offspring.
4. Over many generations the proportion of hares with thick/white fur increases in snowy regions.

Darwin's finches

On the Galapagos Islands, different islands offer different food: seeds, insects, cactus flowers. The finches on each island have evolved beak shapes that match.

1. Variation in beak shape in the original founding population, produced by mutation and sexual reproduction.
2. On each island, the food available acts as a selection pressure: finches whose beak shape matched the local food found more to eat.
3. Better-fed finches survive and reproduce, passing on the alleles for that beak shape.
4. Over many generations each island ends up with a population whose beak shape suits its food source — and the islands now host distinct species.

Rock pocket mice

A classic exam scenario. In the American south-west, populations of rock pocket mice live on either pale sandy desert or dark volcanic rock.

1. Variation in coat colour exists in the original mouse population, produced by mutation.
2. On dark volcanic rock, light-coloured mice are more visible to predators (owls, hawks); on pale sand, dark mice stand out.
3. Dark-coloured mice on dark rock survive to reproduce, passing on their alleles.
4. Over many generations the frequency of the dark colouration allele increases in populations living on volcanic rock.

How to write a full-mark Edexcel answer

Whatever the scenario, walk through the same five lines and name the specifics.

• Start with "variation exists in ___ because of random mutation".
• Identify the selection pressure (predator, food, climate, antibiotic).
• Say which trait is advantageous and why.
• Say the advantaged individuals survive and reproduce, passing on alleles.
• Finish with "over many generations the proportion of ___ increases".

Miss any step and you miss a mark.

Exam tips

• Write natural selection as a FIVE-step sequence every time: variation → selection pressure → survival of the fittest → inheritance → change over many generations. Edexcel mark each step.
• Name mutation as a 'random change in DNA' that may produce a 'new advantageous allele'. Both phrases score marks.
• Always say 'over many generations' — natural selection is not a one-generation event. Edexcel mark schemes use this exact phrase.
• Name the specific selection pressure (antibiotic, predator, snow, food shortage) and the specific advantageous trait. Vague answers like 'the strong survive' do not score.
• Avoid teleological language. Write 'dark moths happened to be camouflaged', not 'moths became dark so they could hide'.
• On antibiotic-resistance questions, put mutation FIRST and selection second. Never write that bacteria 'try' to become resistant.
• Use the word 'allele' (not just 'gene') when describing what is passed on — Edexcel often have allele as a specific marking point.
• On Edexcel 6-mark questions, follow the five-step skeleton AND name the species, the trait and the pressure for full marks.

Mark-scheme phrasing

Common misconceptions

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Worked example

Question:

Answer:

Frequently asked questions