B5.2 Natural selection 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. For OCR Gateway B5.2 you don't need to read Darwin in full, but you do need the mechanism cold. There are three pieces. First, individuals in any population differ from each other — some red, some white, some tall, some short, some thick-furred, some thin. Second, those differences are partly inherited from parents (genes / alleles) and partly caused by the environment, usually a mix of both. Third, when the environment changes — a new predator on the seashore, soot on tree bark, an antibiotic added to a bacterial colony, a colder winter — 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. OCR likes to test this with five rubric-anchored examples: pebble crabs on a beach, longer-necked giraffes, antibiotic-resistant bacteria, peppered moths during industrialisation, and Arctic foxes with thick fur. Evidence for evolution in J247 has three strands — the fossil record (limited because soft-bodied organisms rarely fossilise), antibiotic resistance (evolution we can watch within years) and extinction (the failure mode that natural selection predicts). Speciation, the final piece of B5.2, happens when two populations of the same species are separated for long enough that they can no longer interbreed.

How to learn this topic

Build on what you already know

• GCSE B5.1: DNA, genes, alleles, chromosomes; sexual vs asexual reproduction.
• GCSE B2: cell division — mitosis produces identical cells; meiosis produces genetically different gametes.
• GCSE B3: 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 neutral, occasionally advantageous.
3. Build natural selection step by step: variation → selection pressure → survival → reproduction → inheritance.
4. Walk through the five OCR rubric examples: pebble crabs, giraffes, antibiotic resistance, peppered moths, Arctic foxes.
5. Evidence for evolution: fossils (with limitations), antibiotic resistance, extinction.
6. Speciation — when separated populations stop being able to interbreed.
7. 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. (OCR rewards 'differences in characteristics between individuals'. Only genetic variation is heritable.)

mutation

A random change in the DNA sequence of a cell. Most mutations are neutral, some are harmful, and occasionally one produces an advantage in the current environment. (Always describe mutation as RANDOM. OCR marking phrase: 'random change in DNA'. The 'occasionally advantageous' framing is OCR-specific.)

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/reproduction → inheritance. OCR awards one mark per 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, cold climate) that affects an organism's chances of surviving and reproducing. (Naming the specific pressure (antibiotic, gull predation, sooty bark, cold) makes answers concrete and gains marks.)

advantageous allele

An allele that gives the individual carrying it a better chance of surviving and reproducing in the current environment. The phrase OCR examiners look for in natural selection answers. (Use 'advantageous allele(s) passed on/inherited' verbatim — OCR mark schemes quote this phrase repeatedly.)

speciation

The formation of a new species when two populations are separated for long enough that they can no longer interbreed to produce fertile offspring. (Mention isolation (geographical barrier) plus different selection pressures plus inability to interbreed.)

fossil record

The collection of preserved remains and traces of organisms in rocks, ordered by age. Provides evidence for evolution over very long timescales. (Limitation: soft-bodied organisms decay before fossilising, so the record is incomplete.)

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.)

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.)

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

OCR Gateway wants three outcomes memorised:

• Most mutations are neutral — 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 are harmful — they alter an important amino acid in a protein and the protein no longer works properly.
• Occasionally a mutation produces an advantage in the current environment — for example a bacterium that survives an antibiotic, or a moth that matches a sooty tree. If that new phenotype suits the conditions, it 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 — the OCR mark scheme awards a mark for each.

1. Variation — random mutation produces a population of individuals with different alleles and different phenotypes.
2. Selection pressure — the environment poses a challenge (a predator, a disease, a drought, an antibiotic, a cold winter). Some individuals are better suited to the conditions than others.
3. Survival and reproduction — 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.

OCR rubric example 1 — pebble crabs and a predator

On a stony shore, crabs vary in shell colour: some are brightly coloured, some are grey to match the pebbles. A bird predator hunts by sight.

• Variation in shell colour exists in the population (random mutation, sexual reproduction).
• Brightly coloured crabs are more visible to the predator and are eaten before they can breed.
• Grey crabs reproduce and pass on advantageous alleles for grey shell colour.
• Over many generations, grey colour becomes more common in the population.

OCR rubric example 2 — giraffe necks

• Variation in neck length exists in giraffes due to mutation and sexual reproduction.
• Longer-necked giraffes reach food (leaves high in trees) that shorter-necked giraffes cannot reach.
• Longer-necked giraffes are more likely to survive through droughts when only high leaves are left, and so to reproduce.
• Advantageous alleles are inherited by offspring, so over generations average neck length rises.

OCR rubric example 3 — antibiotic resistance in bacteria

This is the example OCR uses most often and a real-world problem.

• A random mutation gives some bacteria antibiotic resistance — a new allele appears in one bacterium.
• A patient is treated. Bacteria without the resistant allele are killed by the antibiotic.
• The surviving resistant bacteria reproduce and pass on the resistance allele to their daughter cells (and can swap plasmids with other species).
• Over generations the resistant allele increases in frequency until most of the population is 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.

OCR rubric example 4 — peppered moths

Before the industrial revolution, peppered moths were mostly pale and rested on lichen-covered trees. As coal smoke killed lichen and blackened bark:

• Random mutation causes variation in wing colour — some moths happen to be dark.
• Dark moths are better camouflaged against sooty bark and pale moths now stand out to bird predators.
• Dark moths survive longer and reproduce more than pale moths.
• The proportion of dark moths increases over generations. After the Clean Air Acts, the trend reversed.

OCR rubric example 5 — Arctic foxes thick fur

In the Arctic, winters are extreme and cold.

• Variation exists in the population — some foxes have thicker fur than others.
• Thicker fur is an advantage in cold because it insulates better and reduces heat loss.
• The allele for thicker fur is passed on to offspring when thick-furred foxes survive and breed.
• The thicker-fur allele becomes more common over generations in cold environments.

Evidence for evolution

OCR Gateway B5.2 wants three strands of evidence.

• Fossil record — older rock layers contain simpler organisms; younger layers show more modern forms. Limitation: soft-bodied organisms decay before fossilisation, so the record is incomplete and we cannot trace every lineage.
• Antibiotic resistance — evolution observed within years, in real time, in hospitals and farms.
• Extinction — when the environment changes and a population cannot evolve fast enough, it dies out. Extinction is the failure case of evolution and itself counts as evidence the process is happening.

Speciation

Speciation is the formation of a new species. It happens when two populations of the same species are separated — typically by a geographical barrier (a mountain range, a stretch of ocean, an island chain) — and natural selection acts on them in different ways because their environments are different. Over many generations the populations diverge so far genetically that, if reunited, they can no longer interbreed to produce fertile offspring. They are now two species. Darwin's finches on the Galapagos are the classic example: a single ancestral finch species reached the islands, and on each island natural selection favoured different beak shapes for different foods, producing multiple separate species.

Exam tips

• Write natural selection as a FOUR-step sequence every time: variation → selection pressure → survival/reproduction → inheritance. OCR marks each step.
• Use the phrase 'advantageous allele(s) passed on to offspring' — it is the verbatim OCR mark-scheme phrase.
• State mutations are 'random changes in DNA' and that they 'may produce new alleles'. Both score.
• Always say 'over many generations' or 'over time' — natural selection and selective breeding are not one-generation events.
• Name the selection pressure (antibiotic, gull predation, sooty bark, cold winter) and the advantageous trait explicitly. Vague answers like 'the strong survive' do not gain marks.
• For the fossil record, always quote the limitation: 'soft-bodied organisms decay before fossilisation'.
• Distinguish artificial selection (humans choose) from natural selection (environment chooses). The mechanism is the same but the selector is different.
• On antibiotic-resistance questions, mention mutation FIRST, then selection. Never write that bacteria 'try' to become resistant — they don't try.
• For speciation, state that the two populations end up unable to interbreed to produce fertile offspring.

Mark-scheme phrasing

Common misconceptions

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Worked example

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Frequently asked questions

Related topics in OCR GCSE Biology A: Gateway Science (J247)

• B5.1 Inheritance