In simple terms
A friendly intro before the formal notes — no formulas yet.
How Populations Change Over Time
Individuals do not evolve — populations do. Within any population there is heritable variation. Because more offspring are produced than can survive, individuals carrying advantageous alleles tend to survive and reproduce more, so those alleles become more common in the next generation. Repeat this over many generations and the population changes: it evolves.
Think of a population as a huge deck of cards being reshuffled every generation, where each card is an allele. No single card changes its face — but if the red cards happen to help their holders survive and breed a little more often, then each time the deck is dealt again there are slightly more red cards in it. Nobody chose to make the deck redder, and no individual card tried to change colour; the proportions simply drifted toward red because red-holders left more copies behind. Over enough deals the deck looks completely different from the one you started with.
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A population contains heritable variation — differences in alleles that can be passed to offspring.
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Organisms overproduce: more offspring are born than the environment can support, so there is a struggle for limited resources.
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Individuals whose alleles give an advantage in that environment survive and reproduce more successfully (differential survival and reproduction).
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Because the advantage is heritable, the advantageous alleles increase in frequency over generations — the population becomes better adapted. Given reproductive isolation, two such populations can diverge until they become separate species.
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Full topic notes
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What evolution actually means
Evolution is defined as the change in the heritable characteristics of a population over time. Two words in that definition carry most of the weight. 'Heritable' matters because only variation that can be passed to offspring can accumulate across generations — a change caused purely by the environment during an organism's life (a tan, a well-exercised muscle) is not passed on and so is not evolution. 'Population' matters because evolution is a property of a group, not an individual: an individual's alleles are set at fertilisation and cannot change to suit its surroundings, but the PROPORTIONS of alleles in a population can shift as some individuals leave more offspring than others. Biologists therefore describe evolution quantitatively as a change in allele frequencies within a population from one generation to the next.
Evolution = change in the HERITABLE characteristics of a POPULATION over successive generations.
It is measured as a change in ALLELE FREQUENCIES between generations.
Individuals do not evolve; their alleles are fixed at fertilisation. Populations evolve.
Non-heritable changes acquired during life (e.g. a suntan) are not passed on and are not evolution.
Evidence for evolution
The case for evolution does not depend on any single observation. Several independent lines of evidence, gathered by different methods, all point to the same conclusion — descent with modification from common ancestors — and, crucially, they agree with one another. When anatomy, geography and molecular data all reconstruct the same relationships between species, that convergence is what makes the evidence compelling.
Molecular evidence is often treated as the most powerful of these lines because it is quantitative and independent of appearance: two organisms can look very different yet have highly similar DNA, and the number of sequence differences gives an objective measure of how closely related they are.
Fossil record: Fossils in dated rock layers show that life has changed through time, with older strata containing simpler or now-extinct forms and transitional fossils linking major groups. Gaps exist because fossilisation is rare, but the sequence of forms fits descent with modification.
Artificial selection (selective breeding): By choosing which individuals reproduce, humans have rapidly transformed populations — from wild grasses into maize, or wolves into hundreds of dog breeds. This shows directly that selecting heritable variation over generations changes a population, exactly as natural selection does but with humans as the selective agent.
Homologous structures: Structures sharing the same underlying anatomical plan despite different functions — such as the pentadactyl limb of humans, bats, whales and birds — indicate inheritance from a common ancestor. A shared basic plan is hard to explain unless the species descended from a form that already had it.
Biogeography: The geographic distribution of species fits common ancestry. Island species typically resemble the nearest mainland forms rather than ecologically similar species elsewhere, as expected if they descended from colonists that then diverged in isolation.
Molecular (DNA and protein) evidence: All life shares essentially the same genetic code, pointing to a single origin. Comparing DNA base sequences or protein amino-acid sequences shows that species which share a more recent common ancestor differ less; the pattern of differences reconstructs the same evolutionary tree as the anatomical and geographic evidence.
Natural selection: a chain of conditions
Natural selection is the main mechanism by which populations become better adapted to their environments. It is best understood not as a single step but as a chain of conditions, each of which must hold for the process to work. The logic is that variation exists first, by chance, and the environment then acts as a filter — it does not create the variation it selects. Because the chain has distinct links, exam mark schemes award a separate mark for each link you state clearly.
Notice what natural selection is NOT. It is not organisms striving, needing or trying to change; the variation is already present before the environment selects among it. It is not the inheritance of characteristics acquired during life — a giraffe that stretches its neck does not pass a longer neck to its calves. This mistaken 'use and disuse' idea is Lamarckism, and describing selection in those terms is one of the most heavily penalised errors in the exam.
Heritable variation: Individuals in a population differ, and some of these differences are heritable. New variation ultimately arises by MUTATION; sexual reproduction (meiosis and fertilisation) reshuffles existing alleles into new combinations.
Overproduction: Organisms produce far more offspring than the environment can support, so not all can survive.
Competition / struggle for existence: Because resources such as food, space, mates and light are limited, individuals compete and many die before reproducing.
Differential survival and reproduction: Individuals whose heritable characteristics give an advantage in that environment are, on average, more likely to survive and reproduce — this is 'survival of the fittest', where fitness means reproductive success.
Change in allele frequency: Because the advantageous characteristics are heritable, the alleles responsible are passed on more often and increase in frequency over generations. The population becomes better adapted — it evolves.
Adaptation
An adaptation is a heritable characteristic that improves an organism's chances of survival and reproduction in its environment. Adaptations are the RESULT of natural selection acting over many generations, not something an individual designs or acquires on purpose. It is therefore accurate to say 'the population became adapted to cold conditions because individuals with thicker fur survived and reproduced more', and inaccurate to say 'the animals grew thicker fur because they needed to keep warm'. The first describes selection among pre-existing heritable variation; the second smuggles in intention and Lamarckian inheritance. Keeping adaptation firmly in the past tense and at the population level is the safest way to phrase it.
Speciation: reproductive isolation drives divergence
A species can be thought of as a group of organisms that can interbreed to produce fertile offspring. Speciation is the formation of new species, and it happens when a single population is divided into groups that can no longer exchange genes — that is, when gene flow between them stops. Once gene flow is interrupted, mutation, natural selection and genetic drift act on each group independently. Over many generations the groups diverge genetically and phenotypically, and eventually the differences become so great that even if the groups meet again they can no longer produce viable, fertile offspring. At that point they are separate species. Reproductive isolation is therefore the pivotal condition: without it, gene flow keeps the population mixed and prevents lasting divergence.
Allopatric versus sympatric speciation
Biologists classify speciation by the geographical setting in which reproductive isolation arises — specifically, whether a physical barrier is responsible for separating the populations.
Allopatric speciation: Isolation is GEOGRAPHIC. A physical barrier — a mountain range, river, glacier or stretch of ocean — divides a population so that the groups can no longer meet. Cut off from one another, they experience different selection pressures and undergo independent genetic drift, and diverge until reproductively isolated. Island groups descended from a mainland colonist, such as species radiating across separate islands, are classic examples.
Sympatric speciation: Isolation arises WITHIN the same area, with no physical barrier. Reproductive isolation comes instead from behavioural, temporal or genetic mechanisms. Behavioural isolation: differences in courtship or mating signals mean individuals no longer recognise each other as mates. Temporal isolation: groups breed at different times of day, season or year, so they never interbreed. Genetic isolation by polyploidy: an error in cell division doubles the chromosome number, and the resulting polyploid — common in plants — is instantly unable to breed fertilely with the parent population, because crosses give offspring with an odd chromosome number that cannot pair chromosomes correctly at meiosis and so are sterile.
The pace of evolution: gradualism and punctuated equilibrium
How fast does evolutionary change happen? Two models describe the tempo, and the fossil record contains evidence for both. Gradualism, the view associated with Darwin, holds that evolution proceeds by the slow, steady accumulation of many small changes, so a complete fossil record would show long series of intermediate forms. Punctuated equilibrium, proposed by Eldredge and Gould, holds that species often remain little changed for long periods (stasis), and that these periods are interrupted by short bursts of rapid change, typically around speciation events; on this view intermediate fossils are genuinely rare rather than merely undiscovered. The two models are not mutually exclusive — different lineages, in different environments, appear to follow different tempos, so most biologists regard both as real and context-dependent.
Gradualism: slow, continuous accumulation of small changes; predicts many intermediate forms.
Punctuated equilibrium: long stasis interrupted by short, rapid bursts of change; predicts few intermediates and helps explain gaps in the fossil record.
Both patterns are seen in the fossil record; they describe the pace of change, not different mechanisms of inheritance.
Whenever you write about how a characteristic evolved, run a quick language check before moving on. Have you said the variation was already there and HERITABLE? Have you said individuals with the advantageous alleles SURVIVED and REPRODUCED more? Have you said the alleles increased in FREQUENCY in the POPULATION over generations? If any sentence instead says an organism 'tried to', 'wanted to', 'needed to' or 'learned to' change, rewrite it — that phrasing is Lamarckian and scores nothing.
Common mistakes examiners penalise
Saying an individual evolves or 'adapts itself' — evolution is a change in a POPULATION's allele frequencies over generations; an individual's alleles are fixed at fertilisation. Selection acts on individuals, but only the population evolves.
Lamarckian 'need / want / try' wording — organisms do not change because they need to, nor pass on characteristics acquired during life. Variation exists first, by chance, and the environment selects among it. This is the single most heavily penalised error in the topic.
Treating variation as non-heritable — for selection to cause evolution, the variation must be HERITABLE (rooted in alleles). A difference caused only by the environment during life cannot be passed on and does not drive evolution.
Confusing evolution with natural selection — natural selection is a MECHANISM; evolution is the resulting change in the population. State the process step by step, then name the change.
Forgetting that speciation requires the loss of gene flow — without reproductive isolation, gene flow keeps populations mixed and prevents lasting divergence. Always identify the isolating barrier.
Mixing up allopatric and sympatric speciation — allopatric needs a GEOGRAPHIC barrier; sympatric occurs in the same area through behavioural, temporal or genetic (polyploidy) isolation. Name which one you mean.
Describing fitness as strength or size — 'fitness' means reproductive success in that environment, not physical fitness. The 'fittest' are simply those that leave the most fertile offspring.
Model answer — marked the way our engine marks it
A4.1 explanation questions are marked analytically: each distinct valid point is worth one mark, method (M) marks credit correct reasoning at each step of the chain, answer (A) marks credit a correct concluding statement, and error-carried-forward (ECF) means a slip in one step does not cost you the independent marks around it. Study how each mark below is tied to a specific, named idea rather than to loose phrasing — and note exactly which phrasing the engine refuses to credit.
Where this leads
Evolution by natural selection is the thread running through the rest of biology. The same logic — heritable variation filtered by differential reproduction — explains antibiotic resistance in bacteria, pesticide resistance in insects and the diversity revealed by molecular phylogenies. Master the habit of writing evolution as a population-level change in allele frequencies, driven by selection among pre-existing heritable variation and never by an organism's needs, and you have a template that answers evolution questions across the whole course.
Worked examples
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A population of insects lives across a valley. A new, permanently flooded river channel forms and divides the valley, and the insects cannot cross open water. Explain how this could lead to allopatric speciation. [4]
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Model answer. The river is a geographic barrier that splits the single population into two, preventing gene flow between them. The environments on the two sides differ, so the two populations experience different selection pressures; different mutations also arise and genetic drift acts independently in each. Because there is no gene flow between them, the allele frequencies of the two populations change independently over many generations, and the populations diverge. Eventually the genetic differences become so great that, even if the insects met again, they could not interbreed to produce fertile offspring — they have become two separate species.
Explain how natural selection can lead to the evolution of a population that is better adapted to its environment. [4]
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Model answer. Within the population there is heritable variation, which ultimately arises from mutation. More offspring are produced than the environment can support, so individuals must compete for limited resources and not all survive. Individuals whose alleles give characteristics better suited to the environment are, on average, more likely to survive and reproduce. Because these advantageous characteristics are heritable, the alleles responsible are passed on to more offspring and so increase in frequency in the population over successive generations. Over time the population comes to consist mainly of individuals with the advantageous alleles — it has become better adapted, which is evolution.
How it all connects
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Glossary
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Revision flashcards
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Evolution
The change in the heritable characteristics of a POPULATION over successive generations. It is measured as a change in allele frequencies, not as change within one individual.
Key takeaways
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Evolution = change in the HERITABLE characteristics of a POPULATION over successive generations.
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It is measured as a change in ALLELE FREQUENCIES between generations.
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Individuals do not evolve; their alleles are fixed at fertilisation. Populations evolve.
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Non-heritable changes acquired during life (e.g. a suntan) are not passed on and are not evolution.
Practice — then mark it
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Get a Paper 2 question marked: explain natural selection and speciation with each step credited point by point
Get a Paper 2 question marked: explain natural selection and speciation with each step credited point by point
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