Theory of evolution (biology only) — AQA GCSE Biology
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Theory of evolution (biology only) explained
This statement explains how Charles Darwin developed the theory of evolution by natural selection.
Read the full explanation
During a round-the-world expedition, Darwin made observations of living organisms and fossils in many places. He continued to experiment and discuss his ideas for years afterwards. At the same time, knowledge of geology and fossils was developing, which helped him interpret his observations. For example, fossils of extinct species and geological evidence of gradual change supported the idea that living things change over long periods. Darwin proposed that organisms with characteristics best suited to their environment are more likely to survive and reproduce, passing on those characteristics. Over many generations, this natural selection can lead to evolution and to the formation of new species.
Individual organisms within a particular species show a wide range of variation for a characteristic.
Members of the same species share the ability to interbreed and produce fertile offspring, but they are not identical. Within a species, individuals differ in many characteristics, such as height, mass, fur colour, leaf length or disease resistance. This variation arises from differences in genes, including mutations, and from environmental factors such as diet, exercise, light and temperature. Some variation is continuous, where measurements fall along a range and can take any value, for example human height. Other variation is discontinuous, where individuals fall into distinct categories, for example blood group or flower colour. This wide range of variation is important because it provides the raw material on which natural selection can act.
Individuals with characteristics most suited to the environment are more likely to survive to breed successfully.
This is the core of natural selection. Within a population, individuals vary because of differences in their genes, and some variants give a better fit to the local conditions. Those better-suited individuals are more likely to survive, and because they survive longer they are more likely to reproduce and pass their alleles on. A concrete case: in a population of beetles on green leaves, green individuals are harder for birds to spot than brown ones, so more green beetles live to breed. The environment does the selecting, not the organism, and no individual changes to suit the environment during its life. Selection acts on the variation already present, so the population shifts over generations rather than within one lifetime.
The characteristics that have enabled these individuals to survive are then passed on to the next generation.
This statement completes natural selection by explaining inheritance. The survivors breed, and their gametes carry the alleles behind the useful characteristics, so those alleles become more common in the next generation. Over many generations the proportion of well-suited individuals rises and the population changes. The key point is that characteristics are not passed on because they were useful; they are passed on because the alleles controlling them were present in the parents who reproduced. A concrete case: green beetles survive and lay eggs, and their offspring inherit the alleles for green colouration, so the next generation contains more green beetles. Acquired changes during life, such as a suntan or a scar, are not inherited in this way.
Darwin published his ideas in On the Origin of Species (1859). There was much controversy surrounding these revolutionary new ideas.
Charles Darwin spent years collecting evidence, including observations from the voyage of HMS Beagle and studies of finches and tortoises in the Galápagos Islands. He developed the theory of evolution by natural selection and, after receiving a similar idea from Alfred Russel Wallace, published On the Origin of Species in 1859. The book presented evidence that species change over time and are related through common ancestry. It provoked controversy because it challenged literal readings of religious accounts of creation, suggested humans share ancestry with other animals, and was published before genetics could explain how variation arose. Many scientists and religious leaders debated the work, but evidence from fossils, anatomy and later genetics gradually supported it.
The theory of evolution by natural selection was only gradually accepted because: • the theory challenged the idea that God made all the animals and plants that live on Earth • there was insufficient evidence at the time the theory was published to convince many scientists • the mechanism of inheritance and variation was not known until 50 years after the theory was published.
Darwin and Wallace proposed evolution by natural selection in the mid-19th century, yet acceptance took decades. Three barriers explain the delay. First, the theory challenged the widely held religious belief that God created all species in their present form, so many people rejected it on faith rather than evidence. Second, the evidence available in 1859 was limited: Darwin had finches, fossils and artificial selection, but no direct demonstration of how variation arose or how traits passed to offspring. Third, the mechanism of inheritance was unknown; Mendel's work on 'units' was published in 1866 but ignored, and chromosomes were not linked to heredity until the early 20th century, roughly 50 years later. Without a mechanism, natural selection seemed incomplete. Acceptance grew as genetics, the modern synthesis and the fossil record provided supporting evidence.
Other theories, including that of Jean-Baptiste Lamarck, are based mainly on the idea that changes that occur in an organism during its lifetime can be inherited.
Lamarck's theory of evolution proposed that characteristics acquired during an organism's lifetime could be inherited by its offspring. For example, if a giraffe stretched its neck to reach high leaves, Lamarck suggested the lengthened neck would be passed on, so each generation would have a slightly longer neck. This differs from Darwin's theory of natural selection, in which variation already exists and individuals with favourable inherited characteristics are more likely to survive and reproduce. Lamarck's idea is not supported by modern genetics because changes acquired during life, such as muscle growth or scars, generally do not alter the inherited material passed to offspring.
We now know that in the vast majority of cases this type of inheritance cannot occur.
This statement corrects an early idea about how characteristics pass from parents to offspring. Lamarck proposed that features acquired during an organism's life, such as a giraffe stretching its neck to reach high leaves, could be inherited by its young. We now understand that most body changes gained during life are not written into the gametes, so they are not passed on. Only changes to the DNA of gametes, such as new alleles formed by mutation, can be inherited. A stretched neck therefore does not make a longer-necked calf. The rare exceptions involve epigenetic marks or environmental effects on gametes, but these are not the main route of inheritance.
Your focus
- Describe how Darwin's expedition observations contributed to his theory.
- Explain how experimentation, discussion, geology and fossils supported the theory of evolution by natural selection.
- State the main idea of evolution by natural selection.
Show all 24 objectives
- Describe examples of continuous and discontinuous variation within a species.
- Explain how genetic and environmental factors cause variation.
- Relate variation within a species to the process of natural selection.
- Describe how inherited variation within a population leads to some individuals being better suited to their environment.
- Explain why better-suited individuals are more likely to survive and breed successfully.
- Apply the idea of natural selection to an unfamiliar organism and its environment.
- Explain how alleles for useful characteristics are passed to offspring by surviving individuals.
- Describe how the proportion of individuals with a useful characteristic increases over generations.
- Distinguish inherited alleles from characteristics acquired during an organism's lifetime.
- State the title and date of Darwin's major publication.
- Explain why Darwin's ideas caused controversy.
- Describe how evidence from different sources supported the theory over time.
- Describe the three reasons why the theory of evolution by natural selection was only gradually accepted.
- Explain how the lack of a known mechanism of inheritance delayed acceptance of natural selection.
- Relate the eventual acceptance of the theory to later discoveries about genes and chromosomes.
- Describe Lamarck's theory that changes during an organism's lifetime can be inherited.
- Compare Lamarck's theory with Darwin's theory of evolution by natural selection.
- Explain why acquired characteristics are generally not inherited according to modern genetics.
- Describe Lamarck's idea that characteristics acquired during life can be inherited.
- Explain why most acquired characteristics cannot be inherited because they do not change gamete DNA.
- Use a named example to show that this type of inheritance is rejected in the vast majority of cases.
Theory of evolution (biology only) exam tips
Marking Points
- Darwin made observations of organisms and fossils during a round-the-world expedition.
- He backed up his ideas with years of experimentation and discussion.
- His thinking was linked to developing knowledge of geology and fossils.
- He proposed the theory of evolution by natural selection.
- Natural selection explains how species change over many generations as favourable characteristics are passed on.
- Individuals of the same species are not identical; they differ in many characteristics.
- Variation within a species can be continuous, such as height or mass, or discontinuous, such as blood group.
- Genetic causes of variation include different alleles and new mutations.
- Environmental causes of variation include diet, exercise, light intensity and temperature.
- Most characteristics are affected by both genes and the environment.
- Variation within a species is the raw material for natural selection and evolution.
- States that variation exists between individuals in a population, and that this variation is inherited.
- Explains that the environment presents a selection pressure, such as predation, disease or competition for resources.
- Links the characteristic to survival: individuals whose characteristics suit the environment are more likely to survive.
- Links survival to reproduction: survivors are more likely to breed successfully and so contribute offspring to the next generation.
- Uses a specific example, such as green beetles being camouflaged on green leaves, to show the selection pressure acting on a named characteristic.
- Avoids teleological wording: the organism does not choose to change, and the environment does not create the useful characteristic on demand.
- States that the survivors reproduce and so contribute gametes to the next generation.
- Explains that the alleles for the useful characteristics are present in the gametes and are therefore inherited by offspring.
- Links inheritance to a change in the population: the proportion of individuals with the useful characteristic increases over generations.
- Distinguishes inherited alleles from characteristics acquired during an organism's lifetime, which are not passed on in this way.
- Uses a specific example, such as green beetles passing on alleles for green colouration to their offspring.
- Recognises that this process repeated over many generations can lead to evolution of the population.
- Darwin published On the Origin of Species in 1859, setting out evolution by natural selection.
- The book included evidence gathered from observations and specimens, including the Galápagos finches and tortoises.
- The ideas were controversial because they conflicted with religious accounts of creation and with the view that species were fixed.
- Alfred Russel Wallace independently reached similar conclusions, prompting Darwin to publish.
- Acceptance was gradual because the mechanism of inheritance was not understood at the time.
- State that the theory challenged the religious idea that God made all animals and plants, so it conflicted with accepted beliefs.
- Explain that evidence available when the theory was published was insufficient to convince many scientists.
- Explain that the mechanism of inheritance and variation was not understood until about 50 years after publication.
- Link the delay in acceptance to the later discovery of genes, chromosomes and DNA, which supplied the missing mechanism.
- Use the example of Darwin's finches or fossil evidence to show how evidence accumulated gradually.
- Recognise that scientific ideas are accepted when evidence and mechanisms support them, not immediately on publication.
- State that Lamarck's theory is based mainly on the idea that changes occurring in an organism during its lifetime can be inherited.
- Describe the classic example of the giraffe's neck being stretched during life and the lengthened neck being passed on.
- Contrast Lamarck's idea with Darwin's theory of natural selection, where inherited variation already exists in the population.
- Explain that acquired characteristics are generally not inherited because they do not change the inherited material passed to offspring.
- Use a second example, such as a blacksmith developing strong muscles, to show why acquired changes are not passed on.
- State that Lamarck's idea involved characteristics acquired during an organism's lifetime being passed to offspring.
- Explain that most acquired characteristics do not alter the DNA of gametes, so they cannot be inherited.
- Contrast this with inherited variation caused by mutations that produce new alleles in gametes.
- Use a correct example, such as a giraffe's stretched neck not being inherited, to show the idea is rejected.
- Recognise that inheritance depends on genetic material passed from parents to offspring, not on use or disuse of a body part.
Examiner Tips
- 💡Name the expedition and state that Darwin made observations of living things and fossils.
- 💡Mention the roles of experimentation, discussion, geology and fossils in developing the theory.
- 💡Define natural selection clearly, linking survival, reproduction and the passing on of favourable characteristics.
- 💡When describing a graph, name the type of variation and quote a value or category from the data to support your answer.
- 💡Use the terms continuous and discontinuous accurately, and give one example of each.
- 💡For questions on evolution, link variation to survival: individuals with advantageous variations are more likely to survive and reproduce.
- 💡Use the chain variation → selection pressure → survival → reproduction → inheritance, and make each arrow explicit in your answer.
- 💡Name the characteristic and the pressure in your example, for example 'green colouration' and 'predation by birds', rather than writing 'better adapted' on its own.
- 💡If a question says 'explain', give the mechanism; if it says 'describe', state the pattern without needing every causal link.
- 💡Write 'alleles' rather than 'genes' when explaining what is passed on, and say they are carried in the gametes.
- 💡Finish the selection story with the next generation: state that the useful alleles become more common in the population.
- 💡Use the phrase 'over many generations' to show you understand the timescale rather than a single breeding event.
- 💡Quote the date 1859 and the title On the Origin of Species accurately.
- 💡Give a specific reason for controversy, such as conflict with religious accounts of creation, rather than saying people disagreed.
- 💡Mention Wallace as a useful supporting detail when explaining why Darwin published when he did.
- 💡Name all three reasons in the statement and give a short explanation for each to secure full credit.
- 💡Use the phrase 'mechanism of inheritance' when discussing the 50-year gap, and link it to genes and chromosomes.
- 💡If asked why acceptance was gradual, structure your answer around religion, evidence and mechanism rather than writing a general history of Darwin.
- 💡Use the giraffe example to make the difference between Lamarck and Darwin clear in one sentence.
- 💡Include the phrase 'changes during its lifetime' to show you know the key feature of Lamarck's theory.
- 💡When comparing theories, state one similarity and one difference to structure your answer.
- 💡Link the statement to a named scientist, such as Lamarck, so the examiner sees the historical context.
- 💡Use the phrase 'acquired characteristics' and explain why they are not usually passed on through gametes.
- 💡If asked to compare, set out one clear difference between Lamarck's idea and natural selection rather than listing many vague points.
Common Mistakes
- Thinking Darwin proposed his theory immediately after the expedition: he spent years experimenting and discussing before publishing.
- Believing Darwin worked alone without using other scientific knowledge: geology and fossil evidence helped shape his ideas.
- Confusing natural selection with the inheritance of acquired characteristics: natural selection acts on inherited variation, not on characteristics gained during life.
- Confusing variation within a species with differences between species; correct this by stating that members of one species can interbreed, while different species generally cannot.
- Treating all variation as genetic; correct this by explaining that the environment also causes differences, and many characteristics are influenced by both.
- Describing discontinuous variation as a smooth range of values; correct this by stating that discontinuous variation produces distinct categories, while continuous variation produces a range.
- Saying an individual 'adapts' during its lifetime to suit the environment; the correction is that variation already exists and selection acts on it across generations.
- Treating survival alone as the endpoint; the correction is that survival matters because it raises the chance of successful breeding, which is what changes allele frequencies.
- Writing that the environment causes the useful mutation when it is needed; the correction is that mutations arise at random and selection then favours any that happen to be useful.
- Claiming that an organism passes on a characteristic it acquired during its life, such as a stronger muscle from use; the correction is that only alleles in the gametes are inherited.
- Saying the characteristic is passed on because it is useful; the correction is that inheritance depends on which alleles the parents carry, and usefulness only affects who survives to breed.
- Describing change in one individual rather than in the population; the correction is that evolution is a change in the inherited characteristics of a population over generations.
- Stating that Darwin invented evolution; correct this by saying he proposed natural selection as the mechanism and published the evidence.
- Believing the book was rejected by all scientists; correct this by noting that it was widely debated and support grew as evidence accumulated.
- Confusing Darwin with Lamarck; correct this by linking Darwin to natural selection and Lamarck to inheritance of acquired characteristics.
- Thinking the theory was rejected only because it was wrong; the correction is that it lacked a known mechanism of inheritance at the time.
- Believing Darwin published after Mendel; the correction is that Mendel's work came later and was itself unrecognised for decades.
- Assuming all scientists immediately accepted evolution; the correction is that acceptance was gradual and involved religious and evidential objections.
- Stating that Lamarck's theory is the same as Darwin's theory; correction: Lamarck proposed inheritance of acquired characteristics, while Darwin proposed natural selection acting on existing variation.
- Claiming that acquired characteristics are always inherited; correction: changes during life generally do not alter the inherited material passed to offspring.
- Describing Lamarck's theory as disproved by Darwin personally; correction: it was not supported by later understanding of inheritance and genetics.
- Thinking that any feature an organism develops in life is automatically inherited; correct this by stating that only changes in gamete DNA are normally inherited.
- Confusing Lamarck's idea with Darwin's natural selection; correct this by noting that Darwin's mechanism selects existing inherited variation rather than creating it by use.
- Writing that acquired characteristics are never inherited in any circumstance; correct this by saying they are not inherited in the vast majority of cases, leaving room for rare epigenetic effects.