Evolution — AQA GCSE Combined Science
Test yourself on Evolution with AQA GCSE practice questions.
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Evolution explained
Evolution is a change in the inherited characteristics of a population over time.
Read the full explanation
It happens through natural selection: individuals in a population vary because of differences in their genes, and some variants are better suited to the environment than others. Those individuals are more likely to survive, reproduce and pass on the alleles that produce the advantageous characteristics. Over many generations, the frequency of those alleles in the population increases, so the inherited characteristics of the population change. If populations become so different that they can no longer interbreed to produce fertile offspring, they are different species, so natural selection may result in the formation of a new species.
The theory of evolution by natural selection states that all species of living things have evolved from simple life forms that first developed more than three billion years ago.
This statement has two linked clauses: the mechanism (evolution by natural selection) and the timescale (all species descended from simple life forms that arose over three billion years ago). Evolution means the inherited characteristics of populations change across generations. Natural selection explains how: individuals vary, some variants survive and reproduce more successfully, so their alleles become more common. Over immense periods, accumulated changes can produce new species. The timescale is supported by evidence such as fossils of simple cells in ancient rocks, and by molecular comparisons showing shared biochemistry. For example, fossil bacteria-like structures in rocks over three billion years old, plus the universality of DNA and genetic code, indicate common ancestry. So all species, including humans, share ancestors with simple early life.
Students should be able to explain how evolution occurs through natural selection of variants that give rise to phenotypes best suited to their environment.
This statement requires an explanation, not just a definition. Evolution happens when individuals in a population vary because of differences in alleles. Some variants produce phenotypes better suited to the environment, for example longer necks reaching food or camouflage avoiding predators. These individuals are more likely to survive, compete successfully and reproduce, passing their alleles to offspring. Over many generations, the beneficial alleles become more frequent in the population, so the phenotype of the population changes. This is natural selection. The environment selects variants; it does not create them. For example, in a changing habitat, variants with thicker fur may survive cold better and leave more offspring, so thicker fur becomes common. Explaining the full chain from variation to allele frequency change is the skill assessed.
If two populations of one species become so different in phenotype that they can no longer interbreed to produce fertile offspring they have formed two new species.
Speciation happens when populations of one species become reproductively isolated, so gene flow between them stops. Different selection pressures and random mutations then make their phenotypes diverge. Over many generations the accumulated differences become so great that members of the two populations can no longer interbreed successfully to produce fertile offspring. At that point they are classified as two separate species. For example, a river may split a population of beetles; each group experiences different food and climate, so different alleles are favoured. Eventually the two groups may mate but produce infertile young, or not mate at all, showing that new species have formed.
Your focus
- Define evolution as a change in the inherited characteristics of a population over time.
- Describe the process of natural selection leading to changes in allele frequency.
- Explain how natural selection may result in the formation of a new species when populations become reproductively isolated.
Show all 12 objectives
- State that all species have evolved from simple life forms that developed more than three billion years ago.
- Describe evolution as a change in inherited characteristics of populations over generations.
- Use fossil and molecular evidence to support the theory of evolution by natural selection.
- Explain how natural selection acts on variants that produce phenotypes suited to the environment.
- Describe how advantageous alleles increase in frequency over generations.
- Apply the mechanism of natural selection to unfamiliar examples, such as antibiotic resistance.
- Describe how reproductive isolation can lead to the formation of two new species.
- Explain how different selection pressures and mutation cause populations to diverge in phenotype.
- State that two populations are separate species when they can no longer interbreed to produce fertile offspring.
Evolution exam tips
Marking Points
- Evolution is defined as a change in the inherited characteristics of a population over time, not a change in an individual during its lifetime.
- Variation within a population arises from differences in alleles, including mutations, so individuals have different phenotypes.
- Natural selection occurs because individuals with characteristics better suited to the environment are more likely to survive and reproduce.
- The alleles responsible for advantageous characteristics are passed to offspring, so their frequency in the population increases over generations.
- Over long periods, accumulated changes in allele frequencies can make populations so different that they can no longer interbreed to produce fertile offspring, forming a new species.
- Answers should use the sequence: variation, selection pressure, survival, reproduction, inheritance, changed allele frequency over time.
- Evolution is a change in the inherited characteristics of a population over generations, not a change in an individual during its lifetime.
- Natural selection requires variation, inheritance of that variation, and differential survival and reproduction.
- All species share common ancestors; simple life forms existed more than three billion years ago.
- Evidence includes fossils in ancient rocks and molecular evidence such as shared DNA and genetic code.
- The process is gradual and cumulative over very long time periods, allowing new species to arise.
- Variation exists within a population because of differences in alleles.
- Some variants give phenotypes better suited to the environment, improving survival and reproduction.
- Individuals with advantageous phenotypes are more likely to survive and pass on their alleles.
- Over generations, the frequency of advantageous alleles in the population increases.
- The environment selects existing variants; it does not cause new beneficial mutations on demand.
- Reproductive isolation occurs when populations are separated so that they no longer interbreed and gene flow between them stops.
- Different selection pressures in the two environments favour different alleles, so phenotypes diverge over many generations.
- Mutation introduces new alleles, and natural selection acting on these variants increases the differences between the populations.
- Speciation is complete when the two populations can no longer interbreed to produce fertile offspring, so they are recognised as separate species.
- A named example, such as two beetle populations separated by a river, can illustrate how isolation and selection lead to new species.
Examiner Tips
- 💡Define evolution precisely as a change in the inherited characteristics of a population over time.
- 💡When describing natural selection, always link variation to survival, reproduction and the inheritance of advantageous alleles.
- 💡Use the term 'population' rather than 'species' or 'organism' when describing changes in allele frequency.
- 💡Define evolution and natural selection precisely before applying them to an example.
- 💡Use the phrase 'more than three billion years ago' when describing the origin of simple life forms.
- 💡Link each evidence type to the conclusion it supports, such as fossils showing simple life in ancient rocks.
- 💡Write the explanation as a sequence: variation, selection, survival, reproduction, allele frequency change.
- 💡Use a named example, such as antibiotic resistance in bacteria, to make the mechanism concrete.
- 💡Avoid teleological wording such as 'trying to adapt'; describe selection of existing variants instead.
- 💡Use the phrase 'no longer interbreed to produce fertile offspring' when defining the point at which two new species have formed.
- 💡Link each stage of your answer in a logical chain: isolation, different selection pressures, mutation, natural selection, reproductive isolation.
- 💡Include a specific example, such as beetles separated by a river, to show how the process works in practice.
Common Mistakes
- Saying that an individual evolves: correct this by explaining that evolution is a change in the inherited characteristics of a population over time.
- Confusing natural selection with Lamarckian inheritance of acquired characteristics: correct this by stating that only inherited alleles, not characteristics acquired during life, are passed on.
- Thinking that a new species forms whenever a population changes: correct this by explaining that new species form when populations become reproductively isolated and can no longer produce fertile offspring.
- Saying an individual organism evolves during its lifetime; correction: populations evolve across generations as allele frequencies change.
- Claiming humans evolved from modern apes; correction: humans and modern apes share a common ancestor.
- Stating life began only a few million years ago; correction: simple life forms developed more than three billion years ago.
- Saying organisms adapt during their lifetime to suit the environment; correction: variants already exist and selection acts on them across generations.
- Writing that the environment creates the needed mutation; correction: mutations arise randomly, and selection favours beneficial phenotypes.
- Describing only survival without reproduction; correction: passing on alleles to offspring is essential for evolution.
- Thinking that a new species forms as soon as two populations look different; correction: phenotype differences alone are not enough, because reproductive isolation and infertile or no offspring are the defining test.
- Believing that individual organisms change their phenotype to suit the environment; correction: mutations create new alleles, and natural selection favours those already present, so populations change over generations.
- Confusing speciation with extinction; correction: speciation produces two new species from one ancestral species, whereas extinction is the loss of a species.