Evolution — AQA GCSE Biology
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 through natural selection, and it may lead to a new species.
Read the full explanation
The mechanism starts with genetic variation from mutation and sexual reproduction. Individuals whose characteristics suit the environment survive, reproduce and pass on their alleles, so allele frequencies change over generations. A population is all the organisms of one species in a habitat, so evolution is measured in populations, not individuals. If populations become so different, or are separated so they can no longer interbreed to produce fertile offspring, a new species may form. A method is to write the chain: variation, selection pressure, survival and reproduction, inheritance, allele frequency change, possible 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.
Evolution by natural selection explains how the variety of life arose from simple beginnings. The theory states that all species of living things have evolved from simple life forms that first developed more than three billion years ago. Over immense time, random genetic variation plus selection by the environment produced complex organisms from simpler ancestors. Evidence includes fossils showing gradual change, similarities in DNA and biochemistry between organisms, and observed evolution such as antibiotic resistance in bacteria. A useful method is to trace a lineage: simple cells appear in the oldest rocks, then increasingly complex body forms appear later. This does not mean one modern species turns into another today; it means shared ancestry over geological time.
Students should be able to explain how evolution occurs through natural selection of variants that give rise to phenotypes best suited to their environment.
Natural selection acts on variation within a population. Random mutations create new alleles, so individuals differ in phenotype. Those whose phenotypes are best suited to the environment are more likely to survive, compete successfully and reproduce, passing on the advantageous alleles. Over many generations the beneficial alleles become more common, so the population changes. A clear method is to follow one example: a rabbit with longer legs may escape predators, survive and breed, so long-leg alleles increase. Selection is not purposeful; the environment does not create the variant, it only favours it. This explains adaptation and how populations, not individuals, evolve.
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.
A species is often defined by the ability to interbreed and produce fertile offspring. If two populations of one species become separated, for example by a river or mountain range, each experiences different selection pressures. Over many generations their phenotypes and alleles diverge. Eventually they become so different that they can no longer interbreed to produce fertile offspring, so they have formed two new species. A useful method is to compare populations: if mating produces no offspring, or infertile offspring such as a mule, they are separate species. Speciation therefore requires isolation, variation, selection and enough time.
Your focus
- Define evolution as a change in the inherited characteristics of a population over time.
- Describe natural selection as the process that causes evolution.
- Explain how reproductive isolation can lead to the formation of a new species.
Show all 12 objectives
- Describe the theory that all species evolved from simple life forms more than three billion years ago.
- Explain how natural selection over long periods produced complex organisms from simple ancestors.
- Use at least one type of evidence to support the theory of evolution.
- Explain how random variation and environmental selection change a population over generations.
- Describe how advantageous phenotypes lead to greater survival and reproduction.
- Apply natural selection to a familiar example such as antibiotic resistance.
- Define a species in terms of interbreeding and fertile offspring.
- Explain how separated populations can diverge until they become separate species.
- Apply the fertile offspring criterion to decide whether two populations are the same species.
Evolution exam tips
Marking Points
- Evolution is a change in the inherited characteristics of a population over time.
- The mechanism is natural selection acting on genetic variation within the population.
- Individuals with advantageous alleles survive, reproduce and pass those alleles to offspring.
- Allele frequencies in the population change over many generations.
- If populations become reproductively isolated and can no longer produce fertile offspring, a new species may form.
- States that all species of living things have evolved from simple life forms.
- States that these simple life forms first developed more than three billion years ago.
- Explains that variation arises randomly and selection by the environment acts on it over many generations.
- Uses evidence such as fossils, DNA similarities or observed evolution to support the theory.
- Recognises that evolution occurs over very long time scales, not within a human lifetime.
- States that variation arises from random mutation, producing different phenotypes.
- Explains that individuals with phenotypes best suited to the environment are more likely to survive and reproduce.
- Explains that advantageous alleles are passed to offspring and become more common over generations.
- Recognises that selection acts on existing variation rather than creating it.
- Applies the process to a named example, such as antibiotic resistance in bacteria.
- States that a species can interbreed to produce fertile offspring.
- Explains that two populations become different in phenotype because of different selection pressures or isolation.
- States that when they can no longer interbreed to produce fertile offspring they have formed two new species.
- Recognises that speciation requires many generations and usually some form of separation.
- Uses an example such as a mule being infertile to show that two populations are separate species.
Examiner Tips
- 💡Include the words population and over time in any definition of evolution.
- 💡Explain natural selection as a sequence of variation, selection, reproduction and inheritance.
- 💡State that a new species forms only if populations can no longer interbreed to produce fertile offspring.
- 💡Quote the three-billion-year timescale accurately when describing the origin of simple life forms.
- 💡Link each evidence point back to the theory rather than listing evidence without explanation.
- 💡Use the phrase natural selection when describing how simple life forms gave rise to complex ones.
- 💡Use the sequence variation, selection, reproduction, inheritance when explaining natural selection.
- 💡Name the selection pressure, such as predators or antibiotics, to make the explanation specific.
- 💡State clearly that the advantageous allele is inherited by offspring.
- 💡Define species using the fertile offspring criterion before applying it.
- 💡Mention isolation or separation as the starting point for speciation.
- 💡Use the phrase can no longer interbreed to produce fertile offspring in your answer.
Common Mistakes
- Defining evolution as an individual organism changing during its lifetime: correct this by defining it as a change in a population over generations.
- Saying organisms evolve because they want to survive: correct this by stating that variation is random and selection is non-random.
- Treating evolution and natural selection as identical: correct this by describing natural selection as the process and evolution as the change in inherited characteristics over time.
- Thinking evolution means an individual organism changes during its life; correct this by explaining that populations change across generations.
- Believing the first life forms appeared only millions of years ago; correct this by stating more than three billion years.
- Assuming humans evolved from modern apes; correct this by explaining that humans and apes share a common ancestor.
- Saying organisms mutate because they need to adapt; correct this by stating mutations occur randomly and selection favours useful ones.
- Confusing natural selection with selective breeding; correct this by noting that humans choose in selective breeding, while the environment selects in natural selection.
- Claiming individuals evolve; correct this by explaining that populations evolve as allele frequencies change.
- Thinking any two animals that look different are separate species; correct this by using the fertile offspring criterion.
- Believing speciation happens in one generation; correct this by explaining it requires many generations of divergence.
- Confusing infertile offspring with no offspring; correct this by stating that infertile offspring do not count as successful interbreeding.