What you'll learn
Evolution by natural selection is one of the most important ideas in all of biology, explaining how the huge variety of living things came to exist and how species change over time. For AQA GCSE Biology you need to understand Darwin's theory, the steps of natural selection, the evidence that supports evolution, and how new species form. This guide covers variation and its causes, the process of natural selection step by step, the evidence from fossils and antibiotic resistance, and the formation of new species. By the end you should be able to describe natural selection in the correct order, explain the evidence for evolution, and apply the idea to examples such as antibiotic-resistant bacteria.
Key terms and definitions
Evolution — The gradual change in the inherited characteristics of a species over many generations.
Natural selection — The process by which organisms better suited to their environment are more likely to survive and reproduce.
Variation — The differences in characteristics between individuals of the same species.
Adaptation — A feature that helps an organism survive in its environment.
Species — A group of organisms that can breed together to produce fertile offspring.
Gene — A section of DNA that codes for a characteristic.
Mutation — A random change in the DNA that can produce new variation.
Fossil — The preserved remains or traces of organisms from long ago.
Core concepts
Variation and its causes
Individuals of the same species are not identical — they show variation in their characteristics. This variation is caused by differences in their genes (inherited from parents), by their environment, or by a combination of both. New genetic variation arises through mutations, which are random changes in DNA. Variation is essential for natural selection, because it provides the differences that selection can act on.
Darwin's theory of natural selection
Charles Darwin proposed that evolution happens by natural selection. The theory can be broken down into clear steps:
- Individuals in a species show variation because of differences in their genes.
- Organisms produce more offspring than the environment can support, so there is competition to survive.
- Individuals with characteristics that are best suited to the environment are more likely to survive this "struggle for existence".
- These survivors are more likely to reproduce and pass on the genes for their advantageous characteristics to their offspring.
- Over many generations, the advantageous characteristics become more common in the population, and the species gradually changes — it evolves.
The key phrase is "survival of the fittest", where "fittest" means best suited to the environment, not strongest.
Selective breeding compared
It helps to compare natural selection with selective breeding. In selective breeding, humans choose which organisms reproduce, to produce offspring with desired characteristics. In natural selection, the environment does the "choosing" — organisms best suited to their surroundings survive and reproduce. Both change the characteristics of a population over generations, but one is driven by humans and the other by nature.
Evidence for evolution: fossils
Fossils provide important evidence for evolution. They are the preserved remains or traces of organisms from millions of years ago, formed in rocks. By arranging fossils in order of age, scientists can see how organisms have gradually changed over time. Fossils show that many organisms alive today are different from those in the past, and that some species have become extinct. However, the fossil record is incomplete, because many organisms decayed completely or their fossils have not been found, which is one reason it took time for the theory to be accepted.
Evidence for evolution: antibiotic resistance
The evolution of antibiotic-resistant bacteria is powerful evidence for natural selection happening today, and a common exam example. It works like this:
- Bacteria show variation, and by chance a mutation makes some bacteria resistant to an antibiotic.
- When the antibiotic is used, the non-resistant bacteria are killed, but the resistant ones survive.
- The surviving resistant bacteria reproduce, passing on the resistance gene.
- Over time, the whole population becomes resistant, producing a "superbug".
This is why doctors are careful not to overuse antibiotics, and why patients should finish the whole course — to kill all the bacteria before resistance can spread.
The formation of new species
Sometimes populations of a species become so different that they can no longer breed together to produce fertile offspring — at this point, a new species has formed. This can happen when two populations are separated (for example by a geographical barrier) and each is subject to different environmental conditions. Natural selection acts differently on each population, and over a very long time they become so different that they can no longer interbreed.
Why Darwin's theory was slow to be accepted
When Darwin published his theory in 1859, it was controversial and only gradually accepted. There were several reasons. It challenged religious beliefs about how life was created, which many people held strongly at the time. There was also not enough evidence when it was first proposed — the mechanism of inheritance was not understood, because genes and DNA had not yet been discovered, so Darwin could not explain how characteristics were passed on. The incomplete fossil record added to the doubt. Only later, as genetics developed and more evidence accumulated, did the theory become widely accepted. This is a good example of how scientific ideas are tested and take time to be established.
Extinction
Evolution also helps explain extinction, when all members of a species die out. Extinction can happen when the environment changes and a species is no longer suited to it, when a new predator or disease appears, or when a species cannot compete with others for resources. If a species cannot adapt quickly enough through natural selection, it may die out completely. Extinction and the appearance of new species are both natural parts of the long history of life on Earth, and the fossil record shows many species that once existed but are now extinct.
Worked examples
Example 1: Ordering natural selection
Put these steps of natural selection in order: reproduction of survivors, variation, survival of the best suited, competition to survive. The correct order is: variation, competition to survive, survival of the best suited, reproduction of survivors. The advantageous genes are then passed on and become more common.
Example 2: Explaining antibiotic resistance
Explain how a population of bacteria becomes resistant to an antibiotic. By chance, a mutation makes some bacteria resistant. When the antibiotic is used, the non-resistant bacteria are killed but the resistant ones survive. These survivors reproduce and pass on the resistance gene, so over time the whole population becomes resistant.
Example 3: The meaning of "fittest"
A student says natural selection means the strongest animal always survives. Explain why this is not quite right. "Fittest" means best suited to the environment, not necessarily the strongest. An organism might survive because it is better camouflaged, faster, or better at finding food — any characteristic that improves its chances of surviving and reproducing in that environment.
Example 4: Fossils as evidence
Explain how fossils provide evidence for evolution. Fossils can be arranged in order of age, showing how organisms have gradually changed over millions of years. They show that past organisms were different from those alive today, supporting the idea that species evolve over time.
Common mistakes and how to avoid them
The most common error is describing natural selection as organisms "choosing" to change or "trying" to adapt. Organisms do not change on purpose — variation arises randomly through mutation, and the environment then selects which individuals survive. Use the language of chance and selection.
Students often say individual organisms evolve. Individuals do not evolve — populations evolve over many generations, as the proportion of individuals with advantageous genes increases.
Another mistake is misunderstanding "fittest" as strongest. It means best suited to the environment, which could mean best camouflaged or most efficient, not physically strongest.
When explaining antibiotic resistance, do not say the antibiotic "causes" the bacteria to become resistant. The resistance arises by random mutation first; the antibiotic then selects for the already-resistant bacteria by killing the rest.
Finally, remember to mention that advantageous characteristics are passed on to offspring through genes — this inheritance step is essential to the process.
Exam technique for "Evolution by natural selection"
Natural selection questions almost always want the process in order: variation, competition, survival of the best suited, reproduction, and inheritance of the advantageous genes over generations. Practise writing these five steps until they are automatic.
For antibiotic resistance, use the same structure applied to bacteria, and be careful to say the mutation comes first and the antibiotic selects afterwards. This is a very common six-mark question.
When discussing evidence, mention both fossils and antibiotic resistance, and note that the fossil record is incomplete. Use precise terms — variation, mutation, natural selection, species — and always link the survival of individuals to the passing on of genes.
Quick revision summary
- Variation between individuals comes from genes, the environment, and random mutations.
- Natural selection: variation → more offspring than can survive (competition) → best-suited individuals survive → they reproduce → advantageous genes become more common over generations.
- "Fittest" means best suited to the environment, not strongest; populations evolve, not individuals.
- Antibiotic resistance: a random mutation makes some bacteria resistant; the antibiotic kills the rest; survivors reproduce and resistance spreads.
- Fossils show gradual change over time but the record is incomplete.
- A new species forms when populations become so different they can no longer interbreed to produce fertile offspring.