Natural selection and evolution
What you'll learn
- Why there is usually lots of genetic variation within a population.
- How natural selection changes populations over many generations.
- How classification has changed using DNA evidence.
- What evidence supports evolution, including fossils and antibiotic resistance.
The starting point: variation in populations
A species is a group of organisms that can breed together to produce fertile offspring. A population is all the organisms of one species living in the same area at the same time.
Within most populations, individuals are not genetically identical. A gene is a section of DNA that affects a characteristic, and an allele is a different version of a gene. The genome is the complete set of DNA in an organism.
This means a population usually has extensive genetic variation: differences in DNA between individuals. These genetic differences can lead to different phenotypes, meaning observable characteristics such as fur colour, beak shape, height, or resistance to disease.
Variation comes from:
- Mutation: a random change in DNA that can create a new allele.
- Sexual reproduction: alleles are shuffled during meiosis and fertilisation.
- Environmental effects: for example, nutrition can affect height, but not every environmental change is inherited.
Mutation
A mutation is a random change in the DNA base sequence. Most mutations have no effect, some are harmful, and a few can be beneficial in a particular environment.
Natural selection
Natural selection is the process where individuals with phenotypes better suited to their environment are more likely to survive, reproduce, and pass on their alleles.
A selection pressure is something in the environment that affects survival or reproduction. Examples include predators, disease, competition for food, temperature, drought, or antibiotics.
The flowchart below shows the key GCSE idea: individuals do not choose to change. Instead, the proportions of different inherited variants in the population change over generations.
Natural selection in one sentence
Random mutations create genetic variation; the environment selects which variants reproduce most successfully; inherited alleles become more common over many generations.
Explaining selection in a changing environment
A rabbit population contains some rabbits with thick fur and some with thin fur. The climate becomes much colder.
- The starting population has genetic variation: different alleles affect fur thickness, so rabbits show different phenotypes.
- Cold temperature becomes a selection pressure because thin-fur rabbits lose heat faster and are less likely to survive long enough to reproduce.
- Thick-fur rabbits have an advantageous phenotype in this environment, so a larger proportion survive and produce offspring.
- Because fur thickness is inherited, surviving thick-fur rabbits pass the helpful alleles to more offspring.
- Over many generations, thick fur becomes more common in the population, so the population evolves.
Individuals do not evolve
Do not write that “a rabbit evolved thicker fur because it needed to survive”. Mutations are random, and natural selection changes the population over generations, not the body of one individual during its lifetime.
Evolution and new species
Evolution
Evolution is a change in the inherited characteristics of a population over time, through processes such as natural selection.
If natural selection continues for many generations, the population may become noticeably different from its ancestors. If populations become separated, for example by a mountain range, island formation, or different habitats, they may experience different selection pressures.
Over a long time, mutations and natural selection can make the separated populations so different that they can no longer interbreed to produce fertile offspring. This is speciation, meaning the formation of a new species.
Natural selection does not guarantee survival. If the environment changes too quickly, or if no individuals have a useful inherited variation, a species may become extinct, meaning no living individuals of that species remain.
Natural selection has limits
A species can only adapt if useful genetic variation already exists or appears by mutation. If every individual is poorly suited to the new environment, extinction may happen.
Classification and evolutionary relationships
Classification means sorting organisms into groups. Classification systems have changed as biology has developed.
An artificial classification system groups organisms using one or a few features chosen by humans, such as “has wings” or “lives in water”. This can be useful, but it may group unrelated organisms together, such as bats, birds, and insects.
A natural classification system groups organisms using many shared characteristics and their evolutionary relationships. This is more useful for showing how closely related organisms are.
A major modern development is DNA sequencing, which means finding the order of bases in DNA. Molecular phylogenetics uses DNA sequence comparisons to work out evolutionary relationships. Organisms with more similar DNA are usually more closely related and share a more recent common ancestor.
Using DNA evidence to infer relationships
Three species have short DNA sequences:
- Species P: A T G C C A
- Species Q: A T G C C T
- Species R: A C C A C T
- Compare the bases at the same positions in each sequence.
- Species P and Q differ at one position, while P and R differ at four positions, and Q and R differ at three positions.
- The pair with the fewest differences has the most similar DNA, so P and Q are likely to be most closely related.
- Biologists may classify P and Q closer together, even if their outward appearance is not the most similar.
Classification exam wording
If a question mentions DNA sequencing, link it to evolutionary relationships: more similar DNA suggests a more recent common ancestor.
Evidence for evolution
Fossils
A fossil is the preserved remains, traces, or impressions of an organism from the past. Fossils may be bones, shells, footprints, burrows, or even preserved whole organisms.
Fossils provide evidence for evolution because they show that:
- organisms from the past were different from organisms alive today;
- some species have become extinct;
- sequences of fossils in different-aged rocks can show gradual changes over time;
- some fossils have features that link major groups, suggesting common ancestry.
The fossil record is incomplete because many organisms decay before they can fossilise, and many fossils have not yet been found.
Antibiotic resistance in bacteria
Antibiotic resistance is strong evidence for evolution because it can happen quickly enough for humans to observe.
An antibiotic is a medicine that kills bacteria or stops them reproducing. Some bacteria have mutations that make them resistant, meaning the antibiotic no longer kills them effectively.
Antibiotic resistance in bacteria
A patient takes an antibiotic. Most bacteria die, but the infection later returns with resistant bacteria.
- Before treatment, a random mutation has already produced a few bacteria with an allele for resistance.
- The antibiotic acts as a selection pressure: non-resistant bacteria die, while resistant bacteria survive.
- Resistant bacteria reproduce rapidly by binary fission, which is asexual cell division, so they pass the resistance allele to many offspring.
- Over time, the population contains a higher proportion of resistant bacteria, showing evolution by natural selection.
Antibiotics do not create resistance on purpose
Do not say bacteria “become resistant because they need to”. The resistant mutation appears randomly first; the antibiotic then selects the bacteria that already have resistance.
Darwin, Wallace and modern biology
Because you are studying separate Biology J247, you also need the work of Charles Darwin and Alfred Russel Wallace; this is not required for OCR Gateway Combined Science.
Darwin developed the theory of evolution by natural selection after studying variation, fossils, selective breeding, and organisms such as finches. Wallace independently developed very similar ideas after studying animals in different parts of the world. Their work helped biology move towards an evidence-based explanation for biodiversity and common ancestry.
Their ideas affect modern biology in many ways, including:
- understanding how antibiotic resistance evolves;
- explaining why classification should reflect evolutionary relationships;
- conservation of endangered species;
- crop breeding and protection of genetic resources.
Biodiversity means the variety of living organisms, including the variety of species and the genetic variation within species. Seedbanks help store biodiversity by keeping seeds from many plant species and varieties in controlled conditions. This preserves alleles that may be useful in the future, for example for disease resistance or drought tolerance in crops.
In the exam
- For natural selection questions, include variation, selection pressure, survival and reproduction, inheritance of alleles, and change over generations.
- Avoid purposeful language: write “individuals with the advantageous allele were more likely to survive”, not “they adapted because they needed to”.
- For evidence questions, use the named evidence in the question: fossils show change over long time periods, while antibiotic resistance shows evolution happening in observable populations.
Check yourself
- Why does natural selection act on phenotypes but change allele frequencies in a population?
- How can DNA sequencing change how organisms are classified?
- Why is antibiotic resistance an example of evolution by natural selection?
