How does natural selection change populations over time, and what evidence supports evolution?
4.2.2 Evolution: the process of evolution by natural selection acting on variation; the role of mutation in generating variation; the types of natural selection (directional, stabilising and disruptive); the evidence for evolution from fossils, comparative anatomy and molecular biology; and examples such as antibiotic resistance and industrial melanism.
A focused answer to the OCR H420 4.2.2 dot point on evolution. Covers natural selection acting on variation, mutation as the source of variation, directional, stabilising and disruptive selection, the evidence for evolution, and examples such as antibiotic resistance and peppered moths.
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What this dot point is asking
OCR wants you to explain evolution by natural selection acting on variation, the role of mutation in generating variation, the three types of natural selection, the evidence for evolution, and worked examples such as antibiotic resistance and industrial melanism.
The answer
Variation and its source
A population shows variation in phenotype. The ultimate source of new alleles is random mutation in DNA; sexual reproduction (meiosis and random fertilisation) then shuffles these alleles into new combinations. Only heritable variation (caused by genes) can be acted on by natural selection; environmental variation is not passed on.
Natural selection
Natural selection follows a clear logic:
- Organisms produce more offspring than the environment can support, so there is competition for resources.
- There is heritable variation in the population.
- A selection pressure (predation, disease, competition, climate) means individuals with advantageous alleles are more likely to survive and reproduce (have greater fitness).
- They pass on the advantageous alleles, so over generations the frequency of those alleles increases in the population. This is evolution: a change in allele frequency over time.
Types of natural selection
- Directional selection favours one extreme of a phenotype, so the mean shifts towards it (for example antibiotic resistance, or larger beak size in a drought). It often acts when the environment changes.
- Stabilising selection favours the intermediate phenotype and selects against both extremes, so the mean stays the same but the range narrows (for example human birth mass). It acts in a stable environment.
- Disruptive selection favours both extremes against the intermediate, splitting the distribution into two peaks (for example a population on a beach with light and dark backgrounds). It can begin the process of forming new species.
Evidence for evolution
- Fossils show a sequence of forms over geological time and transitional features (for example the evolution of the horse, or feathered dinosaurs).
- Comparative anatomy: homologous structures (the same basic plan adapted to different uses, like the pentadactyl limb of mammals) show descent from a common ancestor.
- Molecular biology: similarities in DNA base sequences and protein (amino acid) sequences between species reflect how recently they shared a common ancestor (the most powerful modern evidence).
Worked examples
- Antibiotic resistance in bacteria: a mutation gives resistance; the antibiotic is the selection pressure; resistant bacteria survive, reproduce and raise the allele frequency (directional selection, observable within years).
- Industrial melanism in the peppered moth: soot darkened the bark; dark moths were better camouflaged and survived predation better; the dark allele increased in frequency in polluted areas.
Examples in context
Example 1. Darwin's finches. On the Galapagos, beak size and shape evolved by directional selection in response to the available food (for example larger, stronger beaks in droughts when only hard seeds remain), a classic demonstration of selection changing a population.
Example 2. Sickle-cell and malaria. In malarial regions the sickle-cell allele persists because heterozygotes resist malaria, an example of selection maintaining an allele that would otherwise be disadvantageous, linking selection to human populations.
Try this
Q1. State the ultimate source of new variation in a population. [1 mark]
- Cue. Random mutation (of DNA).
Q2. Explain why only heritable variation contributes to evolution by natural selection. [2 marks]
- Cue. Only variation caused by genes (alleles) can be passed to offspring; environmental variation is not inherited, so it cannot change allele frequencies over generations.
Q3. Name the type of selection that favours the intermediate phenotype. [1 mark]
- Cue. Stabilising selection.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR H420/02 20195 marksExplain how a population of dark-coloured peppered moths became more common than pale moths in industrial areas during the nineteenth century.Show worked answer →
Run the standard natural-selection sequence on the moth example.
There was variation in the moth population (pale and dark forms) caused originally by mutation. In industrial areas, soot darkened the tree bark, so dark moths were better camouflaged while pale moths stood out.
Pale moths were therefore eaten by birds (predation) more often, while dark moths were more likely to survive and reproduce (the bark colour is the selection pressure). They passed on the allele for dark colour, so over generations the frequency of the dark allele increased and dark moths became more common.
Markers reward variation from mutation, the selection pressure (predation against the less camouflaged form), differential survival and reproduction, and a rise in allele frequency.
OCR H420/02 20224 marksDistinguish between directional selection and stabilising selection, giving an example of each.Show worked answer →
Define each by what it favours and its effect on the distribution.
Directional selection favours one extreme of a phenotype, so the mean of the population shifts towards that extreme over time. Example: antibiotic resistance in bacteria (the resistant extreme is favoured), or the increase in dark peppered moths.
Stabilising selection favours the intermediate phenotype and selects against both extremes, so the mean stays the same but the range narrows. Example: human birth mass (very small and very large babies have lower survival, so intermediate masses are favoured).
Markers reward one extreme favoured and the mean shifting (directional) versus the intermediate favoured and the range narrowing (stabilising), each with a valid example.
Related dot points
- 4.2.2 Classification and evolutionary relationships: the binomial system and the taxonomic hierarchy; the five kingdoms and the three-domain classification; the meaning of phylogeny; and how molecular evidence (DNA base sequences, amino acid sequences) and other evidence are used to clarify evolutionary relationships.
A focused answer to the OCR H420 4.2.2 dot point on classification. Covers the binomial system and taxonomic hierarchy, the five kingdoms and the three-domain system, the meaning of phylogeny, and how molecular and other evidence is used to establish evolutionary relationships.
- 6.1.2 Populations and evolution: the meaning of a gene pool and allele frequency; the use of the Hardy-Weinberg principle to calculate allele and genotype frequencies; the factors that change allele frequencies (natural selection, genetic drift, the founder effect and migration); and the process of speciation (allopatric and sympatric).
A focused answer to the OCR H420 6.1.2 dot point on populations and evolution. Covers gene pools and allele frequency, the Hardy-Weinberg principle and its calculations, the factors that change allele frequencies including genetic drift and the founder effect, and allopatric and sympatric speciation.
- 4.2.1 Biodiversity: the levels of biodiversity (habitat, species and genetic); how to sample plants and animals (random sampling, quadrats, transects and mark-release-recapture); the calculation and interpretation of Simpson's index of diversity; and the ecological, economic and aesthetic reasons for maintaining biodiversity.
A focused answer to the OCR H420 4.2.1 dot point on biodiversity. Covers habitat, species and genetic diversity, sampling methods including quadrats, transects and mark-release-recapture, the calculation and interpretation of Simpson's index of diversity, and the reasons for maintaining biodiversity.
- 4.1.1 The immune response: the structure and function of antibodies; the roles of B and T lymphocytes in the humoral and cell-mediated responses; the primary and secondary responses and the role of memory cells; the principles of vaccination and herd immunity; the differences between active, passive, natural and artificial immunity; and the development of antibiotic resistance.
A focused answer to the OCR H420 4.1.1 dot point on the specific immune response. Covers antibody structure, B and T lymphocytes, the primary and secondary responses, memory cells, vaccination and herd immunity, the four types of immunity, and how antibiotic resistance evolves.
- 6.1.2 Patterns of inheritance: monohybrid and dihybrid crosses; the inheritance of codominant and multiple alleles, sex linkage and epistasis; the use of genetic diagrams to predict phenotypic ratios; and the chi-squared test to compare observed and expected results.
A focused answer to the OCR H420 6.1.2 dot point on patterns of inheritance. Covers monohybrid and dihybrid crosses, codominance and multiple alleles, sex linkage and epistasis, genetic diagrams and phenotypic ratios, and the chi-squared test.
Sources & how we know this
- OCR A Level Biology A (H420) Specification — OCR (2023)