How does natural selection lead to adaptation and changes in allele frequencies over time?
Explain natural selection and how it brings about evolution and adaptation, including directional, stabilising and disruptive selection
A focused answer to the H2 Biology Inheritance and Evolution outcome on natural selection. The logic of selection from variation and differential survival, the three types (directional, stabilising and disruptive), and how selection changes allele frequencies and produces adaptation.
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What this dot point is asking
SEAB wants you to explain natural selection as the mechanism of evolution: how variation, a selection pressure and differential survival lead to changes in allele frequency and to adaptation. You should distinguish directional, stabilising and disruptive selection and apply the logic to real examples such as antibiotic resistance.
The answer
The logic of natural selection
Natural selection follows a clear sequence:
- There is genetic variation within a population (ultimately from mutation, shuffled by sexual reproduction).
- Organisms produce more offspring than can survive, so there is competition for resources (a selection pressure).
- Individuals with advantageous alleles are better suited to the environment, so they are more likely to survive and reproduce (differential survival and reproduction).
- These survivors pass on their advantageous alleles, so the frequency of those alleles increases in the next generation.
Over many generations this changes the population: it becomes better adapted, and that change is evolution.
Three types of selection
- Directional selection favours one extreme of a range, shifting the population mean toward it (for example, antibiotic resistance, or increasing body size).
- Stabilising selection favours the intermediate and selects against both extremes, reducing variation while keeping the mean the same (for example, human birth mass).
- Disruptive selection favours both extremes against the intermediate, which can split a population into two groups and may begin speciation.
Examples in context
Example 1. Antibiotic resistance. Antibiotic use kills susceptible bacteria but spares resistant mutants, which then reproduce, so resistance spreads through the population. This is directional selection in action and a pressing reason to use antibiotics responsibly.
Example 2. Pesticide resistance in insects. Repeated pesticide use selects for rare resistant individuals in the same way, and resistant populations evolve over a few generations. The parallel with antibiotic resistance shows that natural selection is a general principle wherever a strong selection pressure meets genetic variation.
Try this
Q1. State what is meant by a selection pressure. [1 mark]
- Cue. An environmental factor (such as a predator, disease or limited food) that affects the survival and reproductive success of individuals, favouring some phenotypes over others.
Q2. Explain why natural selection can only cause evolution if there is genetic variation in the population. [2 marks]
- Cue. Selection favours individuals with advantageous heritable alleles; without genetic variation there are no different alleles to favour, so allele frequencies cannot change.
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 SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original6 marksExplain how the overuse of an antibiotic can lead to a population of bacteria that is resistant to it, using the principles of natural selection.Show worked answer →
Examiners want the full logic of selection applied to resistance.
Within a bacterial population there is genetic variation, arising from mutation. By chance, some bacteria carry an allele that makes them resistant to the antibiotic, while most do not.
When the antibiotic is applied, it acts as a selection pressure. The non-resistant bacteria are killed, but the resistant bacteria survive. The resistant bacteria are therefore the ones that survive and reproduce, passing on the resistance allele to their offspring.
Over successive generations the proportion of resistant bacteria increases, because each generation the resistant individuals leave more offspring. Eventually most of the population carries the resistance allele, so the population is resistant. Overuse of the antibiotic increases the selection pressure and the frequency with which this occurs.
Markers reward variation from mutation, the antibiotic as a selection pressure, differential survival and reproduction of resistant individuals, inheritance of the allele, and the rise in its frequency over generations.
Original4 marksDistinguish between directional selection and stabilising selection, giving an example of each.Show worked answer →
The answer should contrast which phenotypes are favoured and give examples.
Directional selection favours one extreme of a range of phenotypes, so the mean of the population shifts toward that extreme over time. An example is the evolution of antibiotic resistance, where the resistant extreme is favoured, or the increase in average size where larger individuals survive better.
Stabilising selection favours the intermediate phenotype and selects against both extremes, so the mean stays the same but the variation is reduced. An example is human birth mass, where very small and very large babies historically had lower survival, favouring an intermediate mass.
Markers reward the favoured phenotype in each case (one extreme versus the intermediate), the effect on the population mean and variation, and a correct example of each.
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