QCE Biology - Unit 4 - Continuity of life on Earth
Microevolution, Allele Frequencies and Selection | QCE Biology
Learn microevolution, mutation, gene flow, genetic drift, natural selection, allele frequencies and selection pressures.
Updated 2026-05-18 - 7 min read
QCAA official coverage - Biology 2025 v1.3
Exact syllabus points covered
- Distinguish between microevolution and macroevolution.
- Explain microevolutionary change through the main processes of mutation, gene flow and genetic drift.
- Explain natural selection and identify the three main types of phenotypic selection: stabilising, directional and disruptive.
- Calculate allele frequencies from genotype data.
- Analyse data to determine the effect of a selection pressure on a population, recognising that selection for an allele can be positive or negative.
Microevolution, allele frequencies and selection is part of the way QCE Biology turns living systems into evidence students can describe, analyse and evaluate. The safest way to study it is to connect each term to a data pattern, a biological mechanism and a limitation.
Original Sylligence diagram for biology selection types.
Core explanation
Microevolution
Microevolution is change in allele frequencies within a population over generations. It can occur through mutation, gene flow, genetic drift and natural selection.
Allele frequencies
Allele frequency is the proportion of all alleles at a gene locus represented by one allele. In diploid organisms, each individual contributes two alleles for an autosomal locus.
Natural selection
Selection occurs when heritable variation affects survival or reproduction. Directional selection favours one extreme, stabilising selection favours intermediate phenotypes, and disruptive selection favours both extremes.
Genetic drift and gene flow
Genetic drift is random change, strongest in small populations. Gene flow moves alleles between populations through migration and breeding.
Gene pools and conditions for no evolution
A gene pool is all the alleles present in a population. A genome is the complete DNA sequence of an organism. Microevolution is measured at the population level because allele frequencies describe the gene pool, not one individual.
Hardy-Weinberg reasoning is useful as a null model: if allele frequencies do not change, the population is not evolving at that locus. Allele frequencies are expected to remain constant only when the population is very large, mating is random, mutation is absent or negligible, there is no migration, and there is no natural selection. Real populations often break one or more of these assumptions.
Natural selection requires heritable variation and a selection pressure. Viability selection changes survival to reproductive age. Fecundity selection changes reproductive output. For example, if an insecticide kills susceptible insects before they reproduce, resistant alleles may increase in frequency over generations.
| Mechanism | Direction of change | Typical evidence | | --- | --- | --- | | Mutation | Introduces new alleles | New DNA sequence variant appears | | Natural selection | Non-random increase of alleles linked to higher fitness | Allele changes match a selection pressure | | Gene flow | Alleles move between populations | Migrants breed and allele frequencies become more similar | | Genetic drift | Random allele frequency change | Strongest after chance events in small populations | | Bottleneck effect | Drift after population size collapses | Low diversity remains after numbers recover | | Founder effect | Drift after a few individuals colonise a new area | New population has unusual allele frequencies |
Gene flow only changes allele frequencies if migrants breed successfully and if their allele frequencies differ from the receiving population. Genetic drift can remove alleles even if they are beneficial, especially when the population is small.
Selection patterns and examples
Directional selection shifts a population toward one phenotypic extreme. This can occur when an environmental change consistently favours one trait, such as larger beaks during a period when only large, hard seeds are available. Stabilising selection favours intermediate phenotypes and reduces extremes, such as birth mass being selected away from very low and very high values. Disruptive selection favours both extremes and can increase variation if intermediate phenotypes have lower fitness.
The Hardy-Weinberg model assumes random mating. Non-random mating can change genotype frequencies even if allele frequencies do not immediately change. Selection changes allele frequencies when different genotypes have different survival or reproductive success. Mutation introduces new alleles, but by itself it is usually too rare to cause rapid allele frequency change in a large population.
Gene flow can increase genetic diversity within a population by bringing in new alleles, but it can reduce differences between populations by mixing gene pools. Genetic drift reduces genetic diversity most strongly in small populations, especially after bottleneck and founder events.
When explaining natural selection, use the full chain: variation exists, variation is heritable, a selection pressure affects survival or reproduction, individuals with advantageous phenotypes leave more offspring, and allele frequencies change over generations.
The peppered moth example is useful because it links phenotype, environment and predation. In polluted environments where tree bark became darker, darker moths were better camouflaged and lighter moths were more visible to predators. When pollution decreased and bark became lighter again, the selection pressure changed. The example is not about moths deciding to change colour; it is about existing heritable variation being filtered by predation.
Resistance examples work the same way. Antibiotics, herbicides or insecticides do not create resistance because the organism "needs" it. Resistant variants may already exist or arise by mutation. When the chemical kills susceptible individuals, resistant individuals survive and reproduce, increasing the frequency of resistance alleles.
Homozygous and heterozygous genotype counts can be used to calculate allele frequency. For an allele A, count two A alleles for each AA individual and one A allele for each Aa individual, then divide by the total number of alleles at that locus.
How to use this in data questions
Start by identifying what has been measured. In Biology, a graph or table is rarely just asking for a trend; it is asking whether you can connect the trend to a process. Quote enough data to show the pattern, then use the concept language from the syllabus. If the evidence is limited, name the limitation precisely: sample size, sampling method, uncontrolled variables, measurement precision, population choice or the time scale of the data.
A useful study habit is to turn each heading into a data prompt. Ask what you would expect to happen if the relevant variable increased, decreased or was removed. For ecology topics, think about abundance, distribution, biodiversity, biomass and carrying capacity. For genetics topics, think about genotype, phenotype, gene expression, allele frequency and inheritance pattern. For evolution topics, think about variation, selection pressure, gene flow, isolation and relatedness.
When a question asks you to evaluate, do not just list problems with the experiment. Link the limitation to the confidence of the conclusion. For example, a small sample size matters because a few unusual individuals can distort the pattern. An uncontrolled abiotic factor matters because it gives another possible explanation for the same biological trend. This is the difference between naming a limitation and using it scientifically.
Worked example
Common exam traps
Other traps to watch for:
- using a general word when a syllabus term is available
- ignoring units, sample size or time scale
- treating a model as a perfect copy of the real ecosystem or cell
- writing a memorised paragraph that does not use the given data