Variation and Natural Selection Revision Notes | A Level AQA Biology

1. Why No Two Individuals Are Identical

Picture a meadow full of field mice. Every individual belongs to the same species, yet coat colour, body size, and disease resistance all differ. That variation is not random noise — it is the raw material on which natural selection acts, and understanding where it comes from is the foundation of everything in this topic.

Phenotypic variation — the differences you can observe between individuals — arises from two interacting sources: genetic factors and environmental factors. Genetic factors determine what is possible; environmental factors determine how those possibilities are expressed. This interplay means that phenotypic outcomes cannot be wholly attributed to either source alone. For example, while a genetically predisposed tendency for height can be significantly influenced by environmental factors such as nutrition during growth phases, the extent to which this predisposition is expressed varies across individuals due to specific environmental contexts, demonstrating the complex interaction between genetics and environment.

The primary source of genetic variation is mutation — a heritable change in the base sequence of DNA. A new mutation creates a new allele that did not previously exist in the gene pool, introducing entirely novel variation. Without mutation, there would be no new raw material for selection to act upon.

Mutation alone, however, does not explain the enormous diversity seen within sexually reproducing populations. Two additional processes amplify variation dramatically: meiosis and the random fertilisation of gametes.

• Independent assortment during meiosis I randomly distributes maternal and paternal homologous chromosomes into gametes, producing up to 2²³ different chromosome combinations in humans.
• Crossing over during prophase I of meiosis creates recombinant chromosomes with new combinations of alleles not present in either parent.
• Random fertilisation means any one of millions of genetically distinct sperm can fuse with any one of many genetically distinct eggs, multiplying the possible genotypic combinations further.

Environmental factors add another layer. Two genetically identical plants grown in different light intensities will differ in height and leaf area. A human with a genetic predisposition to a certain body mass will still be shaped by diet and exercise. The phenotype is always the product of genotype interacting with environment.

The key exam point here is precision: mutation is the primary source because it introduces alleles that did not previously exist. Meiosis and fertilisation reshuffle existing alleles but cannot create new ones. Examiners regularly test whether students can make this distinction.

2. Natural Selection: The Filter on Variation

Now that variation exists within the population, something has to act on it. The environment does not treat all phenotypes equally — and that unequal treatment is natural selection.

Three environmental pressures drive differential survival and reproduction: predation, disease, and competition for the means of survival (food, water, mates, territory). These pressures mean that not all individuals survive to reproduce, and those that do survive do not all reproduce equally.

Consider the classic example of the peppered moth (Biston betularia) before industrialisation. The pale, speckled form was well camouflaged against lichen-covered tree bark. Dark (melanic) moths were conspicuous and fell victim to predation at a higher rate. The pale allele was therefore at a selective advantage, and pale moths survived to reproduce more often.

The mechanism links directly to allele frequencies. Individuals with the favourable phenotype survive longer and reproduce more, passing their favourable alleles to a greater number of offspring. Over successive generations, those alleles become more common in the gene pool. Alleles associated with disadvantageous phenotypes are passed on less frequently and decline in frequency — or may be eliminated entirely if the disadvantage is severe enough.

This connection between differential reproductive success and allele frequency is the engine of evolution. Each generation, the gene pool is slightly reshaped by which individuals contributed offspring to it. Over many generations, those small shifts accumulate into significant change.

3. Evolution as a Change in Allele Frequencies

At A Level, evolution has a precise definition that goes beyond the GCSE idea of 'change over time'. The AQA specification defines evolution as a change in the allele frequencies in a population. This definition is worth memorising word for word.

Think of the gene pool as a bag of coloured counters, where each colour represents a different allele. Natural selection is like a rule that says certain colours are more likely to be drawn out and copied into the next generation's bag. After many rounds, the proportions of colours in the bag shift — that shift in proportion is evolution.

Returning to the peppered moth: after industrialisation, soot blackened tree bark and removed the lichen. Now the pale form was conspicuous and the dark form was camouflaged. Predation pressure reversed. The frequency of the melanic allele rose sharply in industrial areas — a measurable, documented change in allele frequency within a population. This is evolution in action, observable within decades.

4. Three Modes of Natural Selection

Natural selection does not always push a population in the same direction. The way selection acts depends on which phenotypes are favoured in a given environment. AQA requires you to compare three distinct modes: stabilising selection, directional selection, and disruptive selection. Each produces a characteristic change in the distribution of phenotypes — and allele frequencies — within the population.

A useful way to visualise these modes is to picture a bell-shaped (normal) distribution curve of phenotype values in the original population. Each mode of selection reshapes that curve in a different way.

Stabilising selection is the most common mode in stable environments — it preserves a well-adapted phenotype and reduces variation. Directional selection is the mode most associated with observable evolutionary change, because it shifts the mean phenotype over time. Disruptive selection is the least common but the most significant for speciation, because it can split a population into two diverging groups.

Worked example: Analysing a Selection Scenario: Identifying the Mode

A population of crabs lives on a rocky shore. Crabs with very pale shells and crabs with very dark shells are both well camouflaged against patches of light rock and dark rock respectively. Crabs with intermediate grey shells are conspicuous against both backgrounds and suffer higher predation. Describe the mode of selection acting on shell colour and explain the expected change in allele frequencies.

1. Pale shells: favoured (camouflaged on light rock). Dark shells: favoured (camouflaged on dark rock). Intermediate grey shells: selected against (conspicuous on both). — Both extreme phenotypes have a selective advantage; the intermediate phenotype is at a disadvantage. This is the defining feature of one specific mode of selection.
2. Both extremes favoured, intermediate selected against → disruptive selection. — Disruptive selection is the only mode in which both extremes are simultaneously favoured. Do not confuse this with directional selection, where only one extreme is favoured.
3. Alleles producing pale shells: increase in frequency. Alleles producing dark shells: increase in frequency. Alleles producing intermediate grey shells: decrease in frequency. — Individuals with favoured phenotypes survive and reproduce more, passing their alleles to more offspring. Alleles associated with the disfavoured intermediate phenotype are passed on less frequently each generation.
4. The original bell-shaped distribution shifts towards a bimodal (two-peaked) distribution, with peaks at pale and dark shell colours and a trough at intermediate grey. — Over generations, the population increasingly consists of pale and dark individuals, with fewer intermediate individuals. Variation within the population increases overall.

Answer: Disruptive selection is acting on shell colour. Alleles producing pale and dark shells increase in frequency; alleles producing intermediate grey shells decrease in frequency. The phenotype distribution becomes bimodal over successive generations.

Notice how the worked example above links each step back to allele frequencies — that is the level of analysis AQA expects. Describing the phenotype change alone is not sufficient; you must connect it to what happens in the gene pool.

5. Connecting the Mechanisms: A Worked Narrative

Earlier, we established that mutation, meiosis, and random fertilisation generate variation. Now we can trace the full chain: variation exists → selection acts → allele frequencies change → the population evolves. Let's walk through a concrete, exam-style scenario to consolidate all the spec points together.

Worked example: Tracing Natural Selection from Variation to Allele Frequency Change

A population of bacteria is exposed to a new antibiotic. Before treatment, 99% of bacteria carry the allele for antibiotic sensitivity (allele S) and 1% carry a mutant allele conferring resistance (allele R). After several rounds of antibiotic treatment, the population is sampled again. Explain, using the concept of natural selection, how the frequency of allele R is expected to change.

1. Allele R arose by mutation — a change in the DNA base sequence. Before treatment, allele R is rare (frequency = 1%) because it conferred no advantage in the absence of the antibiotic. — Mutation is the primary source of genetic variation. The new allele exists in the gene pool before selection begins — selection does not cause the mutation; it acts on pre-existing variation.
2. The antibiotic acts as a selection pressure. Bacteria carrying allele S are killed by the antibiotic (disease/competition analogy: the antibiotic acts like a predator removing susceptible individuals). Bacteria carrying allele R survive. — Predation, disease, and competition are the three named selection pressures in the AQA specification. Here, the antibiotic functions as the environmental pressure causing differential survival.
3. Resistant bacteria (allele R) survive and reproduce. Sensitive bacteria (allele S) are killed before reproducing. Each generation, resistant bacteria contribute far more offspring to the next generation than sensitive bacteria. — Differential reproductive success is the mechanism by which selection changes allele frequencies. It is not enough to survive — individuals must reproduce to pass alleles on.
4. Frequency of allele R increases each generation. Frequency of allele S decreases each generation. After many generations of antibiotic exposure, allele R may approach a frequency of nearly 100% in the population. — This is evolution: a measurable change in allele frequencies within the gene pool of the bacterial population. The mode of selection here is directional — one extreme phenotype (resistance) is strongly favoured.

Answer: The frequency of allele R increases because resistant bacteria survive antibiotic treatment and reproduce, passing allele R to offspring. Sensitive bacteria (allele S) are eliminated before reproducing. Over successive generations, allele R becomes increasingly common in the gene pool — a directional shift in allele frequency that constitutes evolution.

6. Exam Technique and Mark-Winning Strategies

This topic rewards precision. The difference between a 4-mark answer and a 6-mark answer is usually not more facts — it is tighter language and explicit links between mechanisms. Here is how to write answers that collect every available mark.

This topic rewards precision. The difference between a 4-mark answer and a 6-mark answer is usually not more facts — it is tighter language and explicit links between mechanisms. Here is how to write answers that collect every available mark.