Selection, Adaptation, and Evolution Revision Notes | A Level AQA Biology

1. Why Selection Matters: The Engine of Evolution

Every species alive today is the product of billions of years of filtering. Individuals that happened to carry useful alleles survived long enough to reproduce; those that did not, largely did not. That filtering process is natural selection, and understanding its mechanics is central to explaining both the diversity of life and some of the most pressing problems in modern medicine.

The AQA specification asks you to do three things with this topic: explain the mechanism of selection, distinguish between its two main modes (directional and stabilising), and classify the adaptations that result. This lesson builds each of those skills in turn.

2. The Mechanism of Natural Selection

Picture a population of beetles living on bark. Within that population, individuals vary in colour — some are pale brown, some are dark brown — because of genetic variation arising from mutation and sexual reproduction. A predatory bird hunts by sight, and pale beetles are far easier to spot against dark bark.

The pale beetles are eaten at a higher rate, so they leave fewer offspring. The alleles coding for dark colouration are passed on more frequently each generation. Over many generations, the population shifts: dark colouration becomes the norm. This is natural selection in action.

There are four conditions that must all be present for natural selection to operate: variation must exist within the population; that variation must be heritable (genetically based); there must be a selection pressure (an environmental factor that affects survival or reproduction); and individuals with the favourable variant must have greater reproductive success. Remove any one of these conditions and selection cannot drive evolutionary change.

• Variation: individuals differ in phenotype due to genetic differences
• Heritability: the variation has a genetic basis and can be passed to offspring
• Selection pressure: an environmental factor that makes some phenotypes more or less fit
• Differential reproduction: individuals with the favourable phenotype leave more offspring

A key point for exam answers: selection acts on phenotypes, but it is allele frequencies that change as a result. Examiners frequently award marks specifically for stating that the frequency of a favourable allele increases in the population — not merely that 'the organism changes'.

3. Directional Selection — Antibiotic Resistance in Bacteria

Now that the mechanism is clear, we can look at the two modes of selection the specification requires. Directional selection occurs when a selection pressure consistently favours one extreme of the phenotypic range, pulling the population mean in that direction.

The AQA-specified example is antibiotic resistance in bacteria. Within any large bacterial population, random mutations mean that a very small number of individuals carry alleles that confer resistance to a given antibiotic — perhaps by coding for an enzyme that breaks down the drug, or by altering the target site so the drug cannot bind.

When a patient takes an antibiotic, the drug acts as a powerful selection pressure. Susceptible bacteria are killed; resistant individuals survive and reproduce. Because bacteria reproduce asexually and very rapidly, the resistant allele can spread through the population within hours. After several generations, the majority of bacteria in that population carry the resistance allele. The population mean has shifted decisively toward resistance — this is directional selection.

This is why completing a full course of antibiotics matters clinically. Stopping early leaves behind the most resistant individuals — those that survived the partial treatment — and gives them the opportunity to repopulate. Each incomplete course effectively acts as another round of directional selection, accelerating the spread of resistance alleles.

Directional selection can also be illustrated with unfamiliar scenarios in the exam — for example, pesticide resistance in insects, or changes in beak length in birds following a shift in food availability. The mechanism is always the same: a selection pressure favours one extreme, allele frequencies shift, and the population mean moves in that direction over generations.

4. Stabilising Selection — Human Birth Weight

Directional selection shifts the population mean. Stabilising selection does the opposite: it maintains the mean by acting against both extremes simultaneously. The result is a narrowing of the distribution around the optimum phenotype.

The AQA-specified example is human birth weight. Babies born at very low birth weight face increased risk of infection, hypothermia, and respiratory difficulties — they have lower survival rates. Babies born at very high birth weight face increased risk of complications during delivery, including oxygen deprivation. Both extremes therefore experience strong negative selection.

Babies born at an intermediate weight — historically around 3.0–3.5 kg — have historically had the highest survival rates. This intermediate phenotype is therefore favoured, and over many generations the distribution of birth weights has been maintained around this optimum. Variation is reduced; the mean is preserved.

When interpreting data like this in an exam, look for the characteristic bell-curve shape of the frequency distribution and the inverted-U shape of the survival curve. The peak of the survival curve should align with the peak of the frequency distribution — that alignment is the evidence for stabilising selection. If the survival peak were at one extreme, you would be looking at directional selection instead.

5. Comparing Directional and Stabilising Selection

Examiners frequently ask you to compare these two modes. A strong comparison answer addresses both similarities and differences — use the word 'whereas' or 'however' to signal contrast, and make sure you cover the effect on the population distribution, not just which phenotype is favoured.

6. Adaptations: Anatomical, Physiological, and Behavioural

Natural selection produces populations that are better adapted to their environment. The specification requires you to classify adaptations into three categories: anatomical, physiological, and behavioural. Each category describes a different aspect of how an organism's biology is matched to its environment.

Physiological adaptations concern the internal biochemical and metabolic processes of an organism. These are not visible from the outside but are equally the product of natural selection acting on genetic variation. A camel's ability to tolerate large fluctuations in body temperature, or the production of highly concentrated urine by desert rodents to conserve water, are both physiological adaptations.

Behavioural adaptations are inherited patterns of behaviour that increase survival or reproductive success. Hibernation in dormice reduces energy expenditure during winter when food is scarce. The courtship displays of birds of paradise attract mates and signal genetic quality. Both are heritable behaviours shaped by selection over generations.

• Anatomical: structural/physical features — e.g. thick fur in Arctic mammals, hollow bones in birds
• Physiological: internal biochemical processes — e.g. production of antifreeze proteins in Arctic fish, high haemoglobin affinity for oxygen at altitude
• Behavioural: inherited patterns of behaviour — e.g. migration in swallows, warning coloration recognition in prey species

One of the highest-demand skills in this topic is using the mechanism of selection to explain change in a scenario you have never seen before. The specification explicitly requires this. The good news is that the underlying logic is always the same — you are applying the same four-step framework to new data.

7. Applying Selection to Unfamiliar Contexts

Worked example: Applying Selection Logic to an Unfamiliar Scenario

A population of moths lives in a woodland. The moths vary in wing pattern from plain to highly patterned. A new predatory bird colonises the woodland. The bird hunts by detecting movement and pattern contrast. After 20 generations, the proportion of plain-winged moths has increased significantly. Explain how natural selection has produced this change. [5 marks]

1. State that variation in wing pattern exists within the population due to genetic differences (mutation and/or recombination). — [B1] Examiners need you to establish that variation is heritable — not just that it exists. Mention 'genetic' or 'alleles' to secure this mark.
2. The new predatory bird acts as a selection pressure. It detects moths by pattern contrast, so highly patterned moths are more visible and more likely to be predated. — [B1] Name the specific selection pressure and link it to the phenotype being selected against. Vague answers ('the environment changed') do not score.
3. Plain-winged moths are less visible to the predator, so they survive at a higher rate and reproduce more successfully. Highly patterned moths are eaten before they can reproduce. — [B1] The phrase 'reproduce more successfully' is key — selection ultimately acts through reproduction, not just survival.
4. Plain-winged moths pass the alleles for plain wing pattern to their offspring at a higher frequency each generation. The frequency of the allele for plain wing pattern increases in the population. — [B1] This is the mark that many students miss. You must state that allele frequency changes — not just that 'more plain moths exist'.
5. Over 20 generations, the proportion of plain-winged moths increases significantly as the allele for plain wing pattern becomes more common. This is directional selection. — [B1] Naming the type of selection (directional) and linking to the time frame in the question shows higher-order understanding.

Answer: Variation in wing pattern exists due to genetic differences. The predatory bird acts as a selection pressure, making highly patterned moths more visible and more likely to be eaten. Plain-winged moths survive and reproduce at a higher rate, passing the allele for plain wing pattern to offspring more frequently. Over 20 generations, the frequency of the plain wing pattern allele increases — this is directional selection.

Notice how each step maps directly onto the four conditions for natural selection. Practise applying this framework to any scenario: identify the variation, name the selection pressure, state differential reproduction, and conclude with a change in allele frequency. That sequence reliably scores full marks on AQA 'explain how natural selection' questions.

8. Selection, Adaptation, and the Diversity of Life

Zoom out from individual populations and the cumulative effect of selection becomes clear. Different environments impose different selection pressures, so populations in different environments accumulate different adaptations. Over long timescales, populations may diverge so far in their allele frequencies and phenotypes that they can no longer interbreed — they become separate species. This is how selection and adaptation together drive evolution and generate the diversity of living organisms.

The Galapagos finches are a classic illustration. A founding population colonised islands with different food sources. On islands where hard seeds dominated, directional selection favoured individuals with stronger, deeper beaks. On islands where insects in bark were the main food source, selection favoured longer, narrower beaks. Each island population accumulated different anatomical adaptations. Over time, the populations diverged into distinct species — each exquisitely matched to its local environment.

The same principle applies at every scale: the antibiotic-resistant bacterium, the birth-weight distribution in humans, the camouflaged moth. Each is a snapshot of selection in progress. Collectively, across millions of species and billions of years, these processes account for the full diversity of life on Earth.

9. Exam Technique: Getting Full Marks on Selection Questions

Selection questions are among the most consistently answered questions on AQA Biology papers — but also among the most consistently under-marked. Students who know the biology still lose marks through imprecise language. The guidance below addresses the most common sources of mark loss.