Genetic Diversity and Natural Selection Revision Notes | A Level AQA Biology

1. Why Populations Change Over Time

Picture two populations of bacteria — one entirely uniform, one harbouring a handful of individuals with a slightly different enzyme. Introduce an antibiotic and watch what happens. The uniform population is wiped out; the varied one survives. That difference comes down to one thing: genetic diversity, and the mechanism that exploits it is natural selection.

This lesson traces the full chain of events — from a single random mutation, through the filter of the environment, to a measurable shift in allele frequency across generations. Every step is examinable, and the language you use at each step is what separates a 4-mark answer from a 7-mark one.

2. Mutation: The Origin of New Alleles

A mutation is a change in the base sequence of DNA. When a mutation occurs within the coding region of a gene, it can alter the amino acid sequence of the resulting protein — and if that change is heritable, it produces a new allele of that gene. This is the raw material from which all genetic variation ultimately originates.

The word 'random' is doing important work here. Mutations are not directed by the environment — they occur by chance, regardless of whether the resulting allele would be useful. A mutation that arises in a warm, stable habitat is just as likely to produce a heat-resistance allele as a cold-resistance one; the environment does not guide the mutation, it merely determines which mutations are subsequently favoured.

Most mutations are either neutral (no change to protein function) or harmful (disrupted protein structure reduces fitness). For a mutation to affect future generations, it must occur in a germline cell (one that gives rise to gametes — sperm or egg) rather than in a somatic (body) cell; only germline mutations are heritable. Somatic (body cell) mutations occur in non-reproductive cells and cannot be passed to offspring. However, in certain environments, a new allele may produce a protein that functions better than the original — conferring a selective advantage on its possessor. This is the critical junction between mutation and natural selection.

3. Genetic Diversity and Its Role in Natural Selection

Now that we have a mechanism for producing new alleles, we can define the concept that ties everything together. Genetic diversity is the number of different alleles of genes present within a population. The greater the number of different alleles, the higher the genetic diversity of that population.

Here is why this matters for natural selection: if every individual in a population carries identical alleles, there is no variation for the environment to select between. Natural selection requires heritable variation — specifically, variation in alleles — to operate. Without genetic diversity, all individuals respond identically to a new selection pressure, and the population either survives or perishes as a unit. Genetic diversity is therefore a prerequisite for natural selection to occur.

It is worth being precise about what genetic diversity measures. The AQA specification defines it as the number of different alleles of genes in a population — not the number of genes, not the number of chromosomes, and not the number of individuals. Two populations of the same size can have very different levels of genetic diversity depending on how many distinct alleles are present across all their gene loci.

4. The Principles of Natural Selection

Natural selection is best understood as a sequence of linked events. Each step follows logically from the last, and examiners reward answers that make those logical connections explicit. The chain runs as follows.

Worked example: The Mechanism of Natural Selection: Step-by-Step

Describe and explain the full sequence of events by which a new advantageous allele increases in frequency in a population over many generations.

1. A change in the base sequence of a gene arises by chance during DNA replication, producing a new allele at that locus. — This is the source of new genetic variation. The mutation is not directed by the environment — it is random. [B1]
2. The new allele codes for a protein (e.g. an enzyme, receptor, or structural protein) that functions more effectively under the prevailing environmental conditions than the original allele does. — The advantage must be environment-specific. The same allele could be neutral or harmful in a different environment. [B1]
3. These individuals are more likely to survive to reproductive age, find mates, and produce offspring — because the advantageous allele improves their ability to cope with the selection pressure (e.g. predation, disease, resource competition, temperature). — Survival alone is not enough — the individual must reproduce and pass the allele on. Examiners expect you to link survival to reproductive success explicitly. [B1]
4. Because the allele is heritable (carried in the DNA of gametes), it is passed to the next generation. Individuals without the allele are less likely to survive and reproduce, so they contribute fewer offspring. — This is the inheritance step. The allele must be in the germline (gametes) to be inherited — somatic mutations are not passed on. [B1]
5. Because individuals carrying the advantageous allele reproduce at a higher rate, the proportion of that allele in the gene pool increases with each generation. Over many generations, the allele may become the most common allele — or even the only allele — at that locus. — The phrase 'over many generations' is important — natural selection is a gradual process. A single generation rarely produces a detectable shift in allele frequency. [B1]

Answer: Random mutation → new allele → selective advantage in current environment → increased reproductive success → allele inherited by offspring → allele frequency increases over many generations.

Notice how each step feeds into the next. The mutation is meaningless without an environment to test it; the selective advantage is meaningless without reproduction; reproduction is meaningless without inheritance. Examiners are looking for this chain of causation, not a list of isolated facts.

5. Allele Frequency Over Generations: Putting It Together

Earlier we established that genetic diversity — the number of different alleles in a population — is what makes natural selection possible. Now we can see the other side of that relationship: natural selection changes genetic diversity over time. As a favoured allele increases in frequency, other alleles at the same locus become rarer. If selection is strong enough and sustained for long enough, the population may become fixed for the advantageous allele, reducing genetic diversity at that locus.

This has a practical consequence. A population that has been under strong directional selection for a long time may have very low genetic diversity — most individuals carry the same alleles. If the environment then changes, there may be insufficient variation for natural selection to act upon, and the population's ability to adapt is compromised. This is why conservation biologists monitor genetic diversity in endangered species: a genetically uniform population is vulnerable to new diseases, climate shifts, or other novel pressures.

The chart above illustrates the typical S-shaped increase in frequency of an advantageous allele. The rise is slow at first (when the allele is rare and few individuals carry it), accelerates as more individuals inherit it, then slows again as the original allele becomes rare. This pattern is characteristic of directional selection acting on a heritable trait.

6. Applying Natural Selection to Novel Scenarios

Exam questions frequently present a new organism or situation and ask you to apply the principles of natural selection. The key is to map the general mechanism onto the specific context — identifying the selection pressure, the advantageous allele, and the reproductive consequence.

Consider antibiotic resistance in bacteria. A random mutation in one bacterium produces an allele that codes for an enzyme capable of breaking down an antibiotic. In the absence of the antibiotic, this allele confers no particular advantage — the bacterium expends energy producing an enzyme it does not need. When the antibiotic is introduced, however, the selection pressure changes dramatically. Bacteria without the resistance allele are killed; the resistant bacterium survives and reproduces rapidly (bacteria reproduce asexually, so allele frequency can shift over days to weeks depending on conditions). Within a relatively short time, the population is dominated by resistant individuals.

This example also illustrates an important nuance: the environment does not cause the mutation. The antibiotic does not induce resistance mutations — those mutations were already present at low frequency in the population. The antibiotic acts as a selection pressure that changes which alleles are favoured, not as a cause of new mutations. This distinction is frequently tested.

7. Exam Technique: Writing High-Mark Answers

Natural selection questions are among the most predictable in AQA Biology — the same mechanism applies every time, but the marks go to students who apply it precisely rather than vaguely. The following guidance targets the most common ways marks are dropped.

Every mark point in that model answer maps directly to a step in the mechanism covered in Section 4. If you can write that answer fluently under exam conditions, you have mastered the core of this topic. The chain — random mutation, selective advantage, reproductive success, inheritance, frequency change over generations — is the backbone of every natural selection question at A Level.