Understanding Populations and Gene Pools Revision Notes | A Level AQA Biology

1. Why Populations Matter in Genetics

Evolution does not act on individual organisms — it acts on populations. To understand how species change over time, you first need a precise picture of what a population actually is and how its genetic make-up is described.

This lesson builds the vocabulary and conceptual framework that underpins everything in population genetics: from natural selection to speciation. Get these definitions sharp, and the bigger picture clicks into place.

2. Species, Populations, and Where They Live

A species is a group of organisms that share common characteristics and can interbreed to produce fertile offspring. Most species do not exist as one single, continuous mass of individuals — they exist as one or more populations, each occupying a distinct area.

Picture red squirrels in the UK. They survive in isolated pockets — parts of Scotland, Northumberland, and a few islands. Each pocket is a separate population of the same species. The species as a whole spans all those groups, but each population is its own distinct unit with its own local genetic story.

Every word in that definition is doing work. 'Same species' rules out mixed-species groups. 'Particular space' means geography matters — two groups of the same species on opposite sides of a mountain range are two populations, not one. 'Particular time' acknowledges that populations are dynamic; they change across generations. 'Potentially interbreed' is the functional glue — members of the same population can, in principle, exchange genes.

The fact that a species can exist as multiple populations is biologically significant. Separated populations experience different environments, different selection pressures, and different random events. Over time, their gene pools can diverge — which is the starting point for speciation. You will return to this idea when you study isolation and speciation later in the topic.

3. The Gene Pool: A Population's Genetic Library

Now that you have a clear picture of what a population is, the next question is: how do we describe its genetic make-up? The answer is the gene pool.

Think of the gene pool as a library. Each individual organism is like a reader who borrows two copies of each book (one allele from each parent). The library itself — every copy of every book that exists — is the gene pool. No single individual holds all the variation; the variation lives at the population level.

A large, diverse gene pool means the population holds many different alleles for its genes. A small or restricted gene pool — common in isolated or bottlenecked populations — means fewer allele variants are available. This matters enormously for a population's ability to adapt to environmental change.

4. Allele Frequency: Measuring the Gene Pool

Knowing which alleles exist in a gene pool is useful, but knowing how common each allele is tells you far more. That is where allele frequency comes in.

Allele frequency is calculated by counting the number of copies of a specific allele and dividing by the total number of alleles at that locus across the whole population. Because every diploid individual carries two alleles per locus, a population of 50 individuals has 100 alleles at each locus in total.

Worked example: Calculating Allele Frequency in a Population

In a population of 40 field mice, the gene for coat colour has two alleles: B (dark coat) and b (pale coat). Across all 40 mice, there are 56 copies of allele B and 24 copies of allele b. Calculate the frequency of each allele.

1. Total alleles = 40 mice × 2 alleles each = 80 alleles — Every diploid organism carries two alleles at each gene locus, so multiply the number of individuals by 2. This is a common step students skip — always account for diploidy.
2. 56 (B) + 24 (b) = 80 alleles — matches the total — A quick sanity check: the counts of all alleles at a locus must sum to the total number of alleles. If they do not, recheck your data before proceeding.
3. Frequency of B = 56 ÷ 80 = 0.70 — Divide the count of the specific allele by the total allele count. The result is a decimal between 0 and 1.
4. Frequency of b = 24 ÷ 80 = 0.30 — Repeat for the second allele. Notice that 0.70 + 0.30 = 1.00 — allele frequencies at a locus must always sum to 1. Use this as your final check.

Answer: Frequency of B = 0.70 (70%); frequency of b = 0.30 (30%). The two frequencies sum to 1.00, confirming the calculation is correct.

The sum-to-one rule is your built-in error detector. If you have two alleles at a locus and their frequencies do not add to 1 (or 100%), you have made an arithmetic error somewhere. With three or more alleles, the same rule applies — all frequencies at that locus must sum to exactly 1.

5. Comparing Gene Pools Across Populations

With allele frequency as your measuring tool, you can now compare the gene pools of different populations of the same species — and those comparisons reveal a great deal about each population's history and future.

Two populations of the same species can have very different allele frequencies at the same locus. One population might have a high frequency of a particular allele because that allele confers an advantage in its local environment. Another population, geographically isolated, might have a different allele at high frequency due to chance events in its founding group.

A population with a very restricted gene pool — where one allele has a frequency close to 1.0 and all others are near 0 — is described as having low genetic diversity. This is a real conservation concern: cheetahs, for instance, have extremely low genetic diversity due to historical population bottlenecks, making the species vulnerable to disease and environmental change.

6. Exam Technique and Common Mistakes

These concepts appear in AQA exam questions in several ways: as short-answer definitions, as data-interpretation questions involving allele frequency tables, and as the foundation for longer questions on natural selection and Hardy-Weinberg. Getting the language precise here pays dividends across the whole topic.

Understanding populations and gene pools is the foundation for everything that follows in this topic. A population is the unit on which evolution acts; the gene pool is the raw material that evolution works with; and allele frequency is the metric that tells you whether evolution is occurring and in which direction.