Populations (A-level only) — AQA A-Level Biology
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Populations (A-level only) explained
Populations (A-level only) covers 5 specification points, including Species exist as one or more populations.
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and A population as a group of organisms of the…. Open a point below for its explanation and revision guidance.
Species exist as one or more populations.
A species is a group of organisms able to interbreed and produce fertile offspring, but its members are rarely spread evenly across the landscape. They usually occupy separate patches of suitable habitat, and each patch holds a population. Lemurs of one species living on opposite sides of a river, or monkeyflowers growing at different altitudes on the same mountain, are one species divided into several populations. What keeps them a single species is gene flow: if individuals move between the populations, or gametes do, as when a bee carries pollen from one hillside to another, alleles are shared and the two gene pools stay similar.
A population as a group of organisms of the same species occupying a particular space at a particular time that can potentially interbreed.
The definition has four parts and each can earn credit: a group of organisms, of the same species, occupying a particular space at a particular time, which can potentially interbreed. The word potentially matters, because members that never actually mate still belong to the population as long as they could. The time element matters because populations change, so the number of monkeyflowers on a hillside in July is not the number in January, and any measurement of population size is a snapshot of one moment. The space element is why ecologists define an area before sampling it, and why mark-release-recapture estimates assume no migration into or out of that area. Because members can interbreed, they share a gene pool, making the population the unit to which allele frequency measurements and Hardy-Weinberg calculations apply.
The concepts of gene pool and allele frequency.
A gene pool is all the alleles of all the genes of all the individuals in a population at a given time. Allele frequency is the proportion of that gene pool made up by one particular allele, written as a decimal or a percentage, and the frequencies of all the alleles of a gene add up to one. Selection changes allele frequencies by changing which individuals reproduce. When antibiotics are used, only the bacteria carrying a resistance allele survive and reproduce, so the frequency of that allele in the population rises. Two populations that are reproductively separated stop exchanging alleles, so their gene pools become separate and can then change independently of one another.
The Hardy–Weinberg principle provides a mathematical model, which predicts that allele frequencies will not change from generation to generation.
The Hardy-Weinberg principle is a mathematical model, not a law of nature. It predicts that in a population meeting a set of conditions the allele frequencies stay the same from one generation to the next, so no evolution takes place. Two equations carry it: p + q = 1 for the frequencies of a dominant and a recessive allele, and p squared + 2pq + q squared = 1 for the genotype frequencies, where p squared is the homozygous dominant, 2pq the heterozygous and q squared the homozygous recessive. Because only the homozygous recessive phenotype reveals its own genotype, almost every calculation begins by setting q squared equal to the proportion showing the recessive characteristic, taking the square root to find q, then finding p as 1 - q. Its real value is as a null model.
The conditions under which the principle applies.
The model predicts constant allele frequencies only if five conditions hold: the population is large, mating is random, there is no migration into or out of it, no mutations occur, and there is no natural selection for or against any genotype. Each condition removes a route by which allele frequencies could change. A large population removes genetic drift, the chance sampling error that shifts frequencies when only a few individuals reproduce. Random mating stops particular genotypes pairing preferentially. No migration prevents alleles being imported or exported. No mutation stops new alleles appearing. No selection means every genotype leaves the same number of offspring on average. Real populations break at least one of these conditions, which is precisely why they evolve.
Your focus
- Explain how several populations can belong to one species, using the idea of gene flow.
- Interpret a distribution map to identify separate populations and describe what is preventing gene flow between them.
- State what must happen to two populations of one species before they count as two species.
Show all 15 objectives
- Define a population in a single sentence containing all four required elements.
- Explain why the definition says organisms can potentially interbreed rather than do interbreed.
- Decide from information about two groups of organisms whether they form one population or two, and justify the decision.
- Define gene pool and allele frequency, and explain why a frequency is a proportion rather than a count.
- Explain how the use of antibiotics raises the frequency of a resistance allele in a bacterial population.
- Describe what happens to the gene pools of two populations of one species when gene flow between them stops.
- Calculate allele and genotype frequencies from the proportion of a population showing a recessive characteristic.
- Calculate the percentage of heterozygous carriers in a population using 2pq.
- Decide whether the Hardy-Weinberg principle would hold for a described population and justify the decision by reference to the conditions of the model.
- List the five conditions under which the Hardy-Weinberg principle predicts constant allele frequencies.
- Explain how breaking each condition would change allele frequencies, giving the direction of change.
- Suggest, from data showing changed allele frequencies, which of the conditions the population has broken.
Populations (A-level only) exam tips
Marking Points
- one mark for defining a species by the ability to interbreed and produce fertile offspring
- one mark for a population as the members of one species living in the same place at the same time
- one mark for gene flow between populations keeping the gene pools of one species similar
- one mark for recognising that separated populations of one species can later become separate species
- one mark for organisms of the same species
- one mark for occupying the same place at the same time
- one mark for the individuals being able to interbreed, or potentially interbreeding
- one mark for linking a population to a shared gene pool when the question moves on to allele frequencies
- one mark for the gene pool as all the alleles of all the genes in a population at a given time
- one mark for allele frequency as the proportion or percentage of that allele in the gene pool
- one mark for describing a change as an increase or a decrease in the frequency of a named allele in a named population
- one mark for separate gene pools, or no gene flow, once two populations no longer interbreed
- Stating that allele frequencies remain constant from generation to generation when the conditions of the model are met.
- Using q squared as the frequency of the homozygous recessive phenotype to find q, then p = 1 - q.
- Calculating 2pq as the frequency of heterozygotes from p and q rather than measuring it directly.
- Explaining that the principle does not hold when any of its conditions is broken, for example when individuals carrying an allele are selected against, so allele frequency changes.
- Listing the conditions required for the principle to hold: large population, random mating, no migration, no mutation and no selection.
- one mark for each condition named, up to the number the question asks for, from large population, random mating, no migration, no mutation and no selection
- one mark for naming selection for or against a particular phenotype or allele as the reason observed frequencies have changed
- one mark for immigration or emigration, or the population not being isolated, as a cause of change
- one mark for explaining that a small population allows genetic drift, so frequencies change by chance
Examiner Tips
- 💡Check which word the question uses; allele frequency and gene pool questions always concern a population, not a whole species.
- 💡If a map shows two groups divided by a barrier, call them separate populations of one species until the data show they cannot interbreed.
- 💡Name gene flow explicitly, because it is the idea that links separate populations back into one species.
- 💡Even a one-mark definition needs every element, so write the whole sentence rather than a phrase.
- 💡In an ecology question, state the area and the time your figure refers to, because those are part of what a population is.
- 💡If the question uses the word potentially, use it in your answer too.
- 💡Use the word frequency or proportion every time, and always give the direction of the change.
- 💡When you describe selection, finish with the allele frequency, because that is usually the last marking point.
- 💡Check whether the data give numbers of individuals or proportions before you claim that a frequency has changed.
- 💡Write down which value the question has given you, and which symbol it belongs to, before you calculate anything.
- 💡When asked whether the principle would hold, answer yes or no and then justify your answer by referring to the relevant conditions of the model.
- 💡Keep frequencies as decimals to at least three places until the final answer, then round as the question instructs.
- 💡Learn the five conditions as a list you can write out in twenty seconds, then choose the ones the data point to.
- 💡If the question asks why frequencies have changed, state the broken condition positively, as there has been selection against, not as there was no selection.
- 💡Check the number of marks; suggest two reasons means two different conditions, not one condition explained twice.
Common Mistakes
- using species and population as if they meant the same thing in a genetics or ecology answer
- saying two populations are different species simply because they live apart or look different
- writing that a population contains several species, which describes a community
- assuming geographical separation instantly creates a new species, with no reference to accumulating genetic differences
- leaving out the same species, so that the definition describes any group of organisms rather than one species; a community is all the populations of different species in an area, which is a different concept
- leaving out the time, which is what makes population size a value for a stated moment
- writing that the individuals do interbreed rather than can interbreed, which wrongly excludes individuals that have not yet bred
- defining a population as all the members of a species, which ignores that a species can have many populations
- describing a habitat rather than the organisms living in it
- writing that the number of alleles increases; the credit is for the frequency, proportion or percentage increasing
- treating allele frequency as the frequency of a phenotype, when a recessive allele is also hidden in the heterozygotes
- saying a gene pool contains genes rather than alleles
- using the term gene when referring to a specific version of a gene, which must be accurately described as an allele
- describing a gene pool as belonging to a species when the question is about one population
- Giving q when the question asks for q squared; read whether it wants an allele frequency or a genotype frequency. Correction: check the symbol and the units requested.
- Forgetting to take the square root, or taking the square root of the wrong quantity. Correction: q squared is the homozygous recessive frequency, so take the square root to find q.
- Treating everyone showing the dominant characteristic as p squared, when that group also contains the heterozygotes, 2pq. Correction: p squared is only the homozygous dominant.
- Giving the frequency of carriers as pq rather than 2pq. Correction: heterozygotes are represented by 2pq.
- Saying the principle is simply wrong for a real population instead of naming the condition that has been broken. Correction: identify which condition, such as selection or non-random mating, is violated.
- giving the converse of a condition when asked why frequencies did change, for example offering random mating as the reason
- listing births and deaths as a cause without qualification; they only change allele frequencies if they differ between genotypes (selection) or occur by chance in small populations (genetic drift)
- answering no random fertilisation when the condition concerns non-random mating
- naming only one condition when the question asks for two, or restating the same idea in different words
- assuming a population is large enough simply because it is described as a population