Genetic diversity and adaptation
AQA
A-Level
Genetic diversity is defined strictly as the number of different alleles of genes in a population. It is not the number of genes, nor the number of species. All members of a species carry the same genes; what differs is which version, or allele, of each gene they possess. A population is a group of individuals of the same species living in the same area that can interbreed. The greater the number of different alleles present in this population, the greater the genetic diversity. High genetic diversity is crucial because it provides a wider range of phenotypes, meaning there is more variation for natural selection to act upon if environmental conditions change.
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Objectives
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Exam Tips
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Pitfalls
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Key Terms
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Mark Points
Subtopics in this area
Genetic diversity as the number of different alleles of genes in a population.
Genetic diversity is a factor enabling natural selection to occur.
The principles of natural selection in the evolution of populations.
Random mutation can result in new alleles of a gene.
Many mutations are harmful but, in certain environments, the new allele of a gene might benefit its possessor, leading to increased reproductive success.
The advantageous allele is inherited by members of the next generation.
As a result, over many generations, the new allele increases in frequency in the population.
Directional selection, exemplified by antibiotic resistance in bacteria, and stabilising selection, exemplified by human birth weights.
Natural selection results in species that are better adapted to their environment. These adaptations may be anatomical, physiological or behavioural.
Students should be able to: use unfamiliar information to explain how selection produces changes within a population of a species interpret data relating to the effect of selection in producing change within populations show understanding that adaptation and selection are major factors in evolution and contribute to the diversity of living organisms.
Required practical 6: Use of aseptic techniques to investigate the effect of antimicrobial substances on microbial growth.
Genetic diversity and adaptation Revision Guide
Learning Objectives
What you need to know and understand
- Define genetic diversity accurately.
- Distinguish genetic diversity from species diversity and population size.
- Explain the relationship between genetic diversity and the potential for natural selection.
- Explain why natural selection cannot occur in a population with no genetic diversity.
- Describe how different alleles lead to differential survival and reproductive success under a selection pressure.
- Suggest the consequences for a species of low genetic diversity when a new disease appears.
- Explain how a named selection pressure changes the frequency of an allele in a population over several generations.
- Explain why antibiotic resistance spreads faster among bacteria in a hospital than elsewhere.
- Identify and correct a statement that implies an organism mutates in response to its environment.
- Explain how a random mutation produces a new allele of a gene.
- Distinguish between processes that produce new alleles and processes that produce new combinations of existing alleles.
- Explain why a mutation in a body cell does not contribute to the genetic diversity of the next generation.
- Explain why most mutations that alter a protein are harmful, using the link between primary and tertiary structure.
- Use data on a resistant population to explain how a new allele leads to increased reproductive success.
- Explain why an allele that is advantageous in one environment may be disadvantageous in another.
- Explain why survival alone does not change the allele frequency of a population.
- Describe how an advantageous allele passes from parent to offspring.
- Use a named example to explain why individuals carrying a protective allele leave more offspring than those without it.
- Explain in terms of allele frequency how a population changes over many generations of selection.
- Describe data from a graph showing the change in frequency of an allele, quoting figures and units.
- Explain why a bacterial population changes far faster than a human population under an equivalent selection pressure.
- Distinguish directional from stabilising selection using labelled distribution curves before and after selection.
- Explain how the use of antibiotics produces directional selection in a population of bacteria.
- Explain why human birth mass is an example of stabilising selection, naming the disadvantage at each extreme.
- Classify a given adaptation as anatomical, physiological or behavioural and justify the classification.
- Explain how a named adaptation increases an organism's chance of survival and reproduction in a stated environment.
- Explain how an adaptation coded for by alleles becomes more common in a population through natural selection.
- Use unfamiliar information to explain how natural selection produces changes within a population.
- Interpret data relating to the effect of selection in producing change within populations, evaluating its validity.
- Explain how adaptation and selection act as major factors in evolution and contribute to the diversity of living organisms.
- Describe three aseptic techniques used when culturing bacteria and explain the purpose of each.
- Calculate and compare the area of the zones of inhibition produced by different antimicrobial substances.
- Explain why a control disc and a stated maximum incubation temperature are needed in this investigation.
Marking Points
Key points examiners look for in your answers
- one mark for defining genetic diversity as the number of different alleles of genes in a population
- one mark for stating that greater genetic diversity means a wider variety of alleles are present
- one mark for explaining that higher genetic diversity provides more variation for natural selection to act on
- one mark for identifying that a population consists of individuals of the same species in a specific area
- one mark for stating that natural selection requires genetic variation within the population
- one mark for explaining that different alleles result in different phenotypes
- one mark for linking advantageous phenotypes to increased chances of survival and reproductive success
- one mark for explaining that low genetic diversity means a population may lack advantageous alleles when the environment changes
- one mark for random mutation producing a new allele, so there is variation within the population
- one mark for naming the selection pressure and stating that individuals with the allele are more likely to survive
- one mark for those individuals reproducing and passing the advantageous allele to the next generation
- one mark for the frequency of the allele increasing in the population over many generations
- one mark for applying the sequence to the named context, such as non-resistant bacteria dying while resistant bacteria survive and reproduce
- one mark for defining an allele as a different version of a gene with a different base sequence
- one mark for stating that new alleles arise by random mutation
- one mark for stating that mutations occur independently of any need or advantage to the organism
- one mark for stating that only mutations in gametes, or in the cells that form them, are inherited
- one mark for explaining that whether the new allele is advantageous depends on the environment
- one mark for stating that mutation produced the new, for example resistance, allele before the selection pressure was applied
- one mark for identifying the selection pressure in the data, such as the use of DDT or of an antibiotic
- one mark for stating that individuals carrying the allele are more likely to survive and to reproduce
- one mark for the allele being passed on so that its frequency in the population increases
- one mark for recognising that the advantage depends on the environment, so the allele may be disadvantageous elsewhere
- one mark for individuals with the advantageous allele being more likely to survive
- one mark for those individuals reproducing, or being more likely to reproduce
- one mark for the advantageous allele being passed on to offspring (via gametes in sexual reproduction or binary fission in asexual organisms)
- one mark for offspring inheriting the allele, which may be expressed in the phenotype depending on dominance
- one mark for the frequency of the advantageous allele increasing in the population over the generations that follow
- State that the frequency, proportion or percentage of the advantageous allele increases in the population.
- State that the change occurs over many generations.
- State that the frequency of the alternative allele or alleles decreases.
- Relate the rate of change to the strength of the selection pressure or the generation time of the organism.
- Make clear that the change occurs in the population, not within individuals.
- Use the term frequency, proportion or percentage rather than number of alleles when describing the change.
- one mark for directional selection favouring individuals at one extreme, so the mean or mode shifts in that direction
- one mark for stabilising selection favouring individuals near the mean and selecting against both extremes, reducing the range
- one mark for stating that resistant bacteria survive the antibiotic and reproduce, so resistance increases in the population
- one mark for stating that very low and very high birth masses give lower survival, so those alleles are passed on less often
- one mark for interpreting distribution curves correctly, the peak shifted along the axis versus the peak higher and narrower
- one mark for stating that selection increases the frequency of alleles giving characteristics suited to the environment
- one mark for naming an anatomical adaptation and describing the structural feature
- one mark for naming a physiological adaptation, that is a process or substance produced by the organism
- one mark for naming a behavioural adaptation, that is something the organism does
- one mark for explaining how the named adaptation increases survival and reproductive success in that particular environment
- When explaining selection, explicitly state that a random mutation resulted in a new allele, providing a selective advantage against the specific pressure named in the text.
- Describe how organisms with the advantageous allele are more likely to survive and reproduce, passing on this specific allele to their offspring.
- Conclude selection explanations by stating that the frequency of the advantageous allele increases in the population over subsequent generations.
- When interpreting data, explicitly quote data points (with units) to demonstrate the change in the population, before evaluating limitations such as small sample sizes or lack of statistical tests.
- Explicitly link the processes of adaptation and natural selection to being the major driving factors of evolution and the generation of biodiversity.
- Name an aseptic step with its reason, such as flaming the loop to kill microorganisms on it.
- Include a control disc soaked in sterile water or in the solvent alone.
- Quantify the result by measuring the diameter of the clear zone and calculating its area.
- Control a named variable such as disc size, volume of antimicrobial, incubation temperature and time, or the same culture and agar.
- Repeat the plates and calculate a mean to reduce the effect of random errors, improving repeatability.
- Explain why the lid is taped at two points rather than sealed completely, and why the dish is inverted during incubation.
Examiner Tips
Expert advice for maximising your marks
- 💡Always include both 'alleles' and 'population' in your definition of genetic diversity to ensure clarity.
- 💡Link high genetic diversity to an increased ability of a population to adapt to environmental changes.
- 💡Open natural selection answers by stating that there are different alleles in the population, originally produced by random mutation.
- 💡Avoid words like 'want', 'need', or 'try' to survive, as examiners may penalise Lamarckian phrasing.
- 💡Use the five-step chain in order: mutation, variation, selection pressure, differential survival and reproduction, increased allele frequency.
- 💡Name the actual selection pressure given in the stem; a generic answer loses the application mark.
- 💡Write 'more likely to survive' rather than 'survive', because selection is probabilistic.
- 💡Open natural selection answers with 'a random mutation produced a new allele'; it is a mark almost every time.
- 💡Keep the words random and new allele together, because examiners look for both.
- 💡If the question describes a mutation in a body cell, state explicitly that it is not passed on.
- 💡Write 'increased reproductive success' in those words; it is the phrase the specification uses.
- 💡In data questions, quote the figures that show the resistant type increasing over time.
- 💡Explain why the advantageous allele is rare at the start: it has only just arisen by mutation.
- 💡Include all three verbs: survive, reproduce, pass on.
- 💡If you have room, say the allele is passed on 'in the gametes', which shows the mechanism.
- 💡Keep 'more likely to' in front of survive and reproduce throughout.
- 💡Use the words frequency, proportion or percentage, together with the phrase 'over many generations'.
- 💡Where a graph is given, quote the starting and finishing values with units and the time taken.
- 💡Remember a recessive allele persists in heterozygotes, so its frequency falls slowly.
- 💡Sketch both curves before and after selection, labelling the axes as number of individuals against the characteristic.
- 💡Name the favoured extreme in a directional example; that is where the application mark sits.
- 💡For stabilising selection say the range decreases as well as the mean staying the same.
- 💡Adaptation questions almost always want the explanation as well as the example; finish with survival and reproduction.
- 💡If asked for one of each type, check that your three answers genuinely fall in different categories.
- 💡Use the environment described in the stem rather than a textbook example from elsewhere.
- 💡Underline the organism, the allele, and the selection pressure in the question stem, then ensure all three are written into your step-by-step explanation of selection.
- 💡When asked to evaluate a conclusion based on data, look for missing information such as the absence of a control group, small sample sizes, or the lack of a named statistical test.
- 💡Every aseptic step needs its purpose attached; the reason is the mark, not the action.
- 💡Treat the zone of inhibition as the measure of effectiveness, so a larger area means a more effective antimicrobial at that concentration.
- 💡If asked why bacteria still grew on a plate containing an antibiotic, consider too low a concentration, contamination with resistant bacteria, uneven mixing, or mutation.
Common Mistakes
Pitfalls to avoid in your exam answers
- defining genetic diversity as the number of genes, rather than the number of different alleles
- defining genetic diversity as the number of species, which actually describes species diversity
- confusing genetic diversity within a population with the size of the population itself
- writing that organisms adapt or mutate in response to a selection pressure, rather than selection acting on existing mutations
- saying individuals evolve, when it is populations whose allele frequencies change over time
- treating all variation as heritable, when only genetic variation can be passed to offspring
- writing that the antibiotic causes the bacteria to produce a resistance allele, which is explicitly rejected
- saying bacteria become immune to an antibiotic rather than resistant, which examiners reject here
- writing gene where allele is meant
- stopping at 'the resistant ones survive' without reproduction and inheritance
- saying the number of alleles increases rather than the frequency or proportion, as copy number can rise simply through population growth
- writing that the environment or a selection pressure causes the useful mutation to appear
- saying meiosis or crossing over produces new alleles, when they produce new combinations of existing alleles
- using gene and allele interchangeably
- claiming all mutations are harmful, or that every mutation changes the protein
- assuming any mutation occurring in an adult will appear in its children
- claiming the DDT, antibiotic or pathogen caused the mutation to occur
- equating reproductive success with survival alone and omitting the number of offspring produced
- describing the allele as simply 'better' without naming the environment in which it is better
- saying the mutation was needed by the population
- assuming all harmful mutations are quickly removed by selection, forgetting that recessive deleterious alleles can persist in heterozygotes
- treating gene and allele as interchangeable when it is one allele whose frequency changes; a scheme rejecting gene for allele does so once
- stopping at 'they survive' without stating that they reproduce and pass the allele on
- saying parents pass on their characteristics rather than their alleles
- implying offspring acquire the characteristic from the environment rather than inheriting an allele
- assuming every offspring of an advantaged parent must inherit the allele
- Writing that the number of alleles increases rather than the frequency or proportion; mark schemes require frequency/proportion and may reject 'number'.
- Saying individuals change or adapt during their lifetime; selection acts on existing variation, and populations change across generations.
- Expecting the change to be complete within one generation; allele frequency shifts gradually over many generations.
- Claiming the disadvantageous allele disappears at once, when heterozygotes can carry it unexpressed and keep it in the gene pool.
- Forgetting that in small populations allele frequency also changes by chance through genetic drift, not only by selection.
- saying stabilising selection means nothing changes, when allele frequencies at the extremes do fall
- describing directional selection as favouring the strongest rather than one extreme of a measured characteristic
- drawing a curve that changes height but not position for directional selection
- writing that antibiotics make bacteria resistant
- giving birth mass as the example without stating which extremes are disadvantaged and why
- classifying a physiological adaptation as anatomical, for example calling concentrated urine a structure
- describing adaptations as things the organism developed because it needed them
- saying a species is perfectly adapted, when selection only favours the best of the variants available
- giving an adaptation with no link to the environment named in the question
- offering a learned habit as a behavioural adaptation when it is not inherited
- Writing a generic natural selection answer without referencing the specific organism or data. Correction: Always apply the theory to the context by naming the specific allele, organism, and selection pressure provided in the stem.
- Stating that the frequency of the characteristic or gene increases, rather than the allele. Correction: Always specify that it is the frequency of the advantageous allele that increases within the population.
- Treating a correlation shown in data as definitive proof of a causal relationship. Correction: State that correlation does not prove causation and suggest other abiotic or biotic factors that could account for the observed change.
- Sealing the lid all the way round, which creates anaerobic conditions that favour the growth of pathogens. Correction: tape at two points only to allow some gas exchange and prevent anaerobic conditions.
- Incubating above 25 degrees Celsius in a school laboratory, which risks culturing human pathogens. Correction: incubate at or below 25 degrees Celsius to reduce the risk of growing pathogens.
- Comparing zone diameters when the question asks for area, or forgetting to halve the diameter before using pi r squared. Correction: measure the diameter, halve it to get the radius, then use area = pi r squared.
- Omitting the control disc, so a clear zone cannot be attributed to the antimicrobial rather than to the solvent. Correction: always include a control disc soaked in sterile water or the solvent alone.
- Taking the lid fully off while placing the discs, allowing contamination from the air. Correction: lift the lid at an angle and only partially, to minimise entry of airborne contaminants.