Evolution may lead to speciation (A-level only) — AQA A-Level Biology

Speciation describes the evolutionary trajectory whereby one ancestral species splits into two or more distinct, reproductively isolated species. This unit builds upon principles of natural selection, genetic drift, and population genetics, linking microevolutionary changes in allele frequencies to macroevolutionary diversification.

AQA A-Level Biology requires a rigorous understanding of both allopatric (geographical) and sympatric (non-geographical) speciation pathways. Mastering this topic allows students to explain the origin of global biodiversity and evaluate phylogenetic relationships based on evolutionary evidence.

• →Reproductive isolation is the fundamental prerequisite for speciation; it halts gene flow between diverging gene pools.
• →Allopatric speciation requires physical or geographical isolation (e.g., rivers, mountain ranges, islands) followed by independent selection pressures and genetic drift.
• →Sympatric speciation occurs in the same geographical locality, initiated by temporal, behavioural, mechanical, or gametic barriers, often driven by disruptive selection.
• →Speciation is complete only when two populations have diverged genetically to the point where they cannot interbreed to produce fertile offspring.

• one mark for distinguishing continuous variation, measured across a range, from discontinuous variation in distinct categories
• one mark for stating that a continuously varying characteristic is influenced by many genes and by the environment
• one mark for describing the spread of a characteristic using the mean and the standard deviation
• one mark for recognising that selection can act only on variation already present in the population
• one mark for genetic variation arising from different alleles, produced by mutation and rearranged by meiosis and random fertilisation
• one mark for a named environmental or abiotic factor, such as temperature, light intensity or humidity
• one mark for explaining that genetically identical organisms can still differ if their environments differ
• one mark for stating that only genetic differences are passed on to offspring
• Mutation is a change in the base sequence of DNA resulting in the formation of new alleles.
• A change in the base sequence alters the sequence of amino acids (primary structure) in the encoded polypeptide.
• This alters the position of hydrogen, ionic and disulfide bonds, changing the protein's tertiary structure and function.
• Mutations occur randomly and spontaneously, rather than being directed by environmental needs.
• Only mutations occurring during gamete formation can be inherited by the next generation.
• one mark for independent segregation, or random assortment, of homologous chromosomes
• one mark for crossing over between homologous chromosomes, or within a bivalent
• one mark for random fertilisation, or random fusion, of gametes
• one mark for stating that these processes produce new combinations of alleles, or of maternal and paternal chromosomes
• one mark for distinguishing these processes from mutation, which is the source of new alleles
• one mark for naming the selection pressure acting on the population, such as predation, a named pathogen or competition for a named resource
• one mark for variation already existing in the population, caused by mutation
• one mark for individuals with the advantageous phenotype being more likely to survive
• one mark for those survivors reproducing and passing on the advantageous allele
• one mark for the frequency of the advantageous allele increasing over generations
• A mutation produces a new allele, creating variation in the population.
• Individuals with the advantageous phenotype are more likely to survive the selection pressure.
• Survivors reproduce and pass the advantageous allele to their offspring.
• The frequency (proportion or percentage) of the allele in the population increases over successive generations.
• The advantage is specific to the environment, so the same allele may not be favoured elsewhere.
• Individuals with the advantageous phenotype survive and produce more offspring.
• These offspring inherit the advantageous allele.
• The frequency of the advantageous allele in the gene pool increases over successive generations.
• Different selection pressures produce different allele frequencies in separated populations.
• one mark for identifying the type of selection from the shape and position of the distribution curve
• one mark for stabilising selection favouring the modal phenotype and reducing variation in a constant environment
• one mark for directional selection favouring one extreme, so that the mean shifts in that direction
• one mark for disruptive selection favouring both extremes against the intermediates, so that variation increases
• one mark for naming the selection pressure responsible for the change described
• one mark for defining evolution as a change in allele frequency in a population
• one mark for the change being an increase or a decrease, occurring over generations
• one mark for stating that constant allele frequencies mean no evolution is taking place
• one mark for naming a cause of the change, such as natural selection, mutation, migration or genetic drift
• Reproductive separation occurs, preventing gene flow between the two populations.
• The two populations now have separate gene pools.
• Different mutations arise independently in each population.
• Different selection pressures act on each population, favouring different alleles.
• Allele frequencies change differently, accumulating differences until reproductive isolation occurs.
• State that members of the two populations are no longer able to interbreed to produce fertile offspring.
• Explain that any hybrid offspring produced would be sterile due to incompatible chromosomes failing to pair during meiosis.
• Link the inability to interbreed successfully to the accumulation of genetic differences and mutations over time.
• Conclude that the two gene pools remain entirely separate because no alleles can be exchanged between them.
• one mark for a sequence that begins with variation in the original population caused by mutation
• one mark for reproductive separation separating the gene pools
• one mark for different selection pressures changing allele frequencies differently in each population
• one mark for the end point, two populations that cannot interbreed to produce fertile offspring
• one mark for relating the split to common ancestry, as shown by a phylogenetic tree
• one mark for naming geographical isolation, or allopatric speciation, where a physical barrier separates the populations
• one mark for quoting evidence of overlapping distributions before suggesting sympatric speciation
• one mark for naming the isolating mechanism in sympatric speciation, such as different pollinators, different breeding times or polyploidy
• one mark for linking either route to separate gene pools and different allele frequencies
• one mark for genetic drift as a change in allele frequency due to chance rather than selection
• one mark for stating that the effect is greater in a small population
• one mark for explaining that only a sample of the alleles present is passed to the next generation
• one mark for an allele becoming more or less frequent, or being lost entirely, even though it gives no advantage
• one mark for the consequence, a reduction in the genetic diversity of the population

• 💡If a figure shows error bars, say whether they overlap before you claim a difference between the groups.
• 💡Name the type of variation and then say what causes it, because both halves are usually needed for the marks.
• 💡Random sampling is worth naming whenever you are asked how to measure variation in a population.
• 💡Whenever you write different environments, add the specific abiotic factor immediately afterwards.
• 💡In an experiment question, say what you would keep constant, because that is how the two sources of variation are separated.
• 💡Diet, disease and competition all count as environmental factors and are often the ones the data point towards.
• 💡Always link the chain of events logically: altered DNA base sequence -> altered amino acid sequence -> altered tertiary structure -> non-functional protein.
• 💡If asked for the source of new alleles, specify mutation; if asked for new combinations of alleles, specify meiosis or random fertilisation.
• 💡Clearly list the three named processes and explicitly state their outcome to ensure all aspects of the mechanism are covered.
• 💡Write homologous every time you mention the chromosome pairs, because that word is needed for precision.
• 💡If the question has already given you mutation, do not repeat it; it is asking for the other processes.
• 💡Show the calculation 2^23 = 8,388,608 if asked to explain the number of combinations.
• 💡Give the selection pressure by name, then the advantageous phenotype, then survival, then reproduction, then allele frequency.
• 💡Start with variation that already exists, because natural selection explanations normally begin there.
• 💡Use more likely to rather than will, because selection is about probability, not certainty.
• 💡Name the specific resource or pathogen so the selection pressure is clearly identified.
• 💡Four marks usually means four steps: mutation, survival, reproduction, allele frequency. Number them to keep the sequence clear.
• 💡Write 'over many generations' explicitly at the end to secure the timescale point.
• 💡Name the selection pressure and the specific environment, since the advantage is context-dependent.
• 💡Specify the direction of change and the population, for example, 'the frequency of the resistance allele in the hospital population increased'.
• 💡When comparing two populations, state that their allele frequencies now differ, not merely that they changed.
• 💡Sketch the before and after curves in the margin, because deciding which type is acting is far easier from a picture.
• 💡Name the type, then the favoured phenotype, then the selection pressure; three steps usually earn three marks.
• 💡Antibiotic resistance, pesticide resistance and industrial melanism are safe directional examples, and birth mass is the safe stabilising one.
• 💡If a question gives allele frequencies for two generations, comparing them is all the evidence of evolution you need.
• 💡Use the word frequency in the definition, because a change in alleles is too vague to score.
• 💡Link your answer back to the Hardy-Weinberg conditions when the question asks why the frequencies changed.
• 💡Use the phrases 'no gene flow' or 'separate gene pools', as both are directly creditworthy.
• 💡Order matters: state separation first, then independent mutation and different selection pressures, leading to changed allele frequencies and finally reproductive isolation.
• 💡When describing the sequence of speciation, always conclude your explanation by stating that the populations can no longer interbreed to produce fertile offspring.
• 💡Use the mule as a concrete example to illustrate that hybrid sterility, rather than just the physical inability to mate, confirms two populations are separate species.
• 💡Learn the sequence as a chain of six linked statements, because extended speciation questions are marked point by point.
• 💡Begin with the original population, so the examiner can see the new species arising from an existing one.
• 💡Where a tree or a timeline is given, use it to say which species share the most recent common ancestor.
• 💡Use the term and then the evidence, for example allopatric, because the two species are separated by the river shown in Figure 2.
• 💡If the distributions overlap, say so first, because the sympatric marking point is often awarded only once you have quoted that overlap.
• 💡Both routes end the same way, so finish either answer with separate gene pools and an inability to produce fertile offspring.
• 💡Use the word chance or random early in the answer, then add in a small population.
• 💡Link drift to the Hardy-Weinberg condition it breaks, which is the requirement for a large population.
• 💡In a conservation question, connect reduced genetic diversity to a reduced ability to respond to a new selection pressure.
• 💡Structure every speciation essay chronologically: 1. Isolation type -> 2. Mutation/variation -> 3. Selection pressures -> 4. Differential reproductive success -> 5. Allele frequency change -> 6. Inability to produce fertile offspring.
• 💡Always specify that 'allele frequencies change over many generations' rather than using vague terms like 'the animals change'.
• 💡Use precise biological terms for sympatric isolation types: 'temporal' (timing), 'behavioural' (courtship rituals), or 'ecological' (niche preference).

• explaining variation as due to mutation alone, with no mention of meiosis, random fertilisation or the environment
• treating a difference between two sample means as real without checking whether the standard deviations overlap
• saying an organism changes its phenotype to suit its environment, which describes the individual rather than variation within a population
• confusing variation within a population with variation between species
• describing a bar chart of discontinuous data as a normal distribution
• offering different altitudes or different habitats as the environmental factor, when the mark requires the factor itself, such as temperature or humidity
• claiming that an environmentally caused characteristic can be inherited
• naming mutation as the only genetic source and forgetting meiosis and random fertilisation
• saying the environment changes the genes rather than affecting how they are expressed
• giving a factor without saying in which direction it affects the characteristic
• Stating that organisms mutate 'in order to adapt' or 'because they need to'. Correction: Mutations are random and spontaneous; the environment (e.g., antibiotics) selects for existing advantageous alleles rather than causing them.
• Writing that a mutation 'produces different amino acids'. Correction: State that the sequence of amino acids changes, not the amino acids themselves.
• Assuming all mutations alter the phenotype. Correction: Remember that the genetic code is degenerate, so a base substitution may still code for the same amino acid, resulting in no phenotypic change.
• Believing mutations in non-coding DNA cannot affect the phenotype. Correction: Mutations in regulatory non-coding regions can alter gene expression, affecting the phenotype.
• writing random mating when the marking point is the random fertilisation of gametes
• saying meiosis creates new alleles, when it only recombines alleles that already exist
• describing crossing over as occurring between sister chromatids rather than between homologous chromosomes
• naming the processes without saying what they produce, so the new combinations of alleles mark is lost
• placing independent segregation in mitosis, or in meiosis II
• writing that organisms adapt, get stronger or become resistant during their lifetime, instead of the population changing over generations
• saying the fittest survive without saying which characteristic makes them fitter in that particular environment
• referring to immunity when the answer requires a resistance allele
• leaving out reproduction, so the advantage never reaches the next generation
• naming competition without naming the resource being competed for
• Writing that the pathogen caused the mutation. Correction: mutations arise spontaneously; the pathogen is the selection pressure, not the source of the allele.
• Saying the number of alleles increases rather than the frequency. Correction: credit is for frequency, proportion or percentage of the allele in the population rising.
• Stopping at survival without mentioning reproduction. Correction: differential reproductive success is the mechanism; survivors must breed and pass alleles on.
• Treating one generation as sufficient. Correction: allele frequency changes over many generations, so state the timescale explicitly.
• Claiming selection acts directly on alleles. Correction: selection acts on phenotypes; allele frequencies change as a consequence.
• Describing the change as an increase in the number of organisms rather than in the frequency of an allele.
• Stating selection acts directly on alleles, when it acts on the phenotypes those alleles help to produce.
• Ignoring heterozygotes, incorrectly predicting that a recessive allele disappears entirely within one generation.
• describing the curve moving without saying which phenotype had the selective advantage and why
• calling any change in a population directional selection, without checking whether the extremes or the intermediates were favoured
• saying that stabilising selection means no selection is happening, when it is active selection against both extremes
• forgetting that disruptive selection is the one type that increases variation
• misreading a distribution graph, so a taller peak is taken to mean a larger mean
• defining evolution as the formation of new species, which is speciation and only a possible consequence
• saying that an individual organism evolves during its lifetime
• describing evolution as organisms getting better, more advanced or more complex
• writing about a change in the number of organisms rather than in the frequency of an allele
• leaving out the population, so it is unclear what has changed
• Conflating reproductive separation (the initial barrier to gene flow) with reproductive isolation (the final evolutionary outcome).
• Writing 'no inbreeding' instead of 'no interbreeding' between the two populations; the wrong term is rejected.
• Assuming genetic differences appear immediately, rather than accumulating over many generations.
• Stating only that populations 'cannot interbreed' without adding 'to produce fertile offspring'. Correction: Always include the requirement for fertile offspring, as some distinct species can mate but produce sterile hybrids.
• Claiming that physical separation alone makes two populations separate species. Correction: Physical separation causes allopatric isolation, but speciation is only complete when genetic differences prevent successful interbreeding.
• Confusing the cause and effect of genetic differences. Correction: Remember that the inability to interbreed is the consequence of accumulated genetic differences, not the cause of them.
• describing speciation as a single event rather than a sequence taking many generations
• beginning the account with the new species rather than with variation in the original population
• implying that an individual organism becomes a new species
• reading a phylogenetic tree as a ladder of improvement rather than as a record of branching
• leaving out the original population, so the new species appears from nowhere
• describing a geographical barrier without naming geographical isolation or allopatric speciation, when the question specifically asks for the term
• suggesting sympatric speciation in a clearly allopatric context, which is rejected
• claiming sympatric speciation with no evidence that the two populations share an area
• treating different altitudes as a complete explanation, instead of stating the barrier or isolating mechanism it creates
• assuming sympatric speciation is too rare to mention, when polyploidy in plants is a standard example
• explaining drift as selection in disguise, by giving the allele an advantage
• saying drift occurs equally in large populations, when the whole point is that the effect depends on population size
• confusing the founder effect, a few individuals starting a new population, with a bottleneck, a crash in an existing one
• claiming that genetic drift creates new alleles, when it only changes the frequency of alleles already present
• assuming drift always removes harmful alleles
• Believing that geographic isolation alone is speciation: physical separation is only the catalyst; speciation is the resulting genetic divergence that prevents fertile interbreeding.
• Assuming sympatric speciation requires some small physical distance: sympatric organisms inhabit the exact same habitat, but diverge due to ecological, behavioural, or temporal niches.
• Thinking individuals adapt consciously: mutations arise randomly, and environmental pressures passively select for individuals carrying pre-existing or novel advantageous alleles.

1. 1Day 1-2: Review definitions of gene pools, allele frequencies, and the biological species concept.
2. 2Day 3-4: Construct a side-by-side comparison table contrasting allopatric and sympatric speciation with case studies (e.g., Darwin's finches vs. apple maggot flies).
3. 3Day 5-6: Practice writing 4- to 6-mark standard descriptions using the 6-step chronological framework without looking at notes.
4. 4Day 7: Complete past paper questions focusing on data interpretation and experimental evidence for reproductive isolation.

• 📋Extended response (4-6 marks) outlining the stages of either allopatric or sympatric speciation in a named scenario.
• 📋Data analysis questions interpreting graphs of allele frequency changes or phylogenetic trees.
• 📋Suggest and explain questions evaluating novel breeding experiments to determine whether two populations have become distinct species.