Evolution may lead to speciation (A-level only)
AQA
A-Level
No two members of a population are identical. Some characteristics vary in discrete categories, such as blood group or flower colour in a species with two pigment alleles, and this discontinuous variation is usually controlled by one or a few genes with little environmental influence. Others, such as height, body mass or the concentration of a chemical a plant produces, vary continuously across a range, because many genes each make a small contribution and the environment adds more. Plotting a continuous characteristic for a large sample gives a normal distribution, with most individuals near the mean and few at the extremes, and the standard deviation measures how widely they are spread around that mean. This spread is the raw material of natural selection, because selection can only favour phenotypes that already exist in the population.
42
Objectives
40
Exam Tips
63
Pitfalls
71
Key Terms
64
Mark Points
Subtopics in this area
Individuals within a population of a species may show a wide range of variation in phenotype.
This is due to genetic and environmental factors.
The primary source of genetic variation is mutation.
Meiosis and the random fertilisation of gametes during sexual reproduction produce further genetic variation.
Predation, disease and competition for the means of survival result in differential survival and reproduction, ie natural selection.
Those organisms with phenotypes providing selective advantages are likely to produce more offspring and pass on their favourable alleles to the next generation.
The effect of this differential reproductive success on the allele frequencies within a gene pool.
The effects of stabilising, directional and disruptive selection.
Evolution as a change in the allele frequencies in a population.
Reproductive separation of two populations can result in the accumulation of difference in their gene pools.
New species arise when these genetic differences lead to an inability of members of the populations to interbreed and produce fertile offspring.
In this way, new species arise from existing species.
Allopatric and sympatric speciation.
The importance of genetic drift in causing changes in allele frequency in small populations.
Evolution may lead to speciation (A-level only) Revision Guide
Learning Objectives
What you need to know and understand
- Distinguish continuous from discontinuous variation and give an example of each in a named species.
- Use means and standard deviations from a figure to decide whether two groups genuinely differ.
- Explain why a population must show phenotypic variation before natural selection can act on it.
- Explain, for a named characteristic, how genetic and environmental factors each contribute to the variation observed.
- Name a specific abiotic factor rather than a location when accounting for differences between two populations.
- Design a comparison that separates genetic from environmental causes of a difference between two groups of organisms.
- Describe how a change in the base sequence of a gene can produce a non-functional protein, naming each step in the chain.
- Explain why the use of antibiotics does not create resistance alleles but does increase their frequency.
- Explain why only mutations occurring in gametes contribute to genetic variation in the next generation.
- Describe the three processes in sexual reproduction that increase genetic variation and state what each one produces.
- Explain why the independent segregation of 23 homologous pairs generates over eight million chromosome combinations in a gamete.
- Distinguish the variation produced by meiosis and fertilisation from the variation produced by mutation.
- Identify the selection pressure acting in an unfamiliar context and state which phenotype it favours.
- Explain how predation, disease and competition each produce differential survival and reproduction.
- Write a full natural selection explanation in the order variation, selection pressure, survival, reproduction, allele frequency.
- Explain in linked steps why a resistance allele becomes common in a population exposed to a pathogen.
- Explain why a selective advantage in one environment can be a disadvantage in another.
- Rewrite a statement such as 'the organism developed resistance' into correct natural selection language.
- Explain how differential reproductive success changes the allele frequencies of a gene pool over successive generations.
- Explain why a recessive allele falls in frequency only slowly even under strong selection.
- Predict how the allele frequencies of two separated populations diverge when they face different selection pressures.
- Identify stabilising, directional or disruptive selection from distribution curves drawn before and after an environmental change.
- Explain how a change in environment can switch a population from stabilising to directional selection.
- Explain why disruptive selection increases variation and can lead to sympatric speciation.
- Define evolution in terms of allele frequency and explain why the definition applies to populations rather than individuals.
- Use two sets of allele frequencies to decide whether a population has evolved.
- Distinguish evolution from speciation and state the relationship between them.
- Explain how reproductive separation allows two gene pools to change independently of one another.
- Describe, in the correct order, how genetic differences accumulate between two separated populations.
- Identify from data the feature that has stopped gene flow between two populations.
- State the test for whether two populations have become separate species, using the idea of fertile offspring.
- Explain why a sterile hybrid such as a mule shows that two species remain distinct.
- Explain how accumulated genetic differences prevent successful interbreeding between two populations.
- Set out the full sequence from variation within one population to the existence of two species.
- Use a phylogenetic tree to state which species share the most recent common ancestor.
- Explain why populations that interbreed only with difficulty are expected during speciation.
- Distinguish allopatric from sympatric speciation by the type of isolation involved in each.
- Use a distribution map or altitude data to justify allopatric or sympatric speciation for a named pair of species.
- Explain how polyploidy can produce a new plant species in one generation without any geographical separation.
- Explain why chance changes in allele frequency have a greater effect in small populations than in large ones.
- Distinguish the founder effect from a genetic bottleneck, giving an example of each.
- Explain why a population with low genetic diversity is more vulnerable to a new disease or a change in its environment.
Marking Points
Key points examiners look for in your answers
- 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
Examiner Tips
Expert advice for maximising your marks
- 💡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.
Common Mistakes
Pitfalls to avoid in your exam answers
- 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