Evolution may lead to speciation - AQA A-Level Biology (Paper 2) Exam Questions

Practise 18 exam-style questions on Evolution may lead to speciation for AQA A-Level Biology (Paper 2). Write your answer as you would in the exam and get it marked in seconds against the mark scheme, point by point, with the Study Helper on hand if you get stuck.

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18 questions, marked as you go

Sample questions with model answers

Here are 4 of the questions in this topic, each with a model answer, an explanation of why it earns the marks and the mark scheme an examiner would use.

Question 1

2 marks

Natural selection acts on individuals, but evolution happens in populations.

Explain this statement.

Model answer

Natural selection acts on individuals because it is individuals that survive and reproduce, or fail to: an individual with an advantageous phenotype is more likely to survive and have offspring. An individual's alleles do not change during its life, however. Evolution is a change in allele frequencies in a population's gene pool over generations, which happens because the individuals that survive pass on more copies of their alleles.

Explanation

Evolution is defined as a change in the allele frequencies in a population. Selection is the process: some individuals leave more offspring than others because of their phenotypes. The result is seen only in the population, as a change in the proportions of alleles from one generation to the next. Saying that individuals 'evolve' or 'adapt' during their lifetime is a common misconception.

Mark scheme

1 mark
Selection: individuals with advantageous phenotypes (alleles) are more likely to survive and reproduce / an individual's genotype does not change during its life
Max 2. Accept 'differential survival and reproduction of individuals'.
1 mark
Evolution is a change in the allele frequencies in a population (gene pool) over generations, which cannot happen in one individual
Accept 'evolution is measured as a change in allele frequency, which is a property of a population'. Insufficient: 'evolution is change over time'.
1 mark
(Advantageous) alleles passed on by surviving individuals increase in frequency in the next generation of the population

Question 2

3 marks

Students used beads to model the effect of population size on genetic drift. Red beads represented allele R and white beads represented allele W.

They started with a bag of 1000 beads in which the frequency of allele R was 0.5. To model one generation in a population of 10 individuals, they took 20 beads from the bag, one at a time without looking, recorded the colour and returned each bead to the bag. They calculated the frequency of R in these 20 beads, then made a new bag of 1000 beads with this frequency of R to model the next generation. They repeated this for 15 generations. They then did the same for a population of 100 individuals, taking 200 beads each generation.

The graph shows their results.

Use the graph to explain why genetic drift is important only in small populations.

Line graph of Frequency of allele R (y-axis, 0 to 1) against generation (x-axis, 0 to 15). "Population of 10" joins the points (0, 0.5), (1, 0.6), (2, 0.45), (3, 0.55), (4, 0.7), (5, 0.65), (6, 0.8), (7, 0.75), (8, 0.9), (9, 0.85), (10, 1), (11, 1), (12, 1), (13, 1), (14, 1), (15, 1). "Population of 100" (dashed) joins the points (0, 0.5), (1, 0.535), (2, 0.49), (3, 0.465), (4, 0.51), (5, 0.545), (6, 0.52), (7, 0.475), (8, 0.45), (9, 0.495), (10, 0.54), (11, 0.565), (12, 0.53), (13, 0.5), (14, 0.545), (15, 0.58).

Model answer

In the population of 10, the frequency of allele R changed a great deal from generation to generation, rising from 0.5 to 1.0 by generation 10, so allele W was lost completely. In the population of 100, the frequency of R changed much less, staying between about 0.45 and 0.58. In a small population, the alleles passed on to each generation are a small random sample, so chance can change allele frequencies a lot, and alleles can be lost or fixed. In a large population, chance effects average out, so genetic drift has very little effect.

Explanation

Each draw of beads is a random sample of the gene pool, like the gametes that form the next generation. Small samples deviate further from the true frequency by chance, so allele frequencies in small populations wander and may reach 0 or 1 (loss or fixation). This is why genetic drift is important in small populations, such as those formed by a few founders or after a population crash, but is negligible in large ones. Always support the explanation with values from the graph.

Mark scheme

1 mark
In the population of 10, the frequency of R changed a lot / fluctuated widely (e.g. from 0.5 to 1.0) / R became fixed and W was lost (by generation 10)
Max 2 if the answer does not use any values from the graph. Max 3. Accept any correct values from the graph for the population of 10 (frequencies between 0.45 and 1.0).
1 mark
In the population of 100, the frequency of R stayed close to 0.5 / changed much less (between about 0.45 and 0.58)
Accept values within ±0.02 of the graph.
1 mark
In a small population the alleles passed on are a small (random) sample, so chance has a large effect on allele frequencies
Accept 'sampling error is greater in small samples'.
1 mark
In a large population, chance effects tend to cancel out / average out, so drift causes only small changes (and other factors such as selection are more important)
Accept 'random changes are small relative to the size of a large population'.

Question 3

4 marks

In a species of stick insect, body colour is controlled by one gene. The allele for green body, G, is dominant to the allele for brown body, g. A population of 1000 stick insects living on shrubs with green leaves contained 360 GG, 480 Gg and 160 gg stick insects.

During one summer, birds ate 50% of the brown stick insects and 10% of the green stick insects. Assume that no other stick insects died before breeding.

Calculate the frequency of allele g in the population before the birds fed and in the stick insects that survived. Give your answers to 2 decimal places.

Explain what your answers suggest will happen to this population in the next generation.

Model answer

Before: copies of g = 480 + (2 × 160) = 800 out of 2000, so frequency of g = 0.40

Survivors: GG = 360 × 0.9 = 324; Gg = 480 × 0.9 = 432; gg = 160 × 0.5 = 80; total = 836
Copies of g = 432 + (2 × 80) = 592; total copies = 2 × 836 = 1672
Frequency of g in survivors = 592 ÷ 1672 = 0.354 = 0.35

The frequency of allele g has fallen from 0.40 to 0.35 in the stick insects that will breed. Brown stick insects are less well camouflaged on green leaves, so more of them are eaten. In the next generation, the frequency of allele g and of brown stick insects will be lower: the population is evolving by directional selection.

Explanation

Selection acts on phenotypes, but its effect is measured as a change in allele frequency. Count copies of the allele: each gg stick insect has two copies of g and each Gg one, out of two copies per stick insect. Predation removed a larger proportion of brown stick insects, so the survivors, which form the next generation's parents, have a lower frequency of g. Evolution is this change in allele frequency over generations. The usual error is counting gg individuals as one copy each.

Mark scheme

1 mark
Frequency of g before = 0.40 ((480 + 2 × 160) ÷ 2000); or a correct answer of 0.40 (accept 0.4) with no working
Max 4. A correct answer of 0.40 (accept 0.4) for the frequency before predation with no working scores point 1, and a correct answer of 0.35 for the survivors with no working scores points 2 and 3.
1 mark
Correct method for survivors: 324 GG, 432 Gg and 80 gg survive AND (432 + 2 × 80) ÷ (2 × 836); or a correct final answer of 0.35 with no working
Accept for this mark only: 0.31 (432 + 80 copies counted, gg counted as one copy each) OR 0.354 (not to 2 decimal places).
1 mark
0.35
ECF from incorrect numbers of survivors if the method of counting alleles is correct.
1 mark
Frequency of g (and of brown stick insects) will be lower in the next generation / allele frequency changes, so the population evolves
Accept 'directional selection against the brown phenotype'. ECF from their values if the frequency of g falls.
1 mark
Because brown stick insects are more visible to birds on green leaves, so green stick insects survive and reproduce more and pass on allele G
Accept 'green stick insects are camouflaged'.

Question 4

6 marks

Explain how evolutionary change over a long period of time has resulted in a great diversity of species.

Model answer

Mutations produce new alleles, and meiosis and random fertilisation produce new combinations of alleles, so individuals in a population vary. Different environments impose different selection pressures, such as predation, disease and competition. Individuals with advantageous alleles are more likely to survive and reproduce, so they pass on these alleles and allele frequencies change. When populations become reproductively isolated, for example by geographical barriers or by differences in breeding behaviour or timing within the same area, there is no gene flow between them. Different mutations, different selection pressures and genetic drift cause their gene pools to accumulate differences. Eventually the populations can no longer interbreed to produce fertile offspring, so they are separate species. Over millions of years this process has happened again and again, with each new species able to split further, so a few ancestral species have given rise to a great diversity of species adapted to many different niches.

Explanation

This answer links variation, natural selection, isolation and speciation into one chain, then adds the long time scale: speciation repeated many times, each new species able to split again, produces a branching tree of life. Credit comes from the key terms: new alleles from mutation, different selection pressures, changes in allele frequency, reproductive isolation, separate gene pools and 'cannot interbreed to produce fertile offspring'. Answers that describe only natural selection, without isolation and speciation, cannot explain the number of species.

Mark scheme

1 mark
Mutations produce new alleles / genetic variation
Max 6. Reject 'mutations happen in response to the environment'.
1 mark
Meiosis (crossing over/independent segregation) and random fertilisation produce new combinations of alleles
1 mark
Different environments/habitats produce different selection pressures (e.g. predation, disease, competition)
1 mark
Individuals with advantageous alleles/phenotypes are more likely to survive and reproduce, so pass on their alleles / allele frequencies change
Accept 'differential reproductive success'.
1 mark
Populations become reproductively isolated / separated (by geographical isolation or within the same area)
Accept allopatric or sympatric speciation if linked to isolation.
1 mark
No gene flow, so differences accumulate in the separate gene pools
Accept 'genetic drift in small populations adds to the differences'.
1 mark
Eventually the populations cannot interbreed to produce fertile offspring, so new species form
Both parts are needed: cannot interbreed AND fertile offspring.
1 mark
This happens repeatedly (branching) over millions of years / from common ancestors, so the number of species increases / species adapt to many different niches
Accept 'adaptive radiation' or 'new species can themselves give rise to further species'.

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