Syllabus Edition

First teaching 2023

First exams 2025

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Natural Selection (HL IB Biology)

Exam Questions

3 hours37 questions
1a
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2 marks

The following diagram illustrates the development of antibiotic resistance in bacteria.

HwnfSW9B_e-4a

Annotate the bacteria labelled A and B.

1b
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1 mark

State the selection pressure that is applied at C.

1c
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1 mark

The mutation for antibiotic resistance is passed on to other bacteria at point D.

List one of the processes by which this could occur.

1d
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1 mark

State one strategy that could be used to reduce the rate at which resistance evolves in bacteria.

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2a
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3 marks

The blackworm (Lumbriculus variegatus) is a species of worm native to North America and Europe. Blackworm habitat includes marshes, swamps and ponds, and they are a popular food source for fish kept in aquariums.

Each body segment is able to regenerate into a complete individual, and sexual reproduction in blackworms is very rare.

blackworm

Dvortygirl, CC BY-SA 3.0, via Wikimedia Commons

(i)

State the main source of variation in a blackworm population.

[1]

(ii)

List two other sources of variation in other species.

[2]

2b
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2 marks

The blackworm in the image in part a) has grown a second tail. Its ability to regenerate body parts can be considered a useful adaptation.

(i)

Define the term 'adaptation'.

[1]

(ii)

Suggest one role of the regeneration abilities of the blackworm.

[1]

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3a1 mark

Define the term gene pool.

3b2 marks

The number of alleles of one type that occur within a gene pool, expressed as a proportion of the total alleles, is the allele frequency.

State and explain one reason why the frequency of an allele might change over time.

3c1 mark

Different alleles in a population result in different phenotypes. 

State why it is easier to calculate a phenotype frequency than an allele frequency.

3d
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2 marks

In a population of cattle 12 individuals are white, 15 are red, and 28 are roan.

Calculate the frequency of the red phenotype.

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4a
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1 mark

A population of guppies has two alleles of a specific gene in its gene pool. The frequencies of the alleles are shown in the table below. 

A mutation occurs in the population that leads to the formation of a new allele for the gene. 

Complete the table below with the allele frequencies of the new allele between March and July. 

Month Frequency of allele 1 Frequency of allele 2 Frequency of new allele
January 0.81 0.19 0
February 0.78 0.22 0
March 0.77 0.21  
April 0.65 0.27  
May 0.51 0.34  
June 0.43 0.33  
July 0.40 0.24  

4b2 marks

The new allele was a dominant allele that provided the guppies with a survival advantage within their population.  

Predict what will happen to the frequencies of all three alleles for the next five months, from August to December. 

4c2 marks

A few individuals with the mutated allele from the original population travelled to a new area and merged with a different population of guppies. 

In the new population the frequency of the new allele remained low over many generations and then decreased. 

Suggest a reason for the difference in the frequency of the new allele between the population in part a) and this new population.

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5a1 mark

Head circumference in newborn babies is an example of selection in human populations:

  • Babies that are very small at birth have reduced survival chances due to developmental difficulties or infection
  • Babies that are very large at birth have reduced survival chances due to fatal complications during childbirth

State the type of selection that is likely to be acting on head circumference in human babies.

5b2 marks

Selection acting on modern day humans is minimal in the majority of populations. 

Suggest two reasons why selection is minimal in the majority of populations. 

5c2 marks

A scientific study was carried out in 2017 on a small community of people in Indiana known as the Amish. This community mixes very infrequently with the wider US population and children are most often born to parents within the community.

The study found that a large number of individuals in their community possessed a mutated allele that increased their life expectancy by ten years on average. 

Suggest why this allele is found in much higher percentages in this community compared to the rest of the US population. 

5d1 mark

Isolated groups, such as the Amish, make excellent subjects for scientific study. 

Suggest one reason why scientists often focus on isolated groups for studies on inheritance and genetics.

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6a1 mark

Define the term population.

6b4 marks

A population of beetles lives in leaf litter on a forest floor. The beetles are predated on by various species of small birds and rodents.

The beetles have an outer wing case that protect their wings, known as an elytra, which occurs in different colours.

Explain how the beetle population could have evolved from having a large diversity elytra colours to having mostly brown and green. 

6c5 marks

A road was built through the forest in which the beetles lived, causing the two populations to become isolated from each other. 

On one side of the road, the leaf litter is cleared and a population of plants with red berries starts to colonise the area. 

(i)

State the type of speciation that could occur in this instance.

[1]

(ii)

Describe how the speciation named in part (i) could occur.

[3]

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7a3 marks

The image below shows a normal distribution curve.

types-of-selection-graphs-ma

Sketch three graphs to show how this normal distribution curve would change if the population is exposed to directional selection, stabilising selection, and disruptive selection. 

7b4 marks

Explain how natural selection could enable a population of bacteria to become resistant to an antibiotic. 

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1a2 marks

A group of biologists conducted an investigation on a remote archipelago (a collection of islands) in the Pacific Ocean. A species of mouse lives on these islands without any natural predators. The biologists measured the claw length of a large number of these mice.

On half of the islands a species of snake was accidentally introduced that preys on the mice but that cannot climb trees. Several years after the snakes were introduced the biologists returned and found that on the islands with snakes, the claw length of the mice had changed. Some had shorter claws, enabling them to run faster, while others had longer claws, enabling them to climb trees.

Suggest the benefit to the scientists’ investigation of there being islands without any snakes present.

1b3 marks

The evolution of long claws in the mice in part (a) was made possible by a mutation in the gene controlling claw length. 

Explain how a mutation could lead to a change in claw length.

1c3 marks

When the biologists conducted the investigation in part (a), flooding of the islands was very rare. Now, due to climate change, flooding of the islands occurs more regularly. This flooding can regularly wipe out large numbers of ground-living species.

Using this information and the information from part (a), explain how the claw length of the mice on the islands are likely to be changing now.

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2a5 marks

Outline the equivalent features of selective breeding and evolution by natural selection.

2b7 marks

Explain how natural selection can account for the development of antibiotic resistant bacterial strains.

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3a2 marks

Fur colour in rabbits (Oryctolagus cuniculus) is determined by four alleles, each with a varying degree of dominance.

  • Allele C = brown
  • Allele cch = chinchilla
  • Allele ch = himalayan
  • Allele c = white

Scientists investigated the frequency of the different alleles within a population of rabbits. Allele C had a frequency of 0.65 while allele cch had a frequency of 0.25. Allele ch had a frequency that was twice that of allele c.

Calculate the frequency of allele ch.

3b3 marks

The habitat of this rabbit population changed:

  • Temperatures dropped
  • Snowfall became more frequent

Explain the effect that this would have on the allele frequencies in the rabbit population over time.

3c2 marks

The following graph shows the distribution of coat colour within the rabbit population before the environment changed.

q1c_10-3_gene_pools_speciation_medium_ib_hl_biology_sq

State, with a reason, the type of selection that would be observed after the environment changed.

3d2 marks

After several generations the rabbit population accumulated several phenotypic differences to other rabbit populations in nearby habitats.

Explain how scientists could determine whether rabbits from these populations still belong to the same species.

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4a4 marks

Renosterveld is a type of vegetation that is part of South Africa’s fynbos biome and is found within the Cape Floristic Region. Renosterveld is characterised by a variety of species of shrubs and grasses that grow in relatively fertile soil, which make the areas where they grow popular for cultivating crops, such as wheat. Renosterveld is home to the endangered geometric tortoise (Psammobates geometricus), which survives in pockets of natural vegetation.

Wheat crops are cultivated in fields which are securely fenced, preventing the movement of some animals between the natural vegetation and crop fields.

Explain how the cultivation of crops in this region could result in speciation in geometric tortoises over time.

4b2 marks

Scientists investigated one of these tortoise populations and discovered that most individuals were either very large or very small. Very small individuals have the advantage of being able to hide under shrubs from aerial predators, while large individuals have larger shells that make it difficult for aerial predators to get a secure grip on them.

State, with a reason, the type of selection that will be occurring in this population.

4c2 marks

A fire swept through the habitat of the tortoise population, destroying all the vegetation. Only ten tortoises near the edge of the vegetation managed to escape the blaze; all of these individuals had the small phenotype.

The surviving tortoises were rescued by volunteers at a reptile sanctuary and released into a small nature reserve where no other geometric tortoises occurred.

Explain the effect that this event would have on the new tortoise population in the nature reserve.

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5a4 marks

Mining for gold produces waste rocks and mine tailings, which contain sulfur-bearing minerals. When surface water and shallow groundwater come into contact with these minerals, a chemical reaction occurs which produces sulfuric acid. This process is known as acid mine drainage and it can lower the soil pH to a level where very few plants can survive.

Environmentalists studied the area around an abandoned gold mine and discovered a few specimens of earleaf acacia (Acacia auriculiformis) surviving in the acidic soil. This species typically occurs in more neutral or alkaline soils.

Explain how natural selection could produce a population of Acacia auriculiformis that would be tolerant of the acidic soil found around the mine shaft.

5b2 marks

The scientists found that individuals from the population of Acacia auriculiformis that are tolerant to low soil pH flowered at a different time to individuals from the population of Acacia auriculiformis growing in the surrounding habitat, which were not tolerant to low soil pH.

Identify, with a reason, the type of speciation that could occur between the two populations of Acacia auriculiformis.

5c2 marks

State, with a reason, whether the population of Acacia auriculiformis that are tolerant to low soil pH could be considered a stable gene pool.

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6a3 marks

Mosquitoes spread a disease called malaria. DDT is a pesticide used to kill mosquitoes; it is used in many countries in Africa to control the spread of malaria.

Some mosquitoes have an allele that gives them resistance to DDT. A group of biologists studied how frequently this allele occurred in a population of mosquitoes in Uganda over 5 years.

The graph below shows the biologists’ results.

q4a_10-3_gene_pools_speciation_medium_ib_hl_biology_sq

Explain the results shown in the graph.

6b2 marks

Calculate the percentage increase in the percentage of mosquitoes that have the allele for DDT resistance from 2017 to 2020.

6c2 marks

Due to the increase in DDT resistance in mosquitoes, many African countries now use alternative pesticides such as pyrethroids, instead of DDT, to control the spread of malaria.

Explain the effect this may have on the frequency of the allele for DDT resistance over time.

6d2 marks

Describe the effect of stabilising selection on a population.

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7a6 marks

Describe the process of evolution by natural selection.

7b
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2 marks

Outline why some areas of the human genome are more susceptible to detrimental mutations than others.

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13 marks

Outline how artificial selection has led to the domestication of wolves over many years to result in the domestic dogs of the modern day.

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2a
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3 marks

Natural selection would not be possible without the presence of variation within a species.

Explain how variation allows natural selection to occur.

2b
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5 marks

Mutation, meiosis and sexual reproduction are all sources of genetic variation.

Outline the way in which each of these factors contributes to variation within a species.

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3a
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3 marks

Ensatina is the genus name for a group of lungless salamanders (a type of amphibian) that occur in certain regions of the USA. Seven subspecies have been identified across California, each of which shows slight differences in colouration. Some scientists argue that the Ensatina populations represents different species rather than varieties within a single species. This is due to the fact that some of the populations are not able to successfully interbreed.

The map below shows the distribution of the subspecies of Ensatina salamanders across California.

h-3aa

Explain the process that could have resulted in the slight variations in colouration between different Ensatina subspecies.

3b
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2 marks

Some of the adjacent subspecies of Ensatina can successfully breed with one another, but the western subspecies E. eschscholtzii cannot interbreed with the eastern subspecies E. klauberi.

Based on the information provided, evaluate the claim made by some scientists that the different subspecies of Ensatina should be classified as separate species.

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4a
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3 marks

The turtle-headed sea snake, Emydocephalus annulatus, can be found in waters off the coast of Australia, New Zealand and New Caledonia. These snakes usually have banded patterns of white with dark rings, although some individuals exhibit a single dark colour with no banded patterns. The dark parts of the skin contains a high concentration of the pigment melanin, which binds to certain trace elements present in the water. These trace elements are removed from the body when the snake sloughs off the skin. It was found that melanic sea snakes will slough off the skin more frequently than those with banded colouration.

Scientists studied the frequency of melanic sea snakes from several sites in waters surrounding urban-industrial areas and waters from non urban-industrial areas. The results are shown in the table below.

Site Melanic sea snakes in urban-industrial waters / % Melanic sea snakes in non urban-industrial waters / %
A 78 23
B 95 0
C 64 14
D 92 2
E 98 7

Calculate the percentage difference in the mean frequency of melanic snakes found in urban-industrial waters and those that were present in non urban-industrial waters.

4b
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2 marks

Suggest a possible explanation for the results shown in part a).

4c
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3 marks

Based on the information provided in part a), deduce the adaptive advantage of melanism to turtle-headed sea snakes.

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5a2 marks

Malaria is caused by a parasite that attacks red blood cells, producing repeated bouts of serious illness and often causing death.

The allele for normal haemoglobin in red cells is HbA. In the West African country of Burkina Faso, 20% of people are heterozygous for a different allele, HbC, which has no effect on their health.

People homozygous for HbC suffer a very mild anaemia. The graph below shows how the HbC allele affects the chance of getting malaria.

kEFQnmjX_malaria-by-genotype-graph-sq

The HbC allele is increasing in frequency in parts of Africa, such as Burkina Faso.

Suggest an explanation for this.

5b3 marks

The image below shows the prevalence of malaria and the frequency of the HbC allele.

J36r7YTE_africa-diagram-malaria-and-allele-frequencies-sq

Scientists concluded that the HbC allele is more prevalent in areas with a higher risk of mortality from malaria.

Use the information provided to evaluate this conclusion. 

5c
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2 marks

In a village with a population of 500 there were 8 people who were homozygous for the normal adult haemoglobin allele and 96 who were heterozygous. 

Calculate the frequency of the HbC allele in the village.

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6a4 marks

The graph below shows the population sizes of a rabbit and a wolf population over many generations. 

predator-prey-population-graph-sq

Explain the data in the graph for rabbit and wolf populations over the first 300 generations. 

6b4 marks

During the time period of the study a new allele emerged in the rabbit population.

The graph below shows how the frequency of this new allele (allele A) changed over several generations in relation to the existing allele (allele B).

M16uEq77_allele-frequency-graph-for-rabbits-sq

(i)

Use information from both graphs to suggest a reason for the change in frequency of allele A from generation 150 to 200. 

[2]

(ii)

Using the graph above and the graph from part (a), suggest a reason for the change in allele frequency for allele A from generation 200 to 300. 

[2]

6c1 mark

At generation 350 in the graph shown in part b) the frequency of allele A is high, but never 100%. 

State one reason why it is not beneficial for this population to have one allele at 100% frequency.

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7a4 marks

During extended periods of low global temperatures, known as ice ages, the European black bear populations changed from having a small body mass to having a much larger one. 

Explain the concept of directional selection using the example of black bear body mass during an ice age. 

7b1 mark

The evolution of Black bear body mass happened on Earth thousands of years ago.

Suggest how scientists know about their evolutionary past. 

7c4 marks

The tree below is taken from a 2008 study of the mitochondrial DNA of bears.

DD2E1ttJ_bear-phylogenetic-tree-sq

(i)

Suggest how scientists determined the relationships between the bear species. 

[1]

(ii)

The scientists who wrote this study, Krause et al., stated that there was evidence for a "rapid radiation" of bears 5 million years ago. 

Describe the evidence from the figure that supports the idea of "rapid radiation". 

[2]

(iii)

Brown bears and polar bears share a recent common ancestor according to this tree.

Suggest one cause of the speciation event that occurred between the ancestors of brown bears and polar bears.

[1] 

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8a3 marks

A student describes a gene pool as "all the genes that can be found in a particular species". 

Explain why the student's definition is incorrect. 

8b
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5 marks

The evolution of some species can be driven by female mating preferences, e.g. in a species of bird called the long-tailed widowbird, females have a preference for males with long tails. The long tails of male widowbirds have evolved from short-tailed ancestors despite being heavy and limiting male flying ability.

Discuss the selection process that has resulted in long tails in male widowbirds.

8c5 marks

Since the first human genome was sequenced in 2003, scientists have spent a lot of time studying human genomes around the world.

The study of the human genome has involved measuring the frequencies of different alleles in different populations around the world.

Explain some of the reasons for studying and comparing allele frequencies in different human populations, as well as some of the limitations of this process. 

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