Natural Selection (HL IB Biology)

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

Explain how variation allows natural selection to occur.

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.

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.

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

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.

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.

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.

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

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

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 Hb^A. In the West African country of Burkina Faso, 20% of people are heterozygous for a different allele, Hb^C, which has no effect on their health.

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

The Hb^C allele is increasing in frequency in parts of Africa, such as Burkina Faso.

Suggest an explanation for this.

The image below shows the prevalence of malaria and the frequency of the Hb^C allele.

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

Use the information provided to evaluate this conclusion. 

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 Hb^C allele in the village.

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

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

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).

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]

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.

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. 

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

Suggest how scientists know about their evolutionary past. 

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

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] 

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. 

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.

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. 

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

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

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

• Allele C = brown
• Allele c^ch = chinchilla
• Allele c^h = 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 c^ch had a frequency of 0.25. Allele c^h had a frequency that was twice that of allele c.

Calculate the frequency of allele c^h.

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.

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

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

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.

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.

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.

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.

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.

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.

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

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.

Explain the results shown in the graph.

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

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.

Describe the effect of stabilising selection on a population.

Describe the process of evolution by natural selection.

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